Ullmanns Agrochemicals, Vol. 1c 2007 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-31604-5

Fertilizers 3

FertilizersHeinrich W. Scherer, Agrikulturchemisches Institut, Universitat Bonn, Bonn, Federal Republic of Germany(Chap. 1 and 2)Konrad Mengel, Institute for Plant Nutrition, Justus-Liebig-Universitat Giessen, Giessen, Federal Republicof Germany (Chap. 1 and 2)Heinrich Dittmar, BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of Germany (Chap. 3 and 5)Manfred Drach, BASF Aktiengesellschaft, Limburgerhof, Federal Republic of Germany (Chap. 4.1, 4.2 and4.3)Ralf Vosskamp, BASF Aktiengesellschaft, Limburgerhof, Federal Republic of Germany (Chap. 4.1, 4.2 and4.3)Martin E. Trenkel, Eusserthal, Federal Republic of Germany (Chap. 4.4 and 4.5)Reinhold Gutser, Lehrstuhl fur Panzenernahrung, Technische Universitat Munchen-Weihenstephan,Freising, Federal Republic of Germany (Chap. 4.6)Gunter Steffens, Landwirtschaftliche Untersuchungs- und Forschungsanstalt, Oldenburg, Federal Republicof Germany (Chap. 4.7)Vilmos Czikkely, BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of Germany (Chap. 6)Titus Niedermaier, formerly BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of Germany (Chap.6)Reinhardt Hahndel, BASF Aktiengesellschaft, Limburgerhof, Federal Republic of Germany (Chap. 7)Hans Prun, formerly BASF Aktiengesellschaft, Limburgerhof, Federal Republic of Germany (Chap. 7)Karl-Heinz Ullrich, BASF Aktiengesellschaft, Limburgerhof, Federal Republic of Germany (Chap. 8)HermannMuhlfeld, formerly Chemische Fabrik Kalk GmbH, Koln, Federal Republic of Germany (Chap. 8)Wilfried Werner, Agrikulturchemisches Institut der Universitat Bonn, Bonn, Federal Republic of Germany(Chap. 9)Gunter Kluge, Bundesministerium fur Ernahrung, Landwirtschaft und Forsten, Bonn, Federal Republic ofGermany (Chap. 10)Friedrich Kuhlmann, Institut fur Betriebslehre der Agrar- und Ernahrungswirtschaft der Justus-Liebig-Universitat Giessen, Giessen, Federal Republic of Germany (Chap. 11.1)Hugo Steinhauser, formerly Lehrstuhl fur Wirtschaftslehre des Landbaues, Technische Universitat Munchen,Freising, Federal Republic of Germany (Chap. 11.1)Walter Brandlein, BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of Germany (Chap. 11.2)Karl-FriedrichKummer, BASFAktiengesellschaft, Limburgerhof, Federal Republic ofGermany (Chap. 11.3)

See also: Individual fertilizers are also described under the separate keywords Ammonia, AmmoniumCompounds, Nitrates and Nitrites, Phosphate Fertilizers, Potassium Compounds, and Urea

1. Introduction . . . . . . . . . . . . . . 52. Plant Nutrition and Soil Science . 72.1. Plant Nutrients . . . . . . . . . . . . 72.1.1. Denition and Classication . . . . 72.1.2. Function of Plant Nutrients . . . . . 82.2. Soil Science . . . . . . . . . . . . . . . 132.2.1. Soil Classes, Soil Types, and Parent

Material . . . . . . . . . . . . . . . . . 13

2.2.2. Nutrient Retention in Soils . . . . . 152.2.3. Soil pH, Buffer Power, and Liming 182.2.4. Soil Water Plant Relationships . . 192.2.5. OrganicMatter of Soils andNitrogen

Turnover . . . . . . . . . . . . . . . . . 202.3. Nutrient Availability . . . . . . . . . 232.3.1. Factors and Processes . . . . . . . . . 23

4 Fertilizers

2.3.2. Determination of Available PlantNutrients in Soils . . . . . . . . . . . 24

2.4. Physiology of Plant Nutrition . . . 252.4.1. Nutrient Uptake and Long-Distance

Transport in Plants . . . . . . . . . . . 252.4.2. Effect ofNutrition onGrowth,Yield,

and Quality . . . . . . . . . . . . . . . 262.5. Nutrient Balance . . . . . . . . . . . 272.5.1. Gains and Losses of Plant Nutrients 272.5.2. Alternative Plant Nutrition . . . . . . 283. Standard Fertilizers . . . . . . . . . 283.1. Solid Fertilizers . . . . . . . . . . . . 283.1.1. Straight Fertilizers . . . . . . . . . . . 283.1.2. Multinutrient Fertilizers . . . . . . . 283.1.3. Lime Fertilizers . . . . . . . . . . . . 313.1.4. Magnesium Fertilizers . . . . . . . . 323.2. Liquid Fertilizers . . . . . . . . . . . 323.2.1. Nitrogen Liquids . . . . . . . . . . . . 333.2.2. Multinutrient Liquids . . . . . . . . . 363.2.2.1. NP Liquids . . . . . . . . . . . . . . . 363.2.2.2. NPK liquids . . . . . . . . . . . . . . . 393.2.2.3. UAS Liquids . . . . . . . . . . . . . . 393.2.3. Suspensions . . . . . . . . . . . . . . . 404. Special Fertilizers . . . . . . . . . . 424.1. Water-Soluble Nutrient Salts . . . 424.2. Foliar Fertilizers . . . . . . . . . . . 424.2.1. Production . . . . . . . . . . . . . . . . 424.2.2. Application . . . . . . . . . . . . . . . 434.2.3. Combination with Agricultural

Pesticides . . . . . . . . . . . . . . . . 444.3. Micronutrients . . . . . . . . . . . . 444.3.1. Micronutrient Forms . . . . . . . . . 444.3.2. Production . . . . . . . . . . . . . . . . 454.3.3. Commercial Fertilizers . . . . . . . . 464.3.4. Use . . . . . . . . . . . . . . . . . . . . 464.4. Slow- and Controlled-Release

Fertilizers . . . . . . . . . . . . . . . . 474.4.1. Introduction . . . . . . . . . . . . . . . 474.4.2. Urea Aldehyde Slow-Release

Fertilizers . . . . . . . . . . . . . . . . 484.4.2.1. Urea Formaldehyde Condensation

Products. . . . . . . . . . . . . . . . . . 484.4.2.2. Other Urea Aldehyde

Condensation Products . . . . . . . . 494.4.2.3. Further Processing of Urea

Aldehyde Condensates . . . . . . . . 504.4.3. Other Organic Chemicals . . . . . . 514.4.4. Inorganic Compounds . . . . . . . . 524.4.5. Coated and Encapsulated

Controlled-Release Fertilizers . . . 524.4.5.1. Sulfur-Coated Controlled-Release

Fertilizers . . . . . . . . . . . . . . . . 524.4.5.2. Sulfur-Coated, Polymer-Encapsulated

Controlled-Release Fertilizers . . . 53

4.4.5.3. Polymer-EncapsulatedControlled-Release Fertilizers . . . 54

4.4.6. Anti-Float Materials . . . . . . . . . . 554.4.7. Controlled-Release Fertilizers on

Carriers . . . . . . . . . . . . . . . . . 554.4.8. Supergranules . . . . . . . . . . . . . . 554.4.9. Legislation . . . . . . . . . . . . . . . 564.5. Nitrication and Urease

Inhibitors . . . . . . . . . . . . . . . . 564.5.1. Introduction . . . . . . . . . . . . . . . 564.5.2. Types of Nitrication and Urease

Inhibitors . . . . . . . . . . . . . . . . 574.5.3. Pyridines . . . . . . . . . . . . . . . . . 584.5.3.1. Nitrapyrin . . . . . . . . . . . . . . . . 584.5.3.2. Other pyridines . . . . . . . . . . . . . 584.5.4. Dicyandiamide . . . . . . . . . . . . . 594.5.5. Pyrazoles . . . . . . . . . . . . . . . . 604.5.5.1. 1-Carbamoyl-3-methylpyrazole . . 604.5.5.2. Outlook . . . . . . . . . . . . . . . . . 614.5.6. Neem/Neem-Coated Urea . . . . . . 614.5.7. Urease Inhibitors . . . . . . . . . . . . 614.5.8. Environmental Aspects . . . . . . . . 624.5.9. Legal Requirements . . . . . . . . . . 624.6. Organic Fertilizers (Secondary

Raw Material Fertilizers) . . . . . 634.6.1. Fertilizers Based on Peat or

Materials of Similar Stability . . . . 644.6.2. Fertilizers Based onWasteMaterials

of Animal Origin . . . . . . . . . . . . 644.6.3. Fertilizers Based on Wastes of Plant

Origin . . . . . . . . . . . . . . . . . . 654.6.4. Fertilizers Based on Municipal

Waste . . . . . . . . . . . . . . . . . . . 664.7. Manure . . . . . . . . . . . . . . . . . 674.7.1. Composition . . . . . . . . . . . . . . 684.7.2. Manure Nutrient Efciency . . . . . 684.7.3. Environmental Aspects . . . . . . . . 695. Fertilizer Granulation . . . . . . . 705.1. Introduction . . . . . . . . . . . . . . 705.2. Granulator Feedstocks . . . . . . . 725.3. Granulation Equipment . . . . . . 775.3.1. Pug Mill . . . . . . . . . . . . . . . . . 775.3.2. Drum Granulator . . . . . . . . . . . . 775.3.3. Pan Granulator . . . . . . . . . . . . . 815.3.4. The Granulator Mixer . . . . . . . 815.3.5. Roll Presses . . . . . . . . . . . . . . . 825.4. Costs of Agglomeration . . . . . . . 825.5. Bulk Blending . . . . . . . . . . . . . 835.6. Quality Inspection . . . . . . . . . . 845.7. Fertilizer Conditioning . . . . . . . 855.8. Environmental Aspects . . . . . . . 866. Analysis . . . . . . . . . . . . . . . . . 866.1. Sampling and Sample Preparation 866.2. Determination of Nitrogen . . . . . 86

Fertilizers 5

6.3. Determination of Phosphate . . . 886.4. Determination of Potassium . . . . 896.5. Analysis of Calcium, Magnesium,

and Trace Elements . . . . . . . . . 897. Synthetic Soil Conditioners . . . . 897.1. Foams . . . . . . . . . . . . . . . . . . 897.1.1. Closed-Cell Expandable

Polystyrene Foam . . . . . . . . . . . 907.1.2. Primarily Open-Cell Urea

Formaldehyde Resin Foams . . . . . 907.2. Colloidal Silicates . . . . . . . . . . 917.3. Polymer Dispersions and Polymer

Emulsions . . . . . . . . . . . . . . . . 927.4. Tensides . . . . . . . . . . . . . . . . . 938. Storage, Transportation, and

Application . . . . . . . . . . . . . . . 938.1. General Storage Requirements . . 938.2. Application . . . . . . . . . . . . . . . 969. Environmental Aspects of

Fertilizer Application . . . . . . . . 989.1. Nitrogen . . . . . . . . . . . . . . . . . 999.1.1. Ground Water . . . . . . . . . . . . . . 1009.1.2. Surface Waters . . . . . . . . . . . . . 1049.1.3. Atmosphere . . . . . . . . . . . . . . . 104

9.1.4. Biosphere . . . . . . . . . . . . . . . . 1079.1.5. Pedosphere (Soil) . . . . . . . . . . . 1089.1.6. Countermeasures . . . . . . . . . . . . 1089.2. Phosphorus . . . . . . . . . . . . . . . 1099.2.1. Eutrophication . . . . . . . . . . . . . 1099.2.2. Heavy Metals Buildup . . . . . . . . 11110. Legal Aspects . . . . . . . . . . . . . 11111. Economic Aspects . . . . . . . . . . 11311.1. Economics of Fertilization . . . . . 11311.1.1. Input Output Relationships:

The Yield Function . . . . . . . . . . 11311.1.2. Factors Controlling the Optimal

Nitrogen Fertilization Level . . . . . 11411.1.3. Factors Inuencing the Optimal

Nitrogen Fertilization Level . . . . . 11511.1.4. Environmental Aspects of

Fertilization . . . . . . . . . . . . . . . 11511.2. World Consumption, Production,

and Trade . . . . . . . . . . . . . . . . 11611.3. Future Outlook . . . . . . . . . . . . 11911.3.1. Food Situation . . . . . . . . . . . . . 12011.3.2. Development of Fertilizer

Consumption . . . . . . . . . . . . . . 12212. References . . . . . . . . . . . . . . . 123

Fertilizers are products that improve the lev-els of available plant nutrients and/or the chemi-cal and physical properties of soil. An overviewis given over the chemical and physical aspectsof plant nutrition uptake and soil properties. Thedifferent categories of fertilizers are discussed,and special interest is given on production pro-cesses and analyses, including storage and trans-portation as well as environmental, legal, andeconomic aspects.

1. Introduction

Fertilizers in the broadest sense are productsthat improve the levels of available plant nutri-ents and/or the chemical and physical propertiesof soil, thereby directly or indirectly enhancingplant growth, yield, and quality.

Fertilizers are classied as follows in termsof their chemical composition:1) Mineral fertilizers consist of inorganic or syn-

thetically produced organic compounds.2) Organic fertilizers are waste products from

animal husbandry (stable manure, slurry ma-nure), plant decomposition products (com-

post, peat), or products from waste treatment(composted garbage, sewage sludge).

3) Synthetic soil conditioners are productswhose main function is to improve the physi-cal properties of soils, for example, friabilityand water and air transport.The following categories are distinguished

with respect to nutrient content:

1) Straight fertilizers generally contain only oneprimary nutrient.

2) Compound (complex or multinutrient) fertil-izers contain several primary nutrients andsometimes micronutrients as well.

3) Micronutrient fertilizers contain nutrients re-quired in small quantities by plants, as op-posed to macronutrients; quantities rangefrom 1 to 500 g ha1a1.Finally, fertilizers can be classied as solid or

liquid fertilizers and as soil or foliar fertilizers,the latter being applied exclusively by sprayingon an existing plant population.

History. Fertilizing substances were appliedeven in antiquity. Their use can be attributed

6 Fertilizers

to the observation in nature that plants devel-oped especially well in locations where humanor animal excreta, ash residues, rivermud, or dy-ing plants were left. For example, the Egyptiansknew about the fertility of the Nile mud, and theBabylonians recognized the value of stable ma-nure; for example, Homer mentions manure intheOdyssey. Pliny reports that the Ubians northof Mainz used white earth, a calcareous marl,to fertilize their elds. The Romans acknowl-edged the advantages of green manuring, culti-vating legumes and plowing them under. At theend of the rst millenium, wood ash was muchused as fertilizer in Central Europe. Not untilthe beginning of the 19th century did guano,at the suggestion of Alexander von Humboldt(1800), and Chilean caliche, on the recommen-dation of Haenkes (1810), come into use as fer-tilizers. Up to that time, however, it was stillbelieved that the organic matter of soil, humus,was the true source of plant nutrition.

Around 1800, the nutrition problem entereda critical phase in Europe. In 1798, Malthusset forth his pessimistic theses, saying that thequantity of food could increase only in arith-metic progression while the population grew ge-ometrically. Combining results obtained by oth-ers (Sprengel,Boussingault)with his ownpath-breaking studies, J. von Liebig set forth the the-oretical principles of plant nutrition and plantproduction in Chemistry in Its Application toAgriculture and Physiology (1840). He took theview, now considered obvious, that plants re-quire nitrogen, phosphate, and potassium salts asessential nutrients and extract them from the soil.Liebigs mineral theory was well supported byexperimental data of J. B. Boussingault (1802 1887) in France. He and also J. B. Lawes (1814 1900) and J. H. Gilbert (1827 1901) in Eng-land showed that plants benet from inorganicNfertilizers. Liebig thus became the founder of thetheory of mineral fertilizers, and his doctrinesled to an increasing demand for them. A num-ber of companies were subsequently founded inEurope to produce phosphate and potash fertil-izers. Superphosphate was manufactured for therst time in 1846, in England.

In Germany, this industrial developmentstarted in 1855. The importation of saltpeter ona large scale began in the area of the GermanFederation (56 000 t in 1878). Peruvian guanosoon came into heavy use (520 000 t in 1870).

Ammonium sulfate, a coke-oven byproduct, waslater recognized as a valuable fertilizer, and themining ofwater-soluble potassiumminerals wasundertaken in the 1860s [1].

The demand for nitrogen that developed atthe end of the 19th century soon outstrippedthe availability of natural fertilizers. A crucialbreakthrough came aboutwith the discovery andlarge-scale implementation of ammonia synthe-sis byHaber (1909) and its industrial realizationby Bosch (1913).

Around the turn of the century, the tech-nique of hydroponics led to the discovery ofother essential plant nutrients. Research showedthat plants in general require ten primary nutri-ents: carbon, hydrogen, oxygen. nitrogen, phos-phorus, potassium, calcium, magnesium, sulfur,and iron. Javillier and Maze (1908) pointedout for zinc and Agulhon (1910) pointed outfor boron the nutritional effects on plants.War-ington (1923) rst described the symptomsof boron deciency, and Brandenburg (1931)clearly recognized dry rot in the sugar beet asboron deciency.Generallymicronutrientsweremade available to the plant as liquid foliar fer-tilizer, a method rst suggested for iron by Grisin 1844. By 1950, this list of micronutrients hadbeen expanded to include manganese, copper,and molybdenum.

Almost 70 years ago, serious research beganon the best nutrient forms for individual plantspecies under various soil and climatic condi-tions. Besides the classical fertilizers, for ex-ample, controlled-release fertilizers, improvedfoliar fertilizers, nutrient chelates, and nitri-cation inhibitors have been developed in re-cent decades. This development of new nutrientforms is still in full swing in the special fertiliz-ers sector.

In the developed market economies of West-ern Europe, the United States, and Japan, how-ever, the level of mineral fertilizer use has notbeen increasing since the beginningof the 1980s.In some countries, genuine agricultural overpro-duction has occurred recently. Since better de-livery of plant nutrients has led to increasingself-reliance even in the ThirdWorld economies(e.g., China, India, Brazil), these countries arenot so important as purchasers of nutrients onthe world market, so that surpluses cannot beexported without limit. The production of fertil-izers is also on the increase in these countries.

Fertilizers 7

Thus overproduction plus regional environmen-tal problems (nitrates entering the groundwater)are actually leading to a decrease in mineral fer-tilizer use in some areas. This decline will belimited by diminishing soil fertility in localitieswhere fertility has been enhanced by decades ofproper fertilization.

2. Plant Nutrition and Soil ScienceThe science of plant nutrition is situated bet-ween soil science and plant physiology. It com-prises the denition of the elements nutritive forplants; the uptake of plant nutrients and theirdistribution in the plant; the function of the nu-tritive elements in plant metabolism; their effecton plant growth; yield formation and quality pa-rameters in crops; soil nutrient exploitation byplant roots; factors and processes that controlthe plant nutrient availability in soils; toxic ele-ments in soils and their impact on plant growth;the application of plant nutrient carriers (fertiliz-ers) and their turnover in soils; nutrient balance;and the maintenance of soil fertility.

Plant nutrition is consideredmainly from twoaspects, an agronomic one and an ecologicalone. The former is focused on the question offertilizing soil as an efcient means to increasecrop yield and to maintain or even improve soilfertility. The latter, the ecological aspect of plantnutrition, is concerned with the nutritive condi-tion of a soil and a location and with its effecton plant growth and plant communities. Sincefertilizers are the topic of this article, the agro-nomic aspects of plant nutrition are treated withgreater depth.

The science of plant nutrition is closely asso-ciated with the science of soils. The latter com-prises a broad eld of scientic activity and thuscannot be considered here in all its facets. In thisarticle only those problems of soil science rele-vant to understanding plant nutrition are treated.

2.1. Plant Nutrients

2.1.1. Denition and Classication

Froma scientic point of view, the termplant nu-trient is not especially precise. More appropriateis to distinguish between nutritive elements of

plants and nutritive carriers. Essential nutritiveelements for plants are the chemical elementsthat are required for a normal life cycle and thatsatisfy the following criteria:1) A deciency of the element makes it impos-

sible for the plant to complete its life cycle.2) The deciency is specic for the element in

question.3) The element is directly involved in the nutri-

tion of plants because of either its chemicalor its physical properties.

According to this denition, the following chem-ical elements are nutritive elements for plants: C,H, O, N, P, S, K, Ca, Mg, Fe, Mn, Cu, Zn, Mo,B. Further elements, such as Na, Cl, and Si, mayaffect plant growth positively, and there are par-ticular plant species for which these elementsare of great importance. Nevertheless, they arenot essential nutritive elements for plants in thestrict sense of the denition. Cobalt is requiredby some bacteria, e.g., by dinitrogen-xing bac-teria and thus may also benet plant growth in-directly.

Generally it is not the element itself that isprovided to and taken up by the plant, but an ionor a molecule in which the nutritive element ispresent, e.g., C is present inCO2, P inH2PO4 , NinNO3 orNH

+4 , andB inH3BO3. The particular

molecule or ion in which the nutritive element ispresent is termed the nutrient carrier. In the caseof metals, the corresponding ion or salts of ionspecies in question, e.g., K+, Ca2+, Zn2+, canbe considered the carrier. In this sense fertilizersare nutrient carriers.

Plant nutrientsmaybegrouped intomacronu-trients and micronutrients. Macronutrients arerequired in high amounts and thus are present inplant tissues in much higher concentrations thanthemicronutrients. Carbon, H, O, N, P, S, K, Ca,andMg belong to themacronutrients. Their con-centration in the dry plant matter is in the range1 50 mg/g, except for C, H, and O, which havemuch higher concentrations (see Table 2). Theconcentration of the micronutrients in the dryplant matter is in the range 1 1000 g/g.

From the viewpoint of fertilization, those nu-trients that are required by plants in high quanti-ties and that must be regularly supplied by fertil-ization are of particular interest. These nutrientsare N, K, P, and to a minor degree also Ca, Mg,and S. Calcium is a soil nutrient, which means

8 Fertilizers

that it is important for an optimum soil struc-ture. Application of micronutrients is not a com-mon practice, but they are applied at locationswhere soils are low in a particular micronutrientor where soils may bind this micronutrient verystrongly. This is the case for heavy metals (Fe,Mn, Cu, Zn) and B in calcareous and alkalinesoils (soils with a high pH value), while Mo isstrongly xed in acid soils. Acid organic soilsare known for their low available Cu content.

According to the different quantitative re-quirements for macronutrients and micronutri-ents, the former are taken up in much higherquantities than the latter. Thus a wheat standwith a yield potential of 7 t of grain per hectarerequires about 100 kg K but only 100 g Cu.

From a physiological point of view, plant nu-trients are grouped into four groups, as shownin Table 1. The rst group, comprising C, H, O,N, and S, includes all major elementary con-stituents of organic plant matter. Their carriersare presentmainly in the oxidized form, and theymust be reduced during the process of incorpo-ration. The energy required for this reductionoriginates directly or indirectly from photosyn-thetically trapped energy. Assimilation of H isbasically an oxidation process, namely, the ox-idation of water with the help of light energy(photolysis):

The second group (P, B, Si) comprises ele-ments that are taken up as oxo complexes in thepartially deprotonated (P) and protonated (B, Si)form. The oxo complex is not reduced in theplant cell, but may form esters with hydroxylgroups of carbohydrates, thus producing phos-phate, borate, and silicate esters.

The third group comprises metals that aretaken up from the soil solution in ionic form.They are only partially incorporated into the or-ganic structure of the plant tissue: Mg in thechlorophyll molecule, Mn in the electron donorcomplex of photosystem II, and Ca2+ as coun-tercation of indiffusible anions in cell wallsand particularly in biologicalmembranes. Potas-sium is virtually not incorporated into the or-ganic plant matter. It is only weakly adsorbed byCoulombic forces. There exist, however, someorganic molecules that may bind K+ very se-lectively (ionophores, see Section 2.4.1). These

ionophores are likely to be involved in K+ up-take.

The fourth group comprises heavy metals, ofwhich Fe, Cu, and Zn are taken up as ions or inthe form of soluble metal chelates, while Mo istaken up asmolybdate. Thesemolecules are eas-ily incorporated into the organic structure,wherethey serve as essential elements of enzyme sys-tems: Fe in the heme group and in ferredoxin,Mn in arginase [2], Cu in oxidases (polyphenoloxidase, cytochrome oxidase, ascorbate oxidase[3]), Zn in RNA polymerase [4], and Mo in ni-trate reductase [5] and nitrogenase [6].

All nutritive elements of plants, therefore, aretaken up in the form of inorganic complexes,mostly in oxidized form or as metal ions, i.e.,in forms characterized by a low energy level.This is a unique feature of plants, and a fea-ture in which they contrast sharply with animalsand most kinds of microorganisms (bacteria andfungi). Animals and most microorganisms musttake up food that is rich in chemical energy inorder to meet their energy requirements. Plants,at least green plants, meet their energy require-ment by converting radiation energy into chem-ical energy. This energy conversion process ismanifest in the reduction of plant nutrient car-riers (NO3 , SO24 , CO2 ) as already mentioned.Thus important processes of plant nutrition areclosely linked with the unique function of plantsin the great cycle of nature, i.e., the conversionof inorganic matter into organic form. Liebig [7]was correct in commenting on plant nutrition:Die ersten Quellen der Nahrung liefert auss-chlielich die anorganische Natur. (The pri-mary source of nutrition is provided exclusivelyby the inorganic materials in nature.)

2.1.2. Function of Plant Nutrients

Most plant organs and particularly plant partsthat are metabolically very active, such as youngleaves and roots, are rich in water (ca. 80 90wt% of the total fresh matter), while their or-ganic material is ca. 12 18 wt% and their min-eral content is 2 6 wt%. As shown in Table2, in the dry matter of plant material O and Care by far the most abundant elements, followedby H, N, and K. The elements C, O, H, and, tosome extent, N are mainly structural elements inplant matter. They can, however, form chemical

Fertilizers 9Table 1. Physiological classication of plant nutritive elements, nutrient carriers, and form in which the nutrient is taken up

Nutritive element Nutrient carrier UptakeFirst groupC CO2, HCO3 CO2 by leaves, HCO

3 by roots

H H2O H2O by leaves, H2O and HCO3 by rootsO CO2, HCO3 , O2 O2 and CO2 by leaves, HCO

3 and O2 by roots

N NH+4 , NH3, NO3 , NOx NH

+4 and NO

3 by roots, NH3 and NOx by leaves

S SO24 , SO2, SO3, H2S SO24 by roots, SO2, SO3, and H2S by leaves

Second groupP H2PO4 , HPO

24 H2PO

4 and HPO

24 by roots

B H3BO3, borates H3BO3 and B(OH)4 by rootsSi silicates Si(OH)4 by roots

Third groupK K+, K salts K+ by rootsMg Mg2+, Mg salts Mg2+ by rootsCa Ca2+, Ca salts Ca2+ by rootsMn Mn2+, Mn salts Mn2+ by roots

Fourth groupFe, Cu, Zn, Mo ionic form or metal chelates, by roots in ionic form or in the form of soluble metal

minerals containing these chelates, Mo in the form of the molybdateelements

groups that are directly involved in metabolicprocesses, e.g., carboxyl groups, amino groups,hydroxyl groups.

Table 2.Mean content of chemical elements in the dry matter ofgreen plant material

Element Content, g/kgO 440C 420H 60N 30K 20P 4All other elements 26

Since in many soils the available N is low,nitrogen [7727-37-9] is the most important fer-tilizer element, and for this reason its functionin plant metabolism deserves particular inter-est. Nitrogen is an essential element for aminoacids, proteins, nucleic acids, many coenzymes,and some phytohormones. Basic biochemicalprocesses of meristematic growth, such as thesynthesis of proteins and nucleic acids, re-quire N. If this nutrient is not supplied in suf-cient amounts, the growth rate is depressedand the synthesis of proteins affected. Nitrogen-decient plants are characterized by low proteinand high carbohydrate contents. This relation-ship is shown in Table 3 [8].

Nitrogen is also essential for the formationof chloroplasts, especially for the synthesis ofchloroplast proteins. HenceNdeciency is char-acterized by low chlorophyll content; the leaves,

especially the older ones, are pale and yellow;the stems thin and the plants small. Nitrogen-decient plants senescence earlier, probably be-cause of a deciency of the phytohormone cy-tokinin. Abundant N supply increases the pro-tein content, especially the content of free ami-no acids, and often also the content of NO3 inplants. An example of this is shown in Table 4[9]. Excess nitrogen nutrition results in luxuri-ous plants that frequently are susceptible to fungiattack.

Table 3. Effects of N supply on yield of dry matter and the contentof organic N and carbohydrates in the dry matter of young timothyplants (Phleum pratense) [8]Yield and content N supply

Low SufcientYield, g/pot 15.7 20.2Content, mg/gOrganic N, 20.5 31.5Sucrose 46.9 22.6Fructans * 22.2 9.2Starch 32.8 11.7Cellulose 169 184

* Polysaccharides of fructose.

The ratio of N to S in plant matter is ca.10 : 1. Hence sulfur [7704-34-9] is required inmuch lower quantities than N. Their functionsare, however, similar. Sulfur is an elementaryconstituent of most proteins; the SH group in in-volved in various enzymatic processes and it isthe reactive group of coenzymeA.Disulde (S S) bridges are essential structural elements in thetertiary structure of polypeptides and in many

10 FertilizersTable 4. Relationship between N fertilizer rate and nitrogenous fractions in the dry matter of rye grass [9]Nitrogen Nitrogenous fraction, g/kg

fertilizer Total N Protein N Free amino NO3 and NO2 N

rate, kg/ha acid N0 13.2 9.8 1.6 0.4

110 18.9 12.6 2.1 0.6440 37.3 20.6 5.6 3.5

volatile S compounds, such as diallyl disulde,which is the main component in garlic oil. Mus-tard oils occurring in many species of the Cru-ciferae contain a S-glycosidic bond and a sul-furyl group:

Insufcient S supply results in a decrease ofgrowth rate with extremely low levels of SO24and high concentrations of free amino com-pounds and NO3 in the leaves, which is dueto hampered protein synthesis. Sulfur plays animportant role in the baking quality of wheat,since the concentration of S compounds in thegluten fraction is responsible for the linkagesbetween the protein molecules [10]. Sulfur de-ciency may also affect N2 xation of legumesby causing unfavorable conditions in the hostplant or because of the relatively high S contentof nitrogenase and ferredoxin [11]. Deciencysymptoms of S appear at rst in the youngestleaves, which turn light green to yellow. Abun-dant supply with S results in an accumulation ofsulfate in plant tissues.

Sulfur oxide can be taken up by the leaves andmetabolized and thus can contribute to the S nu-trition of plants. Too high SO2 concentrations inthe atmospheremay be toxic. The toxicity symp-toms are necrotic spots in the leaves. Accordingto Saalbach [12], the critical SO2 level in theatmosphere for annual plants is 120 g/m3. Fortrees and other perennials it is about half thislevel. The currently much discussed damage totrees in the forest of the Federal Republic ofGer-many (mainly spruce and silver r) is not causedby toxic SO2 levels.

Phosphorus [7723-14-0] is an essential ele-ment in nucleic acids and various phospholipids(phosphoglyceride and phosphosphingolipids).In both cases. phosphate is esteried with sugars

(nucleic acid) or with alcohol groups of glycerolor sphingosine. Phosphate is also present in var-ious coenzymes; the most prominent is adeno-sine triphosphate (ATP), which carries a kind ofuniversal energy that is used in a number of bio-chemical processes. Metabolites and enzymescan be activated by phosphorylation, a transferof the phosphoryl group from ATP to the meta-bolite according to the following reactions:Activation of glucoseGlucose + ATP Glucose-6-phosphate+ ADPPhosphorylation of an enzymeEnzyme OH + ATP EnzymeOP+ ADP

Such reactions demonstrate the essential roleof P not only in plant metabolism but also in allliving organisms. Undersupply with P results ina reduced growth rate, and seed and fruit forma-tion is affected. The leaves of P-decient plantsoften show a gray dark green color; the stemsmay turn red. The P reserve in seeds is the Mg(Ca) salt of the inositol hexaphosphate (phyticacid):

The physiological role of boron has re-mained obscure until now, and therefore varioushypotheses with numerous modications existconcerning the physiological and biochemicalrole of boron in higher plants. Depending on thepHof the soil, boron seems to be taken upmainly

Fertilizers 11

as undissociated boric acid or as the borate an-ion. Plant species differ in their boron uptakecapacity, reecting differences in boron require-ments for growth. However, there is still somecontroversy about boron translocation in plants.At least in higher plants, a substantial propor-tion of the total boron content is complexed inthe cell walls in in a cis-diol conguration [14].According to Birnbaum et al. [13], B is involvedin the synthesis of uracil and thus affects UTPformation. (UTP is an essential coenzyme for thesynthesis of sucrose and cell-wall components.)Also the synthesis of ribonucleic acid is ham-pered in the case of B deciency. Since uracil isan integral part of ribonucleic acid (RNA), theformation ofRNAmay also be related to the syn-thesis of uracil. Pollard et al. [15] suggest thatB has a specic inuence on plant membranesby the reaction of borate with polyhydroxy com-pounds.

Boron deciency appears as abnormal or re-tarded growth of the apical growing points. Theyoungest leaves aremisshapen andwrinkled andshow a darkish blue-green color. The fact that Bdeciency primarily affects the apex is in accordwith the impaired synthesis of ribonucleic acidsrequired for meristematic growth. High levelsof available B in the soil may cause B toxicityin plants. This is mainly the case in arid areas;however, B toxicity can also be the consequenceof industrial pollution [16]. The toxicity is char-acterized by yellow leaf tips followed by pro-gressive necrosis. The leaves take on a scorchedappearance and drop prematurely.

Silicon [7440-21-3] is not an essential ele-ment for plants; however, it has a benecial ef-fect on various plant species, mainly grasses[17]. In plants well supplied with Si, cuticu-lar water losses are diminshed and resistanceagainst fungal attack is improved [18]. The fa-vorable effect of Si on rice growth iswell known.Silicon-containing fertilizer is frequently ap-plied in rice production.

Among the metal cation species, the potas-sium [7440-09-7] ion, K+, is the nutrient plantstake up from the nutrient medium at the high-est rates. The K+ concentration in the cyto-plasm is about 100 mM and thus much higherthan the concentration of other ion species [19].Probably this high K+ concentration has a fa-vorable inuence on the conformation of vari-ous enzyme proteins [20]. Potassium ions can

easily penetrate plant membranes (see Section2.4.1), which often leads to a depolarization ofthe membranes. Membrane depolarization, it issupposed, has a favorable effect onmeristematicgrowth, photophosphorylation, aerobic phos-phorylation, and phloem loading [21]. These ba-sic processes are important for the long-distancetransport of photosynthates, the synthesis of var-ious organic compounds, and CO2 assimilation.

The data inTable 5 show thatwith an increaseof K+ in alfalfa leaves (Medicago sativa), theCO2 assimilation rate increased, while the mi-tochondrial respiration rate decreased [22]. Inthe case of low K, the respiration was about 2/3of the CO2 assimilation, while with high K theC gained by assimilation was about 11 timeshigher than the C lost by respiration. This typi-cal behavior indicates that under the conditionsof K+ deciency much of the stored carbohy-drates must be respired in order to meet the ATPdemand of the plant. Plants undersupplied withK+ have therefore a low energy status. Suchplants are highly susceptible to fungal attack,water stress, and frost damage.

Table 5. Relationship between K+ concentration in the dry matterof alfalfa leaves, CO2 assimilation, and mitochondrial respiration[22]

Concentration Carbon gain and loss, mg dm2 h1

of K+, mg/g CO2 Mitochondrialassimilation respiration

13 11.9 7.5620 21.7 3.3438 34.0 3.06

Potassium is important in determining the os-motic pressure of plant uids, and K+-decientplants are characterized by inefcient water use.Sodium ions may replace some K+ functions,e.g., the less specic osmotic functions. Impor-tant counterions of K+ in plant tissues are Cl,NO3 , and organic anions. The frequently ob-served favorable effect of Na+ and Cl on plantgrowth is related to their osmotic functions.

Plants suffering from K+ deciency show adecrease in turgor, and under water stress theyeasily become accid. Plant growth is affected,and the older leaves show deciency symptomsas necrosis beginning at the margins of tips andleaves. InK+-decient plant tissue, toxic aminessuch as putrescine and agmatine accumulate.

The most spectacular function ofmagnesium[7439-95-4] is its integral part in the chloro-

12 Fertilizers

phyll molecule. Besides this function, Mg2+ isrequired in various other processes and, the Mgxed in the chlorophyll molecule amounts onlyto about 20% of the Mg present in green planttissues. Magnesium is an essential ion in ribo-somes and in the matrix of the cell nucleus. HereMg2+ is bound by phosphate groups, since theMg2+ is strongly electrophilic and thus attractsoxo complexes such as phosphate [23]. Themagnesium ion activates numerous enzymaticreactions in which phosphate groups are in-volved. The activation is assumed to be broughtabout by bridging the phosphate group with theenzyme or with the substrate. This is an univer-sal function of Mg2+ not only relevant for plantmetabolism but also for practically all kinds oforganisms.

Deciency of Mg2+ affects chlorophyll syn-thesis: leaves turn yellow or red between theveins. The symptoms begin in the older leaves.Protein synthesis and CO2 assimilation are de-pressed under Mg2+ deciency conditions. Re-cent results [24] have shown that the yellowingof spruce needles in the Black Forest is due toa Mg2+ deciency and can be cured by Mg2+fertilizer application.

Calcium [7440-70-2] is the element of theapoplast (cell wall and free space) and of bi-ological membranes. Here it is adsorbed at thephosphate head groups of membrane lipids, thusstabilizing the membranes [25]. Most of theCa2+ present in plant tissues is located in theapoplast and in the vacuole, some in the mi-tochondria and in the chloroplasts, while thecytoplasm is extremely low in Ca2+ (107 to106M). The maintenance of this low cytoplas-mic Ca2+ concentration is of vital importancefor the plant cell [26]. Higher cytoplasmic Ca2+concentrations interfere with numerous enzy-matic reactions and may even lead to a precip-itation of inorganic phosphates. This low Ca2+concentration sufces to form a complex withcalmodulin, a polypeptide of 148 amino acids.The Ca calmodulin complex is a universal en-zyme activator. The activation is brought aboutby allosteric induction.

Direct Ca2+ deciency in plants is rare, sincemost soils are relatively rich in Ca2+. Physi-ological disorders as a consequence of an in-sufcient Ca2+ supply of particular plant parts,however, occur frequently. Calcium is mainlytranslocated by the transpiration stream. Hence

plant parts such as fruits, which mainly feedfrom the phloem and less from the xylem sap,may suffer from an insufcient Ca2+ supply.Shear [27] cites a list of 35 such Ca2+-relateddisorders in fruits and vegetables. Two of themost important ones involve storage tissues andresult in poor crop qualities [28]: bitter pit inapples, characterized by small brown spots onthe surface, and blossom-end rot in tomatoes, acellular breakdown at the distal end of the fruit,which is then susceptible to fungal attack.

Manganese [7439-96-5] is an integral partof the superoxide dismutase and of the elec-tron donor complex of photosystem II. Man-ganesemay activate enzymes in the sameway asMg2+ by bridging the phosphate group with theenzyme or the substrate. Deciency of Mn2+leads to the breakdown of chloroplasts. Char-acteristic deciency symptoms are smaller yel-low spots on the leaves and interveinal chloro-sis. Manganese toxicity may occur, especiallyon ooded soils, because of the reduction andthus solubilization of manganese oxides. Tox-icity symptoms are generally characterized bybrown spots of MnO2 in the older leaves sur-rounded by chlorotic areas [29].

Iron [7439-89-6] is an essential element forhaem and ferredoxin groups. Iron deciencyleads to chloroplast disorders; the synthesis ofthylakoid membranes is disturbed and the pho-tochemical activity affected [30]. Iron deciencyis characterized byyellow leaves. The symptomsare at rst visible in the younger leaves. Thereis evidence that the deciency, mainly occurringin plants growing on calcareous soils, is not in-duced by an insufcient Fe uptake from the soilbut by a physiological disorder in leaves, affect-ing the Fe distribution in the leaf tissue [31].

Iron toxicity can be a problemunder reducingsoil conditions, which prevail in ooded soils.Under such conditions iron(III) oxides are re-duced and the iron is rendered soluble. This mayincrease the Fe concentration in the soil solu-tion by a factor of 102 to 103 [32] so that plantsmay suffer from Fe toxicity, characterized bytiny brown spots on the leaves, which later mayturn uniformally brown. Iron toxicity is knownas bronzing.

Copper [7440-50-8] is an essential elementof various enzymes, such as superoxide dismu-tase, polyphenol oxidases, plastocyanin of thephotosynthetic transport chain, and cytochrome

Fertilizers 13

c oxidase, the terminal oxidase in the mitochon-drial electron transport chain. Deciency in Culeads to pollen sterility and thus affects the fruit-ing of plants. Copper-decient plants often arecharacterized by white twisted leaf tips and atendency to become bushy.

Zinc [7440-66-6] is an integral part of car-bonic anhydrase, superoxide dismutase, RNApolymerase, and various dehydrogenases. It isclosely involved in the N metabolism of plants.In Zn-decient plants, protein synthesis is ham-pered and free amino acids accumulate. Thereis evidence that Zn is involved in the synthe-sis of tryptophan, which is a precursor of indoleacetic acid, an important phytohormone. Zincdeciency is characterized by short internodes,small leaves, and chlorotic areas in the olderleaves. Frequently the shoots die off and theleaves fall prematurely.

Molybdenum [7439-98-7] is present in thenitrate reductase and in the nitrogenase systemthat catalyzes the bacterial xation (reduction)of dinitrogen. Deciency of Mo frequently ap-pears rst in the middle and older leaves as ayellowish green coloration accompanied by arolling of leaf margins. Cruciferae species areparticularly susceptible to Mo deciency. Themost well-known Mo deciency is the whip-tail of cauliower. For further information onthe physiology of plant nutrition, see [3, 23, 33].

2.2. Soil Science

2.2.1. Soil Classes, Soil Types, and ParentMaterial

According to Schroeder [34], soil is the trans-formation product of mineral and organic sub-stances on the earths surface under the inu-ence of environmental factors operating over avery long time and having dened organisationand morphology. It is the growing medium forhigher plants and basis of life for animals andmankind. As a space-time system, soil is fourdimensional.

Soils are complex, quite heterogeneous, andmay differ from each other considerably. Never-theless, all soils have some common features.They possess a mineral, an organic, a liquid,and a gaseous component. In an ideal soil, thepercentage proportions of these components are

45%, 7%, 23%, and 25%, respectively. Thevolumes of the liquid and gaseous componentsmay change quickly. For example, in a water-saturated soil all pores are lled with water, andin a dry soil the soil pore volume is almost com-pletely lled with air. The mineral and organiccomponents contain plant nutrients and adsorbplant nutrients at their surfaces, and they aretherefore of importance for the storage and re-tention of plant nutrients. The liquid phase ofthe soil is the soil solution. It contains dissolvedplant nutrients and is themedium for the translo-cation of plant nutrients from various soil sitestowards the plant roots. The gaseous soil com-ponent is essential for gas exchange, especiallyfor the supply of plant roots with oxygen and forthe release of CO2 from the soil medium into theatmosphere.

For the description, comparison, and assess-ment of soils, a grouping according to generalcriteria is indispensable. There are two maingrouping systems for soils: (1) soil classes orsoil texture and (2) soil types. Textural classesare dened according to the particle size of soils.Soil types relate to the parent material of soils,to the pedological genesis, and to typical proper-ties evident in the soil prole i.e., the horizontallayers of soils, called soil horizons.

Soil Classes. Soil particle sizes as a maincharacteristic of soil classes are grouped intofour major groups as shown in Table 6. The ma-jor groups (sand, silt, and clay) are subdividedinto coarse, medium, ne. Designation of thesoil texture (soil class) depends on the percent-age proportions of the sand, silt, and clay frac-tion in the total ne earth, which is sand + silt +clay. Soils in which the sand fraction dominatesare termed sandy soils, soils consisting mainlyof silt and clay are silty clays, and soils whichcontain all three fractions in more or less equalamounts are called loams. In the German termi-nology, abbreviations for the fractions are used(S = sand, U = silt, T = clay, L = loam). For ex-ample, if the major fraction is silt (U) and thenext sand (S), the abbreviation is sU = sandy silt.Figure 1 shows the designations of the varioussoil classes according to the percentage propor-tion of the three main particle fractions.

In the farmers practice, sandy soils are calledlight soils, soils rich in clay heavy soils. Thisdistinction relates to the force required to work

14 Fertilizers

(plough, cultivate) a soil. Soils rich in clay, butalso silty soils, tend to compaction when driedand hence are heavy to work.

Table 6. Particle size of soil fractions relating to soil textureDiameter, mm Designation Abbrevation> 2 pebbles, gravels0.06 2 sand S0.002 0.06 silt U< 0.002 clay T

Figure 1. Diagram of soil textural classes, German systemof Schroeder [34]The vertical axis shows the percentage of silt, the horizontalthe percentage of clay, and the dashed line the percentageof sand.

Although the grouping according to particlesize is based on a physical factor, particle sizeis also associated with the chemical properties.This can be seen fromFigure 2: the sand fractionconsistsmainly of quartz,which is a sterilemate-rial. Primary silicates (micas, feldspars) containK, Ca, Mg, and other plant nutrients, which arereleased during the process of weathering. Clayminerals are less rich in plant nutrients than theprimary silicates, but they possess large nega-tively charged surfaces that are of the utmostimportance for the adsorption of plant nutrientsand water.

The various soil particles form aggregates inwhich organic matter is also involved. This ag-gregation forming ne pores and holes in thesoil is of relevance for soil structure. A goodsoil structure is characterized by a relatively high

pore volume, ca. 50% of the total soil volume.Soil structure depends much on the Ca satura-tion (see page 16). The richer the soil is in clay,the more important a good soil structure is.

Figure 2. Mineral composition of the sand, silt, and clayfractions [34]

Soil Type. Soil type is related to the parentmaterial fromwhich a soil is developed and fromthe history of development, which is much in-uenced by climate and vegetation.Main groupsof parent material are igneous rocks, sedimen-tary rocks, andmetamorphic rocks. Also organicmatter may be the main parent material. Contentof plant nutrients, capacity to store plant nutri-ents, soil pH, and the rooting depth dependmuchon the parent material, but are also inuenced bysoil development.

In the following, a limited number of impor-tant soil types are considered according to theFAOWorld Soil Classication. Besides this sys-tem there are other systems, e.g., the U.S. SoilTaxonomy. The FAOclassication comprises 26classes.

A distinction can be made between youngsoils and old soils. The latter are generally highlyweathered, their inorganic material consistingmainly of quartz and iron aluminum oxide hy-droxides. Such soils are characterized by poorcation retention capacity (cation exchange ca-pacity), low pH values, and a high phosphatexing power. This soil type, called ferralsol, isfrequent in the tropics, whereas in moderate cli-

Fertilizers 15

mates highly weathered soils belong mainly tothe podsols.

Young soils may be derived from the sed-imentation of rivers and oceans (uvisols) orfrom volcanic ash (andosols). These soils aregenerally rich in plant nutrients and thus formfertile soils. The most fertile soils belong to theblack earths (chernozem). They are frequent inRussia, Central and East Europe aswell asNorthAmerica and are derived from loess. They arecharacterized by a neutral pH, by a well bal-anced content of clay and organic matter and bya deep rooting prole. They are naturally rich inplant nutrients and possess a high nutrient stor-age capacity. Soils in which the parent mate-rial loess is more weathered as compared withthe chernozems belong to the luvisols. This soiltype is common inGermany,Austria, andFrancewhere it represents the most fertile arable land.Gleysols are soils with a high water table, rendz-inas are shallow soils derived from limestone,histosols are rich in only partially decomposedorganic matter.

Under arid conditions salt may accumulate inthe top soil layer. Solonchaks (white alkali soils)are saline soilswith a pHof ca. 8 andwith neutralanions as the most important anion component.Solonetz soils (black alkali soils) possess bicar-bonate and carbonate asmajor anion component.Their pH is in the range 8 10. Crop growth onsaline soils is extremely poor, and in many casesonly a salt ora can grow under such conditions.This is particularly true for the solonetz soils.

For further information on soil texture andsoil types, see [34 37], and the Soil Taxonomyof the Soil Conservation Service of U.S. Depart-ment of Agriculture [38].

2.2.2. Nutrient Retention in Soils

Nutrient retention is an important characteristicof fertile soil.

Cation Exchange. Cations are retained onsoil colloids having a negative charge: thecations are bound at the surface of these par-ticles by Coulomb forces. The most importantcation species are Ca2+, Mg2+, K+, Na+, Al3+,Al(OH)2+, Al(OH)+2 , and H+. This is repre-sented in Figure 3. A distinction can be made

between inorganic and organic soil colloids ca-pable of cation adsorption. Inorganic particlesbelonging to the clay fraction are known as sec-ondary clay minerals because they are mainlyderived by weathering of primary minerals suchas orthoclase, plagioclase, and particularlymica.Organic soil colloids capable of cation adsorp-tion belong to the humic acids. The negativecharge of the inorganic soil colloids originatesfrom the so-called isomorphic substitution andfrom deprotonation. Isomorphic substitution isthe replacement of Si4+ in the crystal lattice byAl3+, Fe2+, or Mg2+, thus leading to a surplusof negative charge, because the anionic groupsof the lattice are not completely balanced byAl3+, Fe2+, or Mg2+. Such a negative charge isa permanent charge, in contrast to labile chargesthat result from deprotonation. Labile chargesare typical for organic colloids (humic acids):carboxylic groups and acid hydroxylic groupsof phenols may be protonated or not dependingon the pH of the environment.

Figure 3. Schematic presentation of cations adsorbed tothe negatively charged surface of a soil colloid

The secondary clayminerals are grouped into1 : 1 clay minerals, in which a Si layer alter-nates with an Al layer, and 2 : 1 clay minerals,in which an Al layer is sandwiched by two Silayers. The most important representatives ofthe 1 : 1 clay minerals are the kaolinites. The2 : 1 secondary clay minerals comprise the il-lite, transitional minerals, vermiculite, chlorite,and smectites ( Clays).Most of these 2 : 1 clayminerals possess inner surfaces. They are there-

16 Fertilizers

fore characterized by a high cation retention (=cation exchange) capacity.

These negatively charged soil colloids, oftenalso called sorption complexes, function like acation exchanger. Adsorbed cations can be re-placed by other cation species. The cation ex-change is stoichiometric. Adsorption and de-sorption depend on the concentrations of thecation species in the surrounding solution. If asoil colloid completely saturated by K+ is ex-posed to increasing Ca2+ concentrations, forexample, adsorbed K+ is more and more re-placed by Ca2+ until eventually the sorptioncomplex is completely saturated by Ca2+ (Fig.4). In soils, such exchange and equilibrium re-actions are complex as numerous cation speciesand sorption complexes with differing prefer-ences for particular cation species are involved.The principle, however, is that cations adsorbedby Coulomb forces at soil colloids equilibratewith free cations in the soil solution.Thus addingcations to a soil by fertilization, e.g., the appli-cation of a potassium salt, results in replacingadsorbed cations with the newly added cationsuntil a new equilibrium is reached. The adsorbedcations are protected against leaching. but theyare available to plant roots. The strength ofcation adsorption increases with the the chargeof the cation species and with the thinness of thehydration shell. Provided that there are no spe-cic adsorption sites, the strength of cation ad-sorption follows Hofmeisters cation sequence:

Ca2+>Mg2+>K+>Na+

Figure 4. Ca2+ K+ exchange, K+ desorption broughtabout by increasing Ca2+ concentration

At equilibrium, cation-exchange reactionsare a helpful tool for predicting the distribu-tion of ions between the adsorbed and solutionphases of the soil as the amounts of cationspresent are changed. When a soil saturated withpotassium is placed in a NaCl solution, the fol-lowing equilibration occurs:

Ksoil+NaClNasoil+KCl

The exchange equation for this reaction is[Na] (K)[K] (Na)

= k1

Brackets refer to ions on the exchange site andparenthesis to the activity of ions in the solution.Since the proportionate strength of adsorption ofthe ions varies with the exchange site, values fork1 differ for different exchange materials.

The divalent/monovalent system, which al-most represents the situation in the soil,withK+,Ca2+, and Mg2+ as the dominant exchangeablecations, is more complex. The following equa-tion, developed by Gapon [39], is widely used todescribe monovalent/divalent exchange:[K] (Ca)1/2

[Ca] (K)= k1

Cation exchange capacity (CEC) is denedas the quantity of cation equivalents adsorbedper unit soil or clay mineral. In Table 7 theexchange capacities of some soil classes areshown. The exchange capacity of the organicsoil appears high if it is based on unit weight ofsoil. A more realistic picture is obtained, how-ever, when the exchange capacity is based onsoil volume, since under eld conditions it issoil volume, not soil weight, that is related to acrop stand. Table 8 shows the cation exchangecapacities of some important clay minerals andof humic acids in relation to the surface of theseparticles.

The cation exchange capacity of kaolinitesand particularly of humic acids depends muchon the pH of the medium. At low pH, mostgroups are protonated and hence the exchangecapacity is low. Increasing soil pH, e.g., by lim-ing, increases the cation exchange capacity ifkaolinites and humic acids are the dominatingexchange complexes.

Cation saturation of negatively charged soilcolloids has some impact on soil structure,whichis dened as the arrangement of soil particles

Fertilizers 17Table 7. Cation exchange capacity based on soil weight and soil volume as well as the specic weight of some soil classes

Soil class Specic weight, kg/L Cation exchange capacity *cmol/kg cmol/L

Sandy soil 1.5 3 4.5Loam 1.5 15 22.5Clay soil 1.5 30 45.0Organic soil 0.3 75 22.5* cmol = centimole.

Table 8. Cation exchange capacity and inner and outer surfaces of some soil colloidsTotal surface, Inner surface,% Cation exchange capacity,m2/g mol/kg

Kaolinite 20 0 10Illite 100 0 30Smectite 800 90 100Humic acids 800 0 200

into aggregates. High percentage of adsorbedCa2+ favors the formation of aggregates. Inwell structured soils, such as in chernozems, 70to 80% of the total cation exchange capacityis occupied by Ca2+. In acid solids, H+ andAl cations (Al3+, Al(OH)2+, Al(OH)+2 ) and insaline soils Na+ and Mg2+ are the dominatingcation species adsorbed to soil colloids.

Anion Exchange. Soil particlesmayalso ad-sorb anions. The adsorption occurs at the OHgroups of aluminum and iron oxides as well asof some clay minerals. One may distinguish bet-ween a nonspecic adsorption and a specic an-ion adsorption. The nonspecic anion (A) ad-sorption originates from protonated hydroxylicgroups.

Protonation depends on soil pH and is particu-larly high under acid conditions. Hence nonspe-cic anion adsorption only plays a role in acidsoils.

The specic anion adsorption is a ligand ex-change. This is, for example, the case for phos-phate. In step 1H2PO4 replaces OH, resultingin a mononuclear bond between the phosphateand the iron oxide. In step 2, a further deproto-nation of the phosphate occurs, followed by asecond ligand exchange (step 3) to form a binu-clear bond between the surface of the iron oxideand the phosphate.

The nal structure is supposed to be very sta-ble, and the phosphate so bound is hardly avail-able to plant roots. This reaction sequence ex-plains why anion (phosphate) adsorption is pro-moted under low pH conditions. In mineral soilswith pH < 7, the adsorbed phosphate representsa major phosphate fraction. Increasing the soilpH, e.g., by liming, increases phosphate avail-ability [40]. The relationship between free andadsorbed anions can be approximately describedby the Langmuir equation:

A = Amaxkc

1+kc

A = surface concentration of adsorbed an-ions

Amax =maximum surface concentrationc = concentration of free anionk = constant related to adsorption energy,

the adsorption strength increasing with k

Adsorption strength depends also on anionspecies decreasing in the order [41]:phosphate>arsenate>selenite = molybdate>sulfate

= uoride>chloride>nitrate

18 Fertilizers

Borate and silicate may also be adsorbed, butonly at high pH. Under these conditions, boricacid and silicic acid may form anions accordingto the following equations:H3BO3+H2OB(OH)4 +H+

H2SiO3+H2OH3SiO4 +H+

This is why in neutral to alkaline soils boroncanbe strongly adsorbed (xed) by soil particles,which may lead to boron deciency in plants.The formation of a silicate anion can improvephosphate availability since H3SiO4 and phos-phates compete for the same ligands at anion-adsorbing surfaces.

2.2.3. Soil pH, Buffer Power, and Liming

Proton concentration (pH) is of vital impor-tance for all living organisms and also has animpact on soils and soil constituents. High H+concentrations (pH

Fertilizers 19

Figure 5. Principle of hydrogen-ion buffering by adsorbed cations

2NO3 +10 e+12H+N2+6H2O

Some species ofAzalea, Calluna,Vaccinium,and also tea (Camellia sinensis) are able to growon acid habitats. These species can mask the Alwith phenols and organic acids and thus avoidAltoxicity. Rye, potatoes, oats, and lupines tolerateweakly acid soils, whereas beets (Beta vulgaris),barley, rape, and most leguminous crops prefermore neutral soils. Wheat takes an intermediaryposition with regard to soil pH. In the case ofleguminous crops, it is not so much the crop it-self, rather it is the Rhizobium bacteria living insymbiosis with the crop that are affected by lowsoil pH. The mulitplication of the Rhizobium inthe soil is depressed by soil acidity.

LowpH levels can be easily overcome by lim-ing, the application of alkalinematerials, mainlyCa/Mg oxides, carbonates, and silicates. Theyreact with soil acidity as follows:

CaO+2H+Ca2++H2O

CaCO3+2H+H2O+CO2+Ca2+

CaSiO3+2H+H2O+SiO2+Ca2+

The quantity of lime required depends on thesoil pH level and the buffer power. The lowerthe pH and the higher the buffer power, the morelime required.

Soil acidity is mainly a problem in humidzones, where the H+ formed in the upper soillayer replaces the adsorbedmetal cations (Ca2+,Mg2+, K+ ), which are then leached. Under aridconditions, salts may accumulate in the top soillayer. If a major part of the anions accumulatedare HCO3 and CO

23 (solonetz soils), high soil

pH levels prevail, which affect plant growth and

soil structure considerably. Such soils canbeme-liorated by heavy applications of elementary sul-fur. Under aerobic conditions the S is oxidizedto H2SO4 by soil microorganisms. The strongacid neutralizes the HCO3 and CO

23

H2SO4+2HCO3 2 H2CO3+SO24 2 H2O+2 CO2+SO24

H2SO4+CO23 SO24 +H2CO3H2O+CO2+SO24

2.2.4. Soil Water Plant Relationships

Plants continuously require water that is takenup from the soil by roots and transported fromthem to the upper plant parts, particularly to theleaves. From here water is released into the at-mosphere. This last process is called transpi-ration. Water in the plant tissues is required tomaintain optimum cell turgor, which is crucialfor most metabolic processes.

Plants frequently have to overcome long dryperiods during which they must feed from thesoil water. Very crudely, the soil can be consid-ered as a sponge that can store water in its poresand holes. The storage water in the soil must beretained against gravitation. The forces respon-sible for this retention are adsorption and capil-lary forces by which the water is sucked to thesurface of the soil particles. This suction forcecan be considered as a negative pressure, andhence the strength of water binding in soils ismeasured in Pascals (Pa), the unit for pressure.The strength of water binding in soils is termedwater potential (in older terminology, water ten-sion). The higher the strength, the lower (more

20 Fertilizers

negative) the soil water potential. Water poten-tials in soils range from 0 to 1 106 kPa.Generally, however, soil water potentials of 10 to 1500 kPa prevail.

The total amount of water that can be ad-sorbed by a soil (all pores and holes lled withwater) is calledmaximumwater capacity. This isof minor importance; more relevant is the waterquantity that can be retained against the gravi-tation force, the eld capacity. Not all water ofthe eld capacity fraction is available to plantroots. A proportion of the eld capacity wateris so strongly adsorbed that it can not be takenup by the roots. For most plant species this iswater with a water potential < 1500 kPa: atsuch a low water potential, plants wilt. There-fore, this critical water potential is also calledwilting point. The soil water fraction not avail-able to plants is called deadwater. The availablesoil water thus equals the difference between theeld capacity and dead water.

Soils differ much in their capacity to storewater. The higher the clay content of a soil, thelarger the total surface of soil particles, and themore water that can be adsorbed. Thewater stor-age capacity of soils increases with the clay con-tent. On the other hand, the water molecules arestrongly adsorbed to the surface of clays andtherefore the fraction of dead water increaseswith the clay content. For this reason, generallymedium textured soils (loamy soils), and not theclay soils, possess the highest storage capacityfor available soil water. Besides soil texture, alsosoil structure and the rooting depth of the soilprole determine the storage capacity for avail-able soil water.

An important criterion of available soil wateris the relationship between the percent water sat-uration of the soil and the water potential. Thisis shown in Figure 6 for a sandy, a loamy, anda clay soil. The section between eld capacity( 10 kPa) and the wilting point ( 1500 kPa)is the highest for the loamy soil.

The capability to use soil water economi-cally differs considerably among plant species.A measure of this capability is the transpira-tion coefcient, the water quantity in kg (orL) required for the production of 1 kg plant drymatter:

Sorghum 277Maize 349Sugar beet 443Spring wheat 491Barley 527Potatoes 575Oats 583Spring rye 634Red clover 698Flax 783Alfalfa 844

Water loss under a vegetation cover resultsfrom evaporation (water release from the soil tothe atmosphere) and transpiration (water releaseof the plant to the atmosphere). Evaporation isunproductive, transpiration productive. The re-lation between the two depends on plant nutri-tion, as can be seen in Table 9, which shows thefavorable effect of N fertilizer on the productiveuse of soil water [43].

2.2.5. Organic Matter of Soils and NitrogenTurnover

Organic matter of soils differs considerably.Soils can be classied according to the contentof organic carbon in the soil (g/kg):

Low 150

Soils with an organic carbon content of 50g/kg are termed organic soils, in contrast tomineral soils. Enrichment of organic matter insoils depends on location and climatic condi-tions. Low temperature and a lack of oxygen fa-vor the accumulation of organic matter in soilsbecause these conditions hamper breakdown bysoil microbes. Therefore, under cold continen-tal climate conditions (frost, long winters) andunder hydromorphic soil conditions (swamps,moors, bogs), organic matter accumulates.

The fertility status of organic soils differsconsiderably. Moors located on the tops ofmountains are generally poor in nutrients, es-pecially in N and K, since they are fed mainlyfrom rain. Moors in lowland fed from rivers andstreamsmay be rich in plant nutrients, especially

Fertilizers 21

Figure 6. Relationship between the soil water content and the soil water potential and the resulting available water (AW) forthree soil classesField capacity is 10 kPa; wilting point is 1500 kPa.Table 9. Relationship between N fertilizer rate, grain yield of barley, evaporation, and transpiration [43]Nitrogen fertilizer rate, Grain yield, Transpiration, * Evaporation,*kg/ha t/ha L/m2 L/m2

30 1.02 85 235125 1.65 121 278225 2.69 217 212* Transpiration and evaporation in liters per square meter of soil surface.

in N. The C :N ratio of the organic matter andsoil pH are suitable indicators of the fertility sta-tus. Fertile soils possess C :N ratios of ca. 20 intheir organic matter and pH values in the weakacid to weak alkaline range. Acid organic soils(highland moors) have much higher C :N ratios,ca. 50 or more in their organic matter. The C :N ratio has direct impact on the decompositionof organic N by soil microbes; the higher theratio the lower the net release of mineral N bymicrobial activity.

Nitrogen turnover in soils is related not onlyto biological processes but also to physicochem-ical processes. In addition, there is a rapid ex-change of N between the biosphere, the soil, andthe atmosphere. The main processes of this ni-trogen cycle are shown in Figure 7. Inorganicnitrogen, mainly NO3 and NH

+4 , including fer-

tilizer N, can be easily assimilated by higherplants as well as by microbes (fungi, bacteria).Also dinitrogen (the N2 of the atmosphere) canbe reduced to NH3 by some soil bacteria. TheN2 xation capacity of the so-called free livingbacteria, mainly species of Azotobacter, Beijer-inckia, Azospirillum, and some species of theCyanobacteria (Anabaena, Nostoc, Rivularia) ismoderate, amounting to ca. 5 50 kg ha1a1.Symbiontic N2-xing bacteria, mainly speciesof Rhizobium and Actinomyces, have a xationcapacity about 10 times higher: for pulses (grain

legumes) 50 100 kg/ha per growth period andfor forage legumes even 200 500 kg ha1a1.They are of utmost importance in the N turnoverand N availability in soils.

Inorganic nitrogen (N2, NH+4 , NO3 ) assimi-lated by living organisms is mainly used for thesynthesis of proteins, amino sugars, and nucleicacids. As soon as these organisms die, the or-ganic N can be attacked by other microorgan-isms, which are able to convert the organic Ninto an inorganic form, a process called nitrogenmineralization. This starts with ammonication,and under aerobic conditions and favorable soilpH ammonication is followed by nitricationin the sequence:

OrganicNNH+4NO2 NO3Ammonication is carried out by a broad

spectrum of heterotrophic organisms; nitrica-tion only by a small number of autotrophic bac-teria [44]. The microbial oxidation of NH+4 toNO2 and NO

2 to NO

3 requires oxygen and

hence proceeds only under aerobic conditions.The oxidation of NH+4 to NO

2 is brought about

by species of Nitrosomonas, Nitrosolobus, andNitrospira, oxidation ofNO2 toNO

3 by species

of Nitrobacter. They all require weak acid toneutral soil conditions; in acid soils nitricationis more or less blocked.

22 Fertilizers

Figure 7. N cycle in nature. Transfer of N between soil, plant, and atmosphere

Ammonium ions produced by microbialbreakdown of organic N, including urea, as wellas NH+4 fertilizer can also be xed by 2 : 1 clayminerals. In this form, NH+4 is protected againstnitrication and leaching, but, depending on thetype of clay minerals, may still be available toplant roots [45]. This xed NH+4 fraction is ofmajor signicance for plant nutrition in soils de-rived from loess.

The concentration of ammonium ion in thesoil solution is governed by the equilibriumNH+4NH3+H+pK = 9.25

At pH < 6, there is virtually no NH3 present;with an increase in pH the deprotonation ofNH+4increases, and so does the risk of NH3 loss byvolatilization. In alkaline and calcareous soilsconsiderable amounts of N can thus be lost bythe soil system [46]. High losses of NH3 mayalso occur from the application of slurries,whichgenerally have an alkaline pH [47].

Ammonium as well as NO3 are taken upby plant roots at high rates, and vigorous cropstands can deplete the NO3 concentration in thesoil to a great extent. Nitrate is very mobile insoils since it is virtually unadsorbed on soil col-loids. It can be leached by rainfall to deeper soillayers or even into the ground water (see Sec-tion 2.5.1). Nitrate losses may also occur un-der anaerobic soil conditions, for some bacterialspecies are able to use the oxygen of the NO3as e acceptor for respiration. Nitrate is thus re-duced to volatile NO, N2O, and N2 [49]:NO3 NO2 NO(g)N2O(g)N2(g)

This process, brought about mainly byspecies of Pseudomonas, Alcalignes, Azospir-illum, Rhizobium, and Tropionibacterium, iscalled denitrication. It may cause considerablesoil N losses particularly in ooded rice soils, inwhich anaerobic conditions prevail [50].

Loss of NO3 by leaching or denitricationcan be reduced by blocking the NO2 formationby application of nitrication inhibitors suchas Nitrapyrin (2-chloro-6-trichloromethylpyri-dine), AM (2-amino-4-chloro-6-trimethylpy-rimidine), or terrazole (5-ethoxy-3-trichlorome-thyl-1,2,4-thiadizole). There are also some nat-ural compounds that are nitrication inhibitors[51]. The most important is neem, which occursin the seeds of Azadirachta indica, a tree com-mon in the tropics.

Some of the organic N in the soil may be in-corporated into a very stable organic form. Thisnitrogen, which mainly occurs in humic acids,is hardly mineralized. In most soils the humus-N fraction is by far the largest, comprising 80 90% of the total soil N. It is of great importancefor soil structure, but has hardly any relevanceas a nutrient reserve.

The fraction of hydrolyzable soil N can bemineralized and thus may serve as a source forN absorbed by plants. The most important frac-tion in this respect is theN of the biomass, whichcomprises ca. 40 200 kg/ha, thus only a smallfraction of total soil N, which may be 2000 8000 kg/ha in arable soils.

Fertilizers 23

2.3. Nutrient Availability

2.3.1. Factors and Processes

From the total amount of N present in the soil,only a small proportion can be made availablefor plants (see above). This is also true for otherplant nutrients. For example, in a clay-rich soilca. 200 000 kgK+may be present in 1 hawithinthe rooting depth of 80 100 cm, but only 1%may be available to plant roots.

Plant nutrient availability depends on physi-cochemical and biological factors. A young rootpushing into a soil directly contacts only a smallamount of macronutrients, which would con-tribute only a few percent of the total nutrientdemand. By far the greatest proportion of nutri-ents (NO3 ,NH+4 ,K+, Ca2+,Mg2+ ) required bythe plant must be transported towards the plantroots. This transport can be brought about bymass ow and/or diffusion. In mass ow, thenutrients are moved with the water ow fromthe soil towards the roots. Therefore mass owdependsmuch onwater uptake and transpirationconditions. At zero transpiration (100% relativehumidity, rainy, or foggy weather) mass ow isalso zero. Mass ow plays a major role for thetransport of Ca2+, Mg2+, and also for NO3 incases where the NO3 concentration of the soilsolution is high, e.g., after fertilizer application.

Nutrients that are taken up at high rates byplant roots (K+, NH+4 ; NO 3 , phosphate) butthat have relatively low total concentration in thesoil solution aremainly transported by diffusion.Uptake of nutrients by roots decreases the nutri-ent concentration near the root surface and estab-lishes a concentration gradient, which drives thediffusive ux of nutrients from the soil towardsthe plant roots. Absorbing roots thus act as a sinkfor plant nutrients. Typical depletion proles areshown in Figure 8 for phosphate [52]. Phosphateand K+ concentrations in the soil solution at theroot surface may be as low as 1 M, whereas inthe bulk soil solution concentrations of 50 to 300Mphosphate and500 to1000 M K+maypre-vail. Fertilizer application increases the nutrientconcentration of the soil solution and hence theconcentration gradient that drives the nutrientstowards the roots. Therefore, the level of the nu-trient concentration in the bulk soil solution isan important factor of nutrient availability.

Figure 8. Phosphate depletion around a corn root: P con-centration in the soil solution as a function of the distancefrom the root surface [52]

Diffusive ux and mass ow in the soil de-pend much on soil moisture. The dryer a soil,the smaller the water cross section (the size ofsoil pores that are still lled with water) and themore the nutrient ux is hampered. Therefore,soil moisture is another important factor of nu-trient availability [53]. A third important fac-tor of nutrient availability is the nutrient bufferpower of a soil, a factor of particular relevancefor phosphate, K+, andNH+4 . Here buffer powermeans the capability of a soil to maintain the nu-trient concentration level in the soil solution (inanalogy to the hydrogen-ion buffer power, seeSection 2.2.3). In well-buffered soils nutrientsabsorbed from the soil solution by roots are re-plenished by nutrient desorption, e.g., by cationexchange.

The most important biological factor for nu-trient availability is root growth [54]. For mostplant nutrients, only the soil volume around theroot can be exploited by the plant. For phosphatethe depletion zone extends only a fewmillimeterfrom the root surface and depends much on thelength of root hairs, as can be seen fromFigure 8.For K+ andNH+4 , the depletion zone is more ex-tended, ca. 1 4 cm from the root surface. Rootmass, root length, and root hairs therefore are ofgreat importance for the portion of soil volumethat can be exploited by a crop stand.

Roots excrete organic materials such as or-ganic acids, sugars, and slimes from which bac-

24 Fertilizers

teria feed. As a result, the bacterial coloniza-tion in the rhizosphere (the volume around theroot) is much denser than in the soil apart fromthe roots. These bacteria are involved in the Nturnover, e.g., for the N2 xation of free-livingbacteria and for denitrifying bacteria [55].

Proton excretion of roots affects bacterial ac-tivity in the rhizosphere, the dissolution of cal-cium phosphates, and the cation exchange. Netproton release of plant roots is strongly affectedby the type of N supply. Ammonium nutritionresults in a high net release of H+, nitrate nutri-tion in release of OH and HCO3 . Leguminousspecies living in symbiosis with Rhizobium areknown for a high H+ release by roots and there-fore have a strong acidifying effect on soils.

Nutrient deciency, e.g., phosphate or Fe de-ciency, also increases net release of H+, whichmay contribute to the dissolution of iron oxidesand calcium phosphates. The release of avenicacid and mugineic acid by plant roots is of par-ticular importance for the mobilization of Fe inthe rhizosphere [56].

Plant growthmay be enhanced after infectionwithmycorrhizal fungi,which leads to increasednutrient uptake due to increases in the effectiveabsorptive surface of the root, mobilization ofsparingly available nutrient sources, or excretionof ectoenzymes or chelating compounds. Fur-thermore, mycorrhizal fungi may protect rootsfrom soil pathogens [57] and and in this way en-hance root growth and nutrient acquisition of thehost plant. This is particularly important whenconsidering the nutrition of plants with immo-bile nutrients such as phosphorus, as fungal hy-phae are known to absorb P and translocate itinto the host plant [58].

2.3.2. Determination of Available PlantNutrients in Soils

The level of available plant nutrients in soils canbe assessed by means of plant analysis and soilanalysis. In the case of plant analysis, the nutri-ent content of a particular plant organ at a certainphysiological stage may reect the nutritionalstatus of the plant, hence also the nutrient statusof the soils [59]. Such diagnostic plant analy-sis is particularly common for perennial crops,including fruit trees and forest trees.

In soil analysis, soil samples are extractedwith special extractants. The quantity of nu-trients extracted reects the level of availablenutrients in the soil. Numerous soil extractantshave been developed. In Central Europe the DLmethod (double lactate method) and the CALmethod (calcium acetate lactate method) arewidely used for the determination of availablesoilK andP. In theNetherlands, available P is ex-tracted bywater (P-water method). In the UnitedStates, the Olsen method (extraction with NH4F+ HCl) is used for the determination of availablesoil phosphate [60]. Ion-exchange resins are alsouseful tools for the determination of availableplant nutrients [61].

Of particular interest is the determination ofavailable soil N early in spring before the rstapplication of N fertilizer. Mineral N (NO3 , ex-changeable NH+4 ) is extracted with a CaCl2 orK2SO4 solution [62]. This technique, called theNmin method, provides reliable information onthe level of directly available soil N. The easilymineralizable N in the soil, which frequently isthe important fraction for the release of availableN during the growth period, is not obtained bythe Nmin method.

Electro-ultraltration (EUF method) hasbeen used for the determination of available soilnutrients [63]. This method uses an electric eldto separate nutrient fractions from a soil suspen-sion. Most plant nutrients can be extracted. Theadvantage of this method is that, besides inor-ganic nitrogen, the readilymineralizable organicN fraction is extracted [64].

In the last decade S has attracted interest as aplant nutrient. Numerous procedures have beenproposed for the determination of plant-avail-able S in soils. The procedures include extrac-tion with water, various salts and acids, and Smineralization by incubation [65].

The relationship between soil analysis dataand the response of crops to fertilizer applica-tion is not always satisfying since other factorsmay interfere, such as rooting depth, root mor-phology, soil moisture, and particularly the claycontent. These factors should be taken into con-sideration in interpreting soil test data.

Fertilizers 25

2.4. Physiology of Plant Nutrition

2.4.1. Nutrient Uptake and Long-DistanceTransport in Plants

Oxygen and CO2 are mainly taken up by above-ground plant parts. The process of uptake is adiffusion ofCO2 andO2 into the plant tissue. Forthe entry of these gases into the plant, the stomataare of major importance. Water and other plantsnutrients are mainly absorbed from the soil so-lution. The rate of nutrient uptake increases withthe concentration of the particular nutrient in thesoil solution, the rate of uptake leveling off athigher soil-solution concentrations.

Plant nutrients in the soil solution, mainlypresent in ionic form, diffuse into the root tissue.The outer plasma membrane of the cells (plas-malemma) is a great diffusion barrier. The trans-port of nutrients across this barrier is the properprocess of ion uptake. This transport is not merediffusion, but is related to specic membranecomponents and to metabolic processes that al-low selective uptake of the plant nutrients, whichis often associated with an accumulation of thenutrient in the cell. For example, the K+ concen-tration in the cell (cytoplasm) may be higher bya factor of 102 103 than the K+ concentrationin the soil solution.

Figure 9. Scheme of plasmalemma-located ATPase,hydrolyzing ATP and pumping H+ into the apoplast(proton pump)

Nutrient uptake is initiated by an enzymelocated in the plasma membrane called AT-Pase (ATP hydrolase). Its substrate is ATP. Hy-drolyzation ofATP results in the splitting ofH2OintoH+ andOH, fromwhich the latter remainsin the cytosol of the cell while H+ is extrudedinto the outer medium (Fig. 9). Thus an electro-chemical potential is created between the twosides of the membrane. The proton motive force(p.m.f.) obtained in this way is described by thefollowing equation [66]:p.m.f. = 50pH+ = electrical charge

The p.m.f. is the driving force for ion up-take. Cations are directly attracted by the nega-tively charged cell. Since the plasmamembrane,however, represents a strong barrier, the entry ofcation species must be mediated by particularcarriers and ion channels. Little is known aboutthese carriers and channels in plant membranes.These are assumed to be ionophores like vali-nomycin, nonactin, or gramicidin, which bindselectively to cation species and hence medi-ate a selective cation transport across the mem-brane. Such a typeof carrier transport is shown inFigure 10. The carrier is hydrophobic and there-fore quite mobile in the membrane, which con-sists mainly of lipids. At the outer side of themembrane it combines selectively with a cationspecies, e.g., K+. The cation carrier complexthen diffuses to the inner side of the membrane,where the K+ is released. Release and combin-ing with K+ depend on the electrochemical dif-ference between the two sides of the membrane.High K+ concentration and a positive chargefavor the combining process; low K+ concen-tration and a negative charge favor the releaseof K+. Net transport rate becomes zero as soonas the electrochemical equilibrium is attained,which is governed by the Nernst equation:

E =RT

zFlnao

ai

E = electrical potential difference betweenthe two sides of the membrane

F =Faraday constantz = oxidation state of the cationao = activity of the cation species in the outer

solutionai = activity of the cation species in the inner

solution

26 Fertilizers

Figure 10. Scheme of K+ carrier transport across theplasmalemma

The uptake of anions (NO3 , H2PO4 ) is alsoassumed to be driven by the plasmalemma-located ATPase. The anions presumably formprotonated carriers at the outer side of the mem-brane and then are selectively tranported acrossthe membrane. The protonated carrier anioncomplexes are positively charged. Hence theelectropotential difference between either sideof the plasmamembrane represents the drivingforce for anion uptake.

Ion absorbed by cells of the root cortex aretranslocated via the symplasm in centripetal di-rection towards the central cylinder, where theyare secreted into the xylem vessels. The actualprocess of this secretion is not yet understood.In the xylem the ions are translocated to theupper plant parts with the transpiration stream.They thus follow the vascular systemof the plantand are distributed along the major and minorvein system of leaves from where they diffuseinto the pores and intercellular spaces of cellwall (apoplast). The transport from the apoplastacross the plasmamembrane into the cytoplasmof leaf cells is a process analogous to the nutrientuptake of root cells.

Some plant nutrients, such as N, P, K, andMg, but not Fe and Ca, may also be translocatedagainst the transpiration stream via the phloemtissue. These nutrients therefore may be trans-ported from the tops to the roots or from olderleaves to younger leaves.

2.4.2. Effect of Nutrition on Growth, Yield,and QualityMeristematic growth requires plant nutrients:N and P for the synthesis of proteins and nu-cleic acids, K and Mg for the activation of en-zymes and for membrane potentials, and all theother nutrients for various processes. The quan-tities required differ greatly but for practicalpurposes mainly N, K, and P, in some casesalso Mg and Ca, limit plant growth. The growthrate is controlled by the nutrient with the low-est availability (Liebigs law of minimum). Ap-plication of this particular nutrient results in agrowth response. This response is not linear, butrather follows a saturation type of curve (Fig.11). Also crop yields as a function of increasingrates of fertilizer applicaton reect this curve,which is also called theMitscherlich curve sinceMitscherlich [67] investigated these relation-ships extensively. The curve is described by thefollowing equation:log (Ay) = log (Acx)

A =maximum yieldy = obtained yieldx = growth factor, e.g., fertilizer ratec = constant

Figure 11.Mitscherlich curve, response curve of diminish-ing increments (i) with each further unit of growth factor

The term (A y) is the increment that is re-quired to attain the maximum yield. This incre-ment becomes smaller as the variable x (growthfactor) becomes greater. Such growth factors in-clude not only plant nutrients, including CO2,

Fertilizers 27

but also light intensity and temperature. Thegrowth response obtained by these factors alsofollows the saturation type of curve shown inFigure 11.

Growth and metabolic processes in plant tis-sues depend not only on the rates of plant nu-trients supplied but also on the ratio in whichthe nutrients are provided. If the N supply isrelatively high as compared with the supply ofother nutrients, the synthesis of N-containingcompounds, such as amino acids and proteins,is promoted. This may have a favorable effecton the protein synthesis in grains of cereals andhence improve their baking quality and their nu-tritional value. In grains for malting purposesand in particular barley, however, low proteincontent and high starch content are required. Inthis case, relatively highN rates have a detrimen-tal effect on grain quality. An analogous case isthe sugar beet, which should be high in sugarand low in N-containing compounds, especiallyamino acids. Relatively highN supply favors thesynthesis of vitamins of the vitamin B group andthe synthesis of carotenes in green plant tissuebut has a negative effect on the content in vitaminC. High N rates may also increase the sensitivityof leaves and culms to fungi attack.

Phosphate and especially K+ have a favor-able impact on the energy status of plants. A rel-atively high supply of both nutrients promotesthe synthesis of carbohydrates and the develop-ment of cell wall material, which increases theresistance against fungi attack.

2.5. Nutrient Balance

2.5.1. Gains and Losses of Plant Nutrients

In order to maintain the level of available plantnutrients in soils, the quantity of nutrients lostfrom the soilmust be replenished. Nutrientsmaybe lost by harvesting of plant material, leaching,transition into a non-available form in the soil,and volatilization. Nutrient gains result from fer-tilizer application, soil weather