428 Z. Pflanzenernaehr. Bodenk. 149, 428-440 (1986)

The N,h-method - an aid to integrating various objectives of nitrogen fertilization

J . Wehrrnann* and H . C. Scharp$ *

* University of Hannover, Institute for Plant Nutrition, Herrenhauser Str. 2,

* * Horticultural Research Station Ahlem, Harenberger Str. 130, 1)-3000 Hannover 91, Federal D-3000 Hannover 21, Federal Republic of Germany

Republic of Germany

Eingegangen: 21. November 1985 Angenommen: 15. Januar 1986

Summary - Zusamrnenfassung

There is increasing interest in the rate and timing of N-fertilization from both an economical and an ecological point of view. Fertilizer recommendations for a variety of aims can be made using the N,,,-method, as shown by experiments on deep rootable loess loam soils.

Yields of cereal crops and sugar were increased by 2-3 dtiha by application of the N,,-method. The N,,,-method led to a reduction in the rate of fertilizer application by 30 kg N h a with sugar beet.

Any stipulated NO3 level in leaf vegetables can be met, If N is fertilized according to N,,,-method. The necessary data for many vegetable crops are given. In particular low nitrate content in spinach and lettuce at high yield was achieved without loss of yield by adjusting both their NO3 and CI nutrition.

Leaching of nitrate out of soils often follows the application of too much fertilizer, but fertilization errors can be avoided and leaching reduced if the N,,-method is used.

Applications and limitations of the N,,,-method are discussed.

Die N&-Methode - Em Hilfsmittel zuc Integration verschiedener Ziele bei der N-Diingung

Die Anforderungen an die Dosierung der N-Diingung sind gestiegen. Neben okonomischen finden okologische Fragen verstarkt Beachtung.

Die N,,,-Methode ist, wie Versuche auf tiefgriindigem Lonlehm zeigen, ein brauchbares Hilfsmittel zur Anpassung der N-Diingung an die jeweiligen Zielvorgaben.

Steigerungen der Ertrage um 2-3 dt/ha werden in der Praxis bei Getreide und Zucker durch Anwendung der N,,-Methode erzielt. Die N,,,-Analyse fiihrt bei Zuckerriiben zu einer Verminderung der Diingung urn ca. 30 kg Nha .

Vorgegebene Hochstwerte der Nitratgehalte fur Blattgemiise lassen sich einhalten, wenn die N-Diingung auf der Grundlage der N~,-Methode dosiert wird. Kenndaten fur viele Gemiisearten werden genannt. Bei Spinat und Kopfsalat fiihrt eine Kombination angepaDter NO3- und CI-Emahrung zu besonders niedrigen Nitratgehalten bei hohen Ertragen.

Die Auswaschung VOR Nitrat aus Boden beruht oft auf zu hohen Diingergaben. Diingungsfehler konnen vermieden werden.

Moglichkeiten und Grenzen der N,,,-Methode werden diskutiert.

M)44-3263/86/040&0428 S M.50/0

0 VCH Verlagsgesellschafl mbH, D-6940 Weinheim, 1986

149, 428-440 (1986) N(min)-analysis 429

Introduction

The main objectives of nitrogen fertilization in intensive farming are: - high yield at the lowest possible fertilizer rates

(high profit minus fertilizer costs) minimum plant diseases and pests (e.g. little use of plant protection agents)

(e.g. low nitrate content in vegetables)

(low nitrate content in drinking water)

-

- high quality products

- minimum nitrate leaching

Product quality and careful management of the environment have increased in importance during the last few years.

To control nitrogen fertilization according to the above objectives, the nitrogen demand of plants and nitrogen supply of soil have also to be taken into account. Following a study of the literature (Soper and Huang, 1963; Borst and Mulder, 1971; Giles et al., 1973; Carter et al., 1974; Rb, 1974; Stumpe and Garz, 1974) this subject was investigated for several years by the Institute for Plant Nutrition at the University of Hannover. This research led to a method to determine nitrogen fertilizer requirement, ,,the N,,,-method.

This paper gives some examples from the results of our experiments on the application of the N,,,-method when the aims of fertilization differed.

The principles of the Nd,,-metbod

The N,i,-method is based upon the finding that different quantities of mineral nitrogen are available in soils at the beginning of growth in spring or at any required fertilization date. 1983 winter wheat fields on loess-soils of the Lower Saxony area contained on average 64 kg N/ha mineral nitrogen in FebruaryMarch, 1985, with a range from 20-567 kg Nha . The mineral nitrogen of loam soils under normal farm conditions consists of nitrate. The nitrate nitrogen mainly originates from mineralization and nitrification in the top-soil. The bulk of the nitrogen is found in the sub-soil at the end of winter due to leaching, as can be seen from Figure 1. In the sub-soil, mineral nitrogen is absorbed by plants according to their rooting

depth and is used like fertilizer nitrogen for yield production. Therefore, the quantity of mineral nitrogen in the rootable soil layer of a particular

crop is measured by the N,i,-methad at the date of fertilizer application. For this, the measured N~n-supply of soil is complemented by fertilizer nitrogen to the optimum N quantity as determined in fertilizer trials. Hence, N fertilization is calculated as the difference between optimum N quantity and N,i,-supply of the soil (Scharpf and Wehrmann, 1975 and 1979; Scharpf, 1977; Wehrmann and Scharpf, 1980; Bohmer, 1980; Bohmer et al., 1982; Molitor, 1982; Kuhlmann et al., 1983).

In the meantime essential data of the Nmin-method have been obtained for several crops and form the basis of a recommendation system. They are summarized in Table 1,

430 Wehrmann, Scharpf Z . Pflanzenernaehr. Bodenk.

0 20 LO 0 20 LO 0 20 LO --- ~c l~ ; ropcer - '~o

100 68

precrop sugar beet

:-4l 4 - n 100

- precrop vegetab les

b3

0 20 LO 0 20 L O k g N l h a - -n

n n

Figure 1: Distribution of mineral nitrogen in 15 loess soils under winter wheat, February 1976 Abbddung 1: Verteilung der Nmi.-Vorrate in 15 Loaboden mit Winterweizen, Februar 1976

which gives informations on date of soil sampling, the soil layer to be analysed, the sum of N,i,-content of soil and fertilizer nitrogen giving optimum yield, and the splitting of the fertilizer dose. The results were obtained on deep rootable loess soils in Southern Lower Saxony. Adjustments to these data are probably necessary at other locations.

The Nd,,-method for dflerent aims of fertilization

The priority objective of fertilization is high yield at low fertilization cost. Data collected on 70 farms located in the loess area of Southern Lower Saxony over several years show that on fields where N,i,-analyses were carried out, yields of winter wheat, winter barley and of sugar were 2-3 dt/ha higher compared to fields on the same farms where N,i,-analyses were not carried out (Table 2). These results were obtained because farmers generally followed the recommendation. However, with sugar beets many farmers tend to fertilize more than is indicated by soil analysis, which results in lower sugar yield and economic loss (Kuhlmann and Wehrmann, 1981).

These yield increases in cereals can be explained by the better adjustment of fertilization to the requirement of the crops on particular fields because the average nitrogen fertilization was not changed substantially as a result of the application of the N,i,-method. This is confirmed by experiments of the Ruhrstickstoff AG with winter cereals which showed that fertilization according to the Nmin-method led to much more reliable optimum yields than fertilization according to conventional methods (Becker and Lunge, 1982).

However, with sugar beets, the application of the N,i,-method resulted in a decrease of nitrogen fertilization by 30 kg Nlha, as Figure 2 shows.

149, 428-440 (1986) N(min)-analysis 431

Table 1: Essential data of the N~"-method for crop plants on deep soils TabeUe 1: Kenndaten der N,,-Methode fur verschiedene Pflanzenarten auf tiefgrundigen Boden

soil analysis Nmin content .f N application kg/ha

1st application total N crop time soil depth or total N supply notes

cm supply in including single dose top dressing

winter and spring Febr./March 90 120 200 splitting of wheat doses higher

winter barley Febr./March 90 100 160 than 80 kg/ha winter rye FebrJMarch 90 100 140 oats 2) 60 100 (130)

sugar beet March/May 90 200 200 net minerali- zation low

sugar beet March/May 90 160 160 net minerali- zation high

potato, sand 2) 60 160 high starch yield

potato, loess 2) 60 250 high tuber yield

cabbage, industrial cabbage, late for storage cabbage, spring/summer brussels sprouts cauliflower, broccoli spinach, spring spinach, autumn celery leek beans ice-lettuce head lettuce peas asparagus, 1. year asparagus, from 2nd

year on onion strawberry

1) 1)

1) April

90 90

60 90 60 60 60 60 60 60 30 30 30 90 90

60 60

350* 250*

200* 350* 300* 250* 200 220 200 140* 140 loo**

80 80

100

130 80

*lo0 kglha in 0-30 cm

** 30-40 kg/ha in addition after heavy rain, 2nd ana- lysis?

1) transplanting time 2) sowing time

432 Wehrmann, Scharpf Z . Pflanzenernaehr. Bodenk.

Table 2: Grain and sugar yield on fields with and without N,,,j,,-soil analysis. Average of 3 or 5 years and 60-330 fields per year for each crop Tabelle 2: Korn- und Zuckerertrag auf Feldern ohne und mit Nmi,-Bodenanalyse. Mittelwerte aus 3- bzw. 5-jahrigen Erhebungen auf jlhrlich 60-330 Feldern pro Pflanzenart

~ _ _ ~

Nmin winter wheat winter barley sugar soil analysis dt/ha dt/ha dt/ha

without 62 with 65

57 73 60 75

kg N l h a

160

Figure 2: Nitrogen fertilization for sugar beets as affected by the N,i,-analysis in 70 farms, 1973-1984

1 L O -. Abbildung 2: Durchschnittliche N-Dungung zu Zuckerriiben ohne und mit N,i,-Analyse in 70

w i t h

r l l l l l l r l l r i

7 5 77 79 8 3 landwirtschaftlichen Betrieben, 1973-1984

In vegetable production, N fertilization could be reduced even more in many cases if the N,i,-method was used (see also Tables 6 and 7) .

No own experiments were done on the relation between nitrogen nutrition and the occurence of plant diseases and pests. However, in the literature the increasing occurence of pathogens with nitrogen is often reported (e.g. powdry mildew on winter wheat, Shanner and Finney, 1977). Consequently, N fertilization by means of the Nmi,-method should help to decrease the infestation. If necessary, the optimum nitrogen quantities given in Table 1 may have to be reduced when optimal chemical plant protection, including growth regulators, is not been applied.

Nitrogen supply influences the quality of plants, e.g. content of sugar, protein and vitamins. This is particularly true in the case of vegetables where nitrate content may be critical for human health.

149, 428-440 (1986) N(min)-analysis 433

20-

10-

y ie ld 30 t FM / h a

10-0

4 0 0 0

-3000

-2000

-1000

mg N O 3 I k g FM

J i I I , l 50 100 150 200 250 300 350 400 N supply, k g N / h a

5 11 27 61 117 126 155 r e s i d u a l NOj-N,kg/ha

Figure 3.: Effect of N-supply ( s u m ' N ~ " 0-60 cm and fertilizer-N) on yield and nitrate content in fresh matter of spinach and on residual nitrate in the soil at harvest Abbddung J: EinfluD des Stickstoffangebotes (Nmi,, M cm + Dunger-Stickstoff) auf Ertrag und Nitratgehalt von Spinat sowie den Nitratgehalt im Boden zum Erntetermin

Since critical levels for nitrate have been introduced by some countries in the last few years, it is necessary to supply N in a way that the stipulated levels can be met. As Figure 3 shows, exceeding the optimum N quantity of about 250 kg N/ha results in a considerable increase of nitrate content in spring spinach. At the highest yield, spinach contains 2300 mg N03/kg fresh matter. With a limitation of the nitrate content to 2000 or 1000 mg N03/kg fresh matter, the N supply has to be reduced to 200 or 100 kg N/ha respectively, resulting in a yield decrease of 10 or 40 % (Table 3). These data are in accordance with extensive investigations in the Netherlands (Refers et al., 1985). Hence, optimum N supply differs according to the objectives.

Table 3: Rate of N-fertilization of spring spinach for different objectives Tabelle 3: Integration verschiedener Ziele bei der Dosierung der N-Diingung zu Friihjahrsspinat

objectives opt. N-supply* yield kg N/ha %

maximum yield, no soil analysis maximum yield, with soil analysis maximum economic yield limitation to 2000 mg NO,/kg FM nitrate limitation in soil 40 kg N/ha limitation to 1000 mg NO, /kg FM

300 280 250 200 120 100

100 100 98 91 65 60

~ ~~ ~~~

* Nmin-amount in soil 0 - 60 cm + N-fertilization

434 Wehrmann, Scharpf Z . Pflanzenernaehr. Bodenk.

Yield decrease due to reduced N supply can be partially avoided by appropriate chloride fertilization, as shown in Figure 4. Chloride uptake causes nitrate in plants to be reduced more and used for synthesis of organic N compounds (Hahndel 1984; Wehrmann and Hahndel, 1984).

2007

I I

loo 150 200 2 50

2ooo1 I LL 1500- C

m .-

0 1000-

E, a

'"1 100 150 200 250 N-supply k g N l h a (Nmin content 0 -60cm

+ N- fer t i l i za t ion 1

Figure 4 Influence of chloride fertilization and N-supply on yield and nitrate content of spring spinach (100 ?6 = yield at 100 kg Nlha without CI-fertilization; mean value from 5 experiments, 1983; the soils contained 66-170 kg CVha in 0-60 cm) Abbildung 4 EinfluS einer Chloriddiingung auf den Ertrag und den Nitratgehalt von Friihjahrsspi- nat

Therefore plants reach their highest yield at a lower N supply. Thus, nitrate content can be considerably decreased. From these experiments the fertilizer doses given in Table 4 can be recommended for the production of spinach and lettuce in Southern Lower Saxony.

Under the climate of the Federal Republic of Germany nitrate leaching mainly depends on the amount of nitrate in the soil during the winter drainage period

149, 428-440 (1986) N(min)-analysis 435

Table 4 Optimum N- and C1-supply to reduce the nitrate content of vegetables at maximum yield Tabelle 4 Optimale N- und C1-Diingung zur Verminderung des Nitratgehaltes von Spinat und Kopfsalat ohne ErtragseinbuBen

N-suppb', kg N/ha (Nmin+ fert.N) c1-supply (Clsoil +Ckert.)

kg Cl/ha

soil depth (cm)

without C1 with C1

spring spinach 0 - 60 250 200 350 autumn spinach 0 - 60 200 150 350 lettuce 0 - 30 120 90 150

(Wehrmann and Scharpf, 1983). The decrease in the amount of nitrate in the soil between December and March (Table 5) was due to leaching, which was probably higher in many cases than the calculated difference since nitrate formation by mineralization was not included.

Table 5: Changes of the nitrate content in soils between December and March after vegetables, cereals or sugar beet. Tabelle 5: Veranderungen der Nitratgehalte von Dezember bis Marz in Boden nach Gemiise, Getreide oder Zuckerriiben

~

December March Difference kg N/ha kg N/ha kg N/ha

carrot 139 31 - 108 carrot 91 24 - 61 table beet 161 31 - 130 lupine-leek 197 73 - 124 celery 191 113 - 78 winter wheat* 20 29 + 9 winter barley* 25 36 + 11 sugar beet* 25 35 + 10

* mean values for 5 years, loess soil

N fertilization has to be carried out in such a way that soils contain as little nitrate as possible at the end of the growing season. This is mostly the case when cereals and sugar beets are grown on deep rootable soils. In most cases these crops are not overfertilized for economical reasons such as avoidance of lodging and low sugar content. Furthermo- re, cereals and sugar beets are capable of exhausting nitrate from deep in the soil. High nitrate contents at the end of the growing season are often found in soils under vegetable production and in soils with high organic fertilization. In vegetable produc- tion, fertilization frequently exceeded the optimum N quantity, as can be seen from Table 6, where fertilizer doses used in practice are compared to optimum N quantity according to the N,i,-method.

436 Wehrmann, Scharpf Z . Pflanzenernaehr. Bodenk.

Table 6: N-fertilization of vegetable crops in practice compared to recommendations according to the N,,,in-method and N,i,-content in the soil at the beginning of winter Tabelle 6 N-Diingung zu Gemiise in der Praxis im Vergleich zur Empfehlung nach N,,,in-Methode und Nfin-Gehalt im Boden zu Winteranfang

crop rotation total fertilization kg N/ha NO, Nov./Dec. kg N/ha (0-90 cm)*

according to in difference Nmin-method practice

cauliflower cauliflower 384 813 + 429 564

cauliflower 70 267 + 197 198

150 432 + 282 248 kohlrabi leek

celery

Savoy

62 96 + 34

232 307 + 75 156

71

* after practice fertilization

Reasons for excessive fertilization may be relatively low fertilizer costs, lack of toxicity symptoms and insurance fertilization by producers. The excess in fertilization (difference between the applied dose and the optimum N quantity) increases nitrate in the soil at the beginning of winter. This nitrogen is readily leached in sandy soils or with high rainfall during the winter (Wehrrnann and ScharpJ 1984).

The application of the N,i,-method leads to low nitrate contents in soils under most vegetable crops at the time of harvest. This was shown in an investigation of the nitrate quantity in the soil during the winter on a vegetable farm. This consultation started in 1983 and resulted in a decrease of fertilization by 50 %, as Table 7 shows. As a result, the nitrate content of soils in danger of leaching could be reduced from more than 200 kg to 55 kg Nlha. This result was obtained without decreasing the yield.

Some vegetable crops, like spinach, only reach maximum yield if the soil contains more than 100 kg NOs-Nha at harvest, as can be seen from Figure 3. High nitrate concentrations in the soil solution are obviously necessary to ensure that 15-20 kg N1ha.d are transported to the roots by mass flow and diffusion. Spinach crops take up nitrogen at this rate in the days before harvest (Hahndel and Wehrmann, 1986). Such crops should be placed at the beginning of an annual crop rotation, so that the residual nitrogen can be used by the following crop.

Limitations of the N,i.-method

Each method for the determination of fertilizer requirement has its limitations. This is also true for the N,i,-method. It is necessary to realize what the limitations are if these are to be further reduced. A surprisingly strong relationship between the

149. 428440 (1986) N(min)-analvsis 437

Table 7: Effect of fertilization by the N~n-method (starting in 1983) on the nitrate content of the soil in the following January Tabelle 7: Auswirkung einer Diingung nach Nd,,-Methode, Beginn 1983, auf die Stickstoffdiingung und den Nitratgehalt des Bodens im Januar

crop (2 fields)

~ ~~ ~~

N-fertilization, kg/ha NO, -N in soil, kg/ha 0 - 90 cm on Jan. 18th

1982 1983 1984 1983 1984 1985

lettuce lettucel)

120 75 78 285 35 58 120 65 0

cabbage 200 110 115 191 71 62

mean value 220 125 96 238 53 60

1) 1983 Savoy instead of lettuce

N,,,-content of the soil and fertilizer requirement was found in many experiments over several years especially with winter wheat (Wehrmann and Scharpf, 1979; Becker and Lung, 1982). A high reliability in forecasting fertilizer requirements can be achieved if N supply is only affected in the usual manner by leaching, denitrification, mineralization or immobilization after soil sampling for N,i,-analysis and after fertilization. The corresponding conditions are listed on the left side in Table 8, conditions quoted on the right side decrease reliability. These are mainly factors which influence net mineraliza- tion and N leaching.

Nitrogen losses by leaching after the date of soil sampling can be estimated by means of correction tables and adjusted by top dressing (Wehrmann and Scharpf, 1983). Net mineralization during the growing season is not taken account of by the measurement of N~,-quantity in soil at the beginning of the growing season. However, fertilizer recommendations are based upon fertilizer trials on fields with typical crop rotations, so that net mineralization of nitrogen on particular fields was included in the result and had not to be considered separately. The data given in Table 1 are valid for locations in Southern Lower Saxony on loess soils delivering on the average 90 kg Nlha from March till August under the crop rotation cereal, cereal, sugar beet (Kiihler, 1983). The range is between 50 and 160 kg Niha. The optimum N quantity is reduced for sugar beet from 200 to 160 kg Nlha and for spinach from 250 to 200 kg Nha, if more than the average net mineralization can be expected, e.g. after ploughing grass land or after high organic fertilization, or where there are nitrogen rich p!ant residues, as can be seen from Table 1. Nitrogen fertilizer for intensive cereal production in locations in Southern Lower Saxony with optimum water supply is usually applied in three doses, at the beginning of the growing season (FebruaryMarch), at stem elongation and at ear emergence. Different net mineralization would probably influence fertilizer requirement at the two

438 Wehrmann, Scharpf Z. Pflanzenemaehr. Bodenk.

Table 8 Reliability of fertilizer requirement forecast by means of the Nfin-method TabeUe 8 Die Genauigkeit der Diingerbedarfsprognose mit Hilfe der Nfin-Methode

~~

increases with - rapid and early rooting in the soil

- short culture periods

- moderate rainfall - deep rootable soils with high water holding

capacity - deep rooting of the soil

- low organic manuring

- low temperatures

- uniform rotation

decreases with - time between soil sampling and rooting of

the soil

- long culture periods

- high rainfall

- shallow or sandy soils

- high or strongly changing organic manuring

- high temperatures

- large differences between varieties

example: winter wheat on loess soils in Southern Lower Saxony

example: sugar beet, lettuce on sand soils

later dressings. At these dates the evaluation of crop appearance and Nmin-analysis as well as the determination of the nitrate content in sap pressed from the stem basis of plants have proved to be useful parameters of nitrogen supply and fertilizer demand on plants (Wehrmann et al., 1982; Wollring, 1983). Farmers do the plant field test themselves. Table 9 gives fertilizer recommendations based upon the plant field test which were determined in fertilizer experiments.

Investigations on net mineralization were carried out on 53 loess loam soils in Southern Lower Saxony with wheat. The aim of these investigations was to forecast net mineralization between the time of Nmin-analysis (Februarymarch) and the end of N

Table 9 Recommendation for the nitrogen top dressing of winter wheat and winter barley by using the nitrate rapid test TabeUe 9 Empfehlung fur die Stickstoff-Kopfdiingung zu Winterweizen und Wintergerste auf der Grundlage des Nitratsafttestes

~

test value* at stem elongation or ear emergence 0 - 1 1 - 2 2 - 3

N-application at stem elongation, kg N/ha 5 0 - 40 40 - 20 2 0 - 0

N-application at ear emergence, kg N/ha 9 0 - 60 60-30 3 0 - 0 ~

* test value: intensity of blue colour (0 -- 3) due to reaction between NO, in plant sap and dipheny lamine

149, 428440 (1986) N(min)-analysis 439

uptake by biological and chemical soil test methods, as well as by calculation. No relationship was found between the net mineralization, which was determined either from the increase in Nmin on sheltered fallow plots or by plant analysis, and laboratory values of 11 different methods (Kiihler, 1983). Therefore, the above mentioned plant sap test seems t o be the only way to determine the nitrogen fertilizer demand of wheat and barley at the late dressings.

Acknowledgement

The authors thank Dr. Barraclough of Rothamsted for improvement to the English.

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