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BAHAN ORGANIK DAN KESUBURAN TANAH

Bahan kajian MK. Agroekologi Diabstraksikan oleh Prof Dr Ir Soemarno MS

Jurs Tanah FP UB September 2011

Soil organic matter (SOM) is the organic matter component of soil. It can be divided into three general pools: living biomass of microorganisms, fresh and partially decomposed residues, and humus: the well-decomposed organic matter and highly stable organic material. Surface litter is generally not included as part of soil organic matter

(Juma, N. G. 1999. Introduction to Soil Science and Soil Resources. Volume I in the Series "The Pedosphere and its Dynamics: A Systems Approach to Soil Science." Salman Productions, Sherwood Park. 335 pp.).

Organic matter (or organic material, Natural Organic Matter, or NOM) is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds. The definition of organic matter varies upon the subject for which it is being used. Organic matter is broken down organic matter that comes from plants and animals in the environment (Natural Organic Matter," GreenFacts, 22 Apr, 2007 <http://www.greenfacts.org/glossary/mno/natural-organic-matter-NOM.htm). Organic matter is a collective term, assigned to the realm of all of this broken down organic matter. Basic structures are created from cellulose, tannin, cutin, and lignin, along with other various proteins, lipids, and sugars. It is very important in the movement of nutrients in the environment and plays a role in water retention on the surface of the planet. These two processes help to ensure the continuance of life on Earth (http://en.wikipedia.org/wiki/Organic_matter).

Peranan Bahan Organik Tanah (BOT)

Kesuburan tanah dapat dideskripsikan sebagai kapabilitas suatu tanah untuk mensuplai unsur hara kepada tanaman dalam jumlah dan proporsi yang dibutuhkan tanaman. Konsep ini merupakan kesuburan tanah secara kimiawi. Dalam makna yang lebih luas, kesuburan tanah juga mencakup kesuburan tanah secara fisika, yang merupakan kapabilitas suatu tanah untuk mensuplai air kepada tanaman, mensuplai udara kepada akar tanaman dan menyediakan tempat untuk “pegangan” akar

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tanaman. Kadangkala kesuburan kimia dan fisika tanah dikombinasikan menjadi konsep “produktivitas tanah”.

Peranan BOT dalam kesuburan kimiawi tanah adalah sebagai penyangga penyediaan N, P dan S, serta sebagai penjerap Ca, Mg, K dan Na. Sedangkan peranan BOT dalam kesuburan fisika adalah meningkatkan kemampuan tanah menahan air (WHC) dan memperbaiki stabilitas struktur tanah.

Pengaruh BOT terhadap sifat-sifat tanah:Organic matter affects both the chemical and physical properties of the

soil and its overall health. Properties influenced by organic matter include: soil structure; moisture holding capacity; diversity and activity of soil organisms, both those that are beneficial and harmful to crop production; and nutrient availability. It also influences the effects of chemical amendments, fertilizers, pesticides and herbicides (Alexandra Bot and José Benites. 2005).

What is the impact of incorporating organic matter into the soil? (http://www.soilhealth.com/organic/)Incorporating organic matter into soil can have several impacts because it disturbs the physical, chemical and biological balances in the soil. It can:1. Mengubah jumlah N-tanah yang tersedia bagi tanaman 2. Mengubah jumlah hara lain yang tersedia bagi tanaman3. Mengubah agregasi tanah4. Mengubah jumlah dan tipe organism tanah .

WHAT DOES ORGANIC MATTER DO? (http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html#do)

Siklus Unsur Hara Meningkatkan KTK tanah Merupakan cadangan hara tanaman. Chelates (binds) nutrients, preventing them from becoming permanently

unavailable to plants. Is food for soil organisms from bacteria to worms. These organisms hold on to

nutrients and release them in forms available to plants. Ketersediaan Lengas Tanah

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Memperbaiki infiltrasi air. Menurunkan evaporasi. Increases water holding capacity, especially in sandy soils.

Struktur Tanah Reduces crusting, especially in fine-textured soils. Memacu pertumbuhan dan perkembangan akar Memperbaiki agregasi, meminimumkan erosi tanah. Mencegah Pemadatan (Kompaksi tanah).

Pengaruh lainnya dari BOT Pesticides break down more quickly and can be "tied-up" by organic matter

(and clays). Dark, bare soil may warm more quickly than light-colored soils, but heavy

residue may slow warming and drying in spring. Many of the effects of organic matter are related to the activity of soil

organisms as they use soil organic matter. See Soil Biology (BU-7403 in this series) for more information.

Plant residues and other organic material may support some diseases and pests, as well as predators and other beneficial organisms.

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BOT memperbaiki agregasi tanah

Struktur tanah, mempresentasikan keberadaan bacteri, bahan anorganik, dan bahan organic, air dan udara. Gambar dipetik dari Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer

Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com). Sumber: http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookplanthorm.html

BOT MEMPERBAIKI EFISIENSI PEMUPUKAN The two major soil fertility constraints are low inherent nutrient

reserve and rapid acidification under continuous cultivation as a consequence of low buffering or cation exchange capacity (Jones and Wild, 1975). Generally, these constraints are tackled by applying chemical fertilizers and lime. However, the application of inorganic fertilizers on depleted soils often fails to provide the expected benefits. This is basically because of low organic matter and low biological activity in the soil. The chemical and nutritional benefits of organic matter are related to the cycling of plant nutrients and the ability of the

Kuarsa

Kuarsa

Kuarsa

Koloni Bakter

i

Komplek agregat tanah

udara

udara

Partikel Liat

Partikel Liat

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soil to supply nutrients for plant growth. Organic matter retains plant nutrients and prevents them leaching to deeper soil layers. Micro-organisms are responsible for the mineralization and immobilization of N, P and S through the decomposition of organic matter (Duxbury, Smith and Doran, 1989). Thus, they contribute to the gradual and continuous liberation of plant nutrients. Available nutrients that are not taken up by the plants are retained by soil organisms. In organic-matter depleted soils, these nutrients would be lost from the system through leaching and runoff.

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Benefits of soil organic matter and humus1. The process that converts raw organic matter into humus feeds the soil

population of microorganisms and other creatures, thus maintains high and healthy levels of soil life (Elo, Maunuksela, Salkinoja-Salonen, Smolander, and Haahtela, 2006).

2. The rate at which raw organic matter is converted into humus promotes (when fast) or limits (when slow) the coexistence of plants, animals, and microbes in soil .

3. Effective humus and stable humus are further sources of nutrients to microbes, the former provides a readily available supply, and the latter acts as a longer-term storage reservoir.

4. Decomposition of dead plant material causes complex organic compounds to be slowly oxidized (lignin-like humus) or to break down into simpler forms (sugars and amino sugars, aliphatic, and phenolic organic acids), which are further transformed into microbial biomass (microbial humus) or are reorganized, and further oxidized, into humic assemblages (fulvic and humic acids), which bind to clay minerals and metal hydroxides. There has been a long debate about the ability of plants to uptake humic substances from their root systems and to metabolize them. There is now a consensus about how humus plays a hormonal role rather than simply a nutritional role in plant physiology (Eyheraguibel, Silvestrea, and Morard, 2008).

5. Humus is a colloidal substance, and increases the soil's cation exchange capacity, hence its ability to store nutrients by chelation. While these nutrient cations are accessible to plants, they are held in the soil safe from being leached by rain or irrigation (Szalay, 1964).

6. Humus can hold the equivalent of 80–90% of its weight in moisture, and therefore increases the soil's capacity to withstand drought conditions (Olness and Archer, 2005).

7. The biochemical structure of humus enables it to moderate – or buffer – excessive acid or alkaline soil conditions (Kikuchi, 2004).

8. During the humification process, microbes secrete sticky gum-like mucilages; these contribute to the crumb structure (tilth) of the soil by holding particles together, and allowing greater aeration of the soil. Toxic substances such as heavy metals, as well as excess nutrients, can be chelated (that is, bound to the complex organic molecules of humus) and so prevented from entering the wider ecosystem (Huang, , Zeng, Feng, Hu, Jiang, Tang, Su, Zhang, Zeng, and Liu, 2008).

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9. The dark color of humus (usually black or dark brown) helps to warm up cold soils in the spring.

Substansi humik membantu ketersediaan hara dalam tanah

Fungsi-fungsi humus tanah (Alexandra Bot and José Benites. 2005):1. improved fertilizer efficiency;2. longlife N – for example, urea performs 60–80 days longer;3. improved nutrient uptake, particularly of P and Ca;4. stimulation of beneficial soil life;5. provides magnified nutrition for reduced disease, insect and frost

impact;6. salinity management – humates “buffer” plants from excess sodium;7. organic humates are a catalyst for increasing soil C levels.

Humus tanah terdiri atas berbagai substansi humik:1. Fulvic acids: the fraction of humus that is soluble in water under all pH

conditions. Their colour is commonly light yellow to yellow-brown.2. Humic acids: the fraction of humus that is soluble in water, except for

conditions more acid than pH 2. Common colours are dark brown to black.

3. Humin: the fraction of humus that is not soluble in water at any pH and that cannot be extracted with a strong base, such as sodium hydroxide (NaOH).

4. Biasanya berwarna gelap kehitaman.

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Karakterisasi BOT dan Mineralisasi

Bahan organik tanah hampir seluruhnya berasal dari residu tanaman. Dengan demikian diharapkan BOT ini mengandung unsur-unsur hara yang sama dengan yang ada dalam tanaman, dan proporsinya relatif sama dengan yang ada dalam tubuh tanaman. Namun pada kenyataannya komposisi BOT berbeda dengan komposisi tubuh tanaman. Segera setelah material tanaman yang mati jatuh ke tanah, ia segera mengalami perubahan. Komponen-komponen yang mudah larut segera tercuci ke luar.

Sebagai sumber cadangan unsur hara, BOT sangat penting , utamanya dalam hal unsur hara N, P, dan S yang terikat secara organik. Unsur-unsur ini dapat menjadi tersedia bagi tanaman melalui proses mineralisasi, yaitu konversi senyawa organik menjadi an-organik dengan melibatkan mikro-organisme tanah. Seringkali dilakukan pembedaan antara bahan organik yang stabil dan yang tidak stabil. Bahan organik yang tidak stabil juga disebut “nutritive, labil, aktif, atau humus-muda”, merupakan bahan organik yang masih baru terbentuk dari biomasa tanaman yang masuk ke tanah (yaitu selama 10-20 tahun terakhir). Bahan organik yang stabil, pasif atau humus tua, merupakan bahan organik yang telah berada dalam tanah selama waktu yang panjang. Perbedaan di antara keduanya tidak tajam, karena humus-labil secara bertahap berubah menjadi humus-stabil.

Perubahan Bentuk BOT 1. Penambahan ke

tanah. When roots and leaves die, they become part of the soil organic matter.

2. Transformations. Soil organisms continually change organic compounds from one form to another. They consume plant residue and other organic matter, and then create by-products, wastes, and cell tissue.

3. Microbes feed plants. Some of the wastes released by soil organisms are nutrients that can be used by plants. Organisms release other compounds that affect plant growth.

4. Stabilization of organic matter. Eventually, soil organic compounds become stabilized and resistant to further changes.

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Sumber: http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html#do

Bahan organik stabil mempunyai komposisi yang kompleks. Komponen yang sangat penting ialah humin, asam humat, dan asam fulvat; ada juga hasil polimerisasi dari senyawa fenolik dan senyawa yang mengandung nitrogen. Sebagian besar BOT tanah secara intensif berhubungan dengan mineral liat dan ini menjadi salah satu alasan mengapa dekomposisi humus berlangsung sangat lambat. Senyawa seperti polisakarida, protein, asam amino, yang mudah terdekomposisi, akan “lenyap “ selama proses konversi bahan organik labil.

Unsur C, N, P dan S dalam bahan organik stabil lazimnya adalah 100:10:1:1, tetapi seringkali terjadi penyimpangan dari proporsi ini. Sifat vegetasi, ilkim, landuse, umur bahan organik dan faktor lainnya sangat berpengaruh. Demikian juga dalam sel-sel mikro-organisme, unsur C, N, P, dan S mempunyai proporsi 100:10:1:1.

Banyak organisme tanah bersifat C-heterotrofik, mereka tidak mampu mengasimilasi CO2 dari udara. Mereka menggunakan senyawa karbon yang ada dalam bahan organik tanah. Demikian juga untuk respirasinya mereka menggunakan senyawa karbon dalam bahan organik tanah. Secara rata-rata fungi menggunakan 2/3 bagian bahan organik untuk mendapatkan energi dan 1/3 bagian untuk membangun jaringan tubuhnya. Dengan kata lain, bahan organik digunakan oleh fungi secara disimilasi (2/3 bagian) dan secara asimilasi (1/3 bagian).

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Dekomposisi bahan organic tanah

Dekomposisi bahan organic merupakan proses biologis yang terjadi secara alamiah. Kecepatan proses dekomposisi ini ditentukan oleh tiga faktor: organism tanah, lingkungan fisik, dan kualitas bahan organiknya. Dalam proses dekomposisi hahan organic ini dilepaskan berbagai hasil, seperti CO2, energy, air, hara tanaman dan senyawa-senyawa organic hasil re-sintesis. Pada akhirnya proses dekomposisi bahan organic akan menghasilkan bahan organic yang lebih kompleks, disebut humus; proses ini lazim disebut “humifikasi”. Humus ini mempengaruhi berbagai sifat dan cirri tanah. Humus ini warnanya gelap, mampu memperbaiki agregasi tanah dan stabilitas agregat tanah; meningkatkan KTK tanah (kemampuan menahan unsure hara); dan menymbang unsure hara N, P dan lainnya.

Humus tanah berfungsi ganda: Memperbaiki efisiensi pupuk; longlife N - for example, urea performs 60-80 days longer; improved nutrient uptake, particularly of P and Ca; stimulation of beneficial soil life; provides magnified nutrition for reduced disease, insect and frost

impact; salinity management - humates “buffer” plants from excess sodium; organic humates are a catalyst for increasing soil C levels.

Crop residues contain mainly complex carbon compounds originating from cell walls (cellulose, hemicellulose, etc.). Chains of carbon, with each carbon atom linked to other carbons, form the “backbone” of organic molecules. These carbon chains, with varying amounts of attached oxygen, H, N, P and S, are the basis for both simple sugars and amino acids and more complicated molecules of long carbon chains or rings. Depending on their chemical structure, decomposition is rapid (sugars, starches and proteins), slow (cellulose, fats, waxes and resins) or very slow (lignin). (http://www.fao.org/docrep/009/a0100e/a0100e05.htm).

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Composition of leaves and roots of leguminous and grass species. Sumber: Primavesi, A. 1984. Manejo ecológico del suelo. La agricultura en regiones tropicales. 5ta Edición. El Ateneo. Rio de

Janeiro, Brazil. 499 pp. (http://www.fao.org/docrep/009/a0100e/a0100e05.htm)

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Sumber: The importance of soil organic matter....... (http://www.fao.org/docrep/009/a0100e/a0100e05.htm)

6 0 0 bahan organik

3 0 0 C 3 0 N 3 P

3 S

Dissimilasi

1/3 asimilasi Respirasi Mineralisasi

Imobilisasi

HumusMine

ralisasi

Unsur

Hara

Organisme Tana

h

Organisme Tana

hProtein

dan Polisakarida

Struktur tana

h

Seresah tanaman Oks

igen

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1 0 0 C 2 0 0 C 2 0 N 1 0 N 2 P 1 P 2 S

1 S

Gambar 1. Bagan distribusi C, N, P dan S untuk asimilasi dan disimilasi, dalam konversi 600 unit masa bahan organik.

CO2 dan H20 terbentuk melalui proses respirasi, misalnya:

C6H12O6 + 6O2 -------- 6 CO2 + 6 H2O + energi

Senyawa organik N, P dan S berubah menjadi bentuk anorganik NH4+, NO3-, H2PO4- dan SO4=, yang tersedia bagi tanaman. Proses perubahan seperti ini disebut mineralisasi. Laju mineralisasi tergantung pada faktor-faktor seperti suhu, pH, aerasi tanah, kelengasan tanah, kesuburan tanah dan sifat vegetasi serta sistem pertanian yang berlaku. Suatu indikasi laju mineralisasi dapat diperoleh dengan jalan mengukur jumlah CO2 atau nitrogen anorganik yang dihasilkan per unit tanah per unit waktu.Kemungkinan lainnya ialah dengan jalan mengukur kandungan bahan organik atau nitrogen organik, fosfor dan belerang organik dari waktu ke waktu. Periode waktu ini harus cukup panjang (beberapa tahun), karena laju mineralisasi relatif tidak tinggi, sekitar 2% setahun di daerah temperate dan sekitar 8% di daerah dataran rendah tropis. Hasil penelitian Jenkinson dan Aynabe (1977) , Ladd dan Amato (1985) membuktikan pentingnya pengaruh suhu terhadap laju mineralisasi bahan organik tanah. Laju mineralisasi relatif meningkat dua kali setiap kenaikkan suhu 9oC, yaitu 2% pada 9oC, 4% pada 18oC, dan 8% pada 27oC. Di atas 30oC dan di bawah 6oC ketentuan ini tidak berlaku.

Kalau C/N rasio bahan organik sama dengan 10; maka konversi 300 unit masa karbon, atau disimilasi 200 unit masa karbon , melibatkan konversi 30 unit masa N. Dari jumlah ini 10 unit diikat dalam sel mikroorganisme dan 20 unit dilepaskan ke dalam larutan tanah. Dalam kaitan ini kita

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mengukur mengukur lenyapnya C atau disimilasi. Per 10 kg C yang telah lenyap dibarengi dengan 1 kg N yang mengalami mineralisasi. Dengan demikian di daerah yang laju dekomposisi relatif BOT 2% setahun , pelepasan N per % bahan organik per tahun per 20 cm topsoil (1 ha, 20 cm = 2.5 x 106 kg) sama dengan:

1/10 x 50/100 x 10-2 x 2.5x106 x 2x10-2 = 25 kg N

N=1/10 C C/BOT 1% BOT masa tnh laju dec. reltf

Dengan anggapan bahwa fraksi massa P dan S adalah 1/10 dari massa N, maka jumlah P dan S yang dilepaskan adalah: 2.5 kg per tahun per ha per 20 cm topsoil per % bahan organik.

Pada umumnya tanaman tidak dapat menggunakan jumlah hara ini secara keseluruhan. Sebagian n dan S tercuci dalam bentuk NO3- dan SO4=, atau menguap sebagai NH3, N2, H2S; sedangkan fosfat diikat oleh partikel tanah dalam bentuk H2OP4- atau PO4=- .Kalau C/N rasio lebih tinggi dari 10, maka lebih sedikit nitrogen yang dilepaskan ke dalam larutan tanah. Kalau C/N rasio lebih dari 30, biasanya tidak cukup N untuk proses asimilasi oleh mikroba. Nitrogen diambil dari larutan tanah dan tidak tersedia lagi bagi tanaman, proses seperti ini disebut imobilisasi nitrogen. Seringkali immobilisasi hanya bersifat sementara, karena kemudian bangkai sel-sel mikroba mengalami proses mineralisasi.

Persenyawaan Humik: Apa manfaatnya?(sumber: http://www.foliarfert.co.nz/pages/humic_substances.htm)

1. Substansi Humik merupakan hasil akhir dari pelapukan bahan organic, dan biasanya mengandung banyak unsure mikro. Substansi ini mengandung energy hingga 5,000 calories per gram, menyediakan energy yang dapat dimanfaatkan untuk pertumbuhan tanaman.

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2. Humates (senyawa kompleks antara logam dengan asam humik) mensuplai tanaman dengan makanan, ia juga mampu membuat tanah lebih subur dan produktif.

3. Humic substance increases the water holding capacity of soil; therefore, it helps plants resist droughts and produces better crops in reduced water conditions.

4. Humic substance breaks up unproductive clay soils, turning them into profitable soils.

5. Humic substance helps retain water soluble inorganic fertilisers, releases them, as needed, to the growing plants, and helps prevent soil leaching.

6. Humic acid stimulates seed germination and viability, and root respiration, formation and growth.

2. Humic acid reduces other fertiliser requirements and increases yield in crops such as potatoes, wheat, tomatoes, corn, beets, etc.

3. Substansi humik memperbaiki drainage tanah. 4. Substansi humik meningkatkan aerasi tanah. 5. Humic acids increase the protein and mineral contents of most

crops. 6. Humates menyediakan lingkungan yang diperlukan bagi

perkembangan mikroba tanah. 7. Substansi humik menghasilkan tanaman yang lebih subur, hijau dan

sehat.

Efeknya pada Kesuburan Tanah 1. Senyawa humik alamiah dalam tanah dapat membantu

pertumbuhan tanaman , baik secara langsung maupun tidak langsung.

2. Secara fisika, substansi ini membantu memperbaiki struktur tanah dan meningkatkan kemampuan tanah menahan dan menyimpan air.

3. Biologically, they affect the activities of microorganisms. 4. Chemically, they serve as an adsorption and retention complex for

inorganic plant nutrients. 5. Nutritionally, they are sources of nitrogen, phosphorus, and sulphur

for plants and microorganisms. All of these effects increase the productivity of the soil.

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Efeknya pada Tanaman 1. Humic acids can have a direct positive effect on plant growth in a

number of ways. 2. Both plant root and top growth have been stimulated by humates,

but the effect is usually more prominent in the roots. A proliferation in root growth, resulting in an increased efficiency of the root system, is a likely cause of higher plant yields seen in response to humic acid treatment.

3. Humic matter has been shown to increase the uptake of nitrogen by plants, and to increase soil nitrogen utilization efficiency. It can also enhance the uptake of potassium,calcium, magnesium and phosphorus.

4. Asam-asam humik dan fulvik sangat penting dalam membantu memperbaiki ketersediaan air dan hara (unsur mikro) bagi tanaman.

5. Salah satu sifat baik dari substansi humik adalah kemampuannya menyerap dan menahan banyak air. Selain itu, asam fulvik juga membantu penetrasi air dan menembus sel-sel tanaman, membantu penyerapan hara dan menyimpan air untuk digunakan selama kondisi kering.

A typical humic substance is a mixture of many molecules, some of which are based on a motif of aromatic nuclei with phenolic and carboxylic substituents, linked together; the illustration shows a typical structure. The functional groups that contribute most to surface charge and reactivity of humic substances are phenolic and carboxylic groups (Stevenson, 1994). Humic acids behave as mixtures of dibasic acids, with a pK1 value around 4 for protonation of carboxyl groups and around 8 for protonation of phenolate groups. There is considerable overall similarities among individual humic acids (Ghabbour, dan Davies, 2001). For this reason, measured pK values for a given sample are average values relating to the constituent species. The other important characteristic is charge density. The molecules may form a supramolecular structure held together by non-covalent forces, such as Van der Waals force, π-π, and CH-π bonds (Piccolo, 2002).

Adanya gugusan karboksilat dan fenolat menyebabkan asam humik mempunyai kemampuan membentuk senyawa kompleks dengan kation seperti Mg2+, Ca2+, Fe2+ dan Fe3+. Asam-asam humik mempunyai

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dua atau lebih gugusan ini yang tersusun sedemikian rupa sehingga memungkinkan pembentukan kompleks khelate (Tipping, 1994). Pembentukan kompleks khelate ini merupakan aspek penting dari peranan biologis asam humik dalam mengendalikan ketersediaan hara ion logam.

Contoh tipikal asam humik, bempunyai berbagai komponen, termasuk quinone, phenol, catechol dan sugar moieties (Stevenson, 1994).

Kapasitas Tukar Kation (KTK)

BOT dapat menahan kation karena ia mengandung gugusan karboksilat dan fenolat yang dapat mengalami disosiasi sbb:

R – COOH ======= RCOO- + H+

R – OH ====== RO- + H+ (hanya pada pH > 7)

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Muatan negatif pada gugusan karboksilat dan fenolat ini dapat mengikat kation. Proses disosiasi tersebut tergantung pH, sehingga kemampuan BOT mengikat kation juga tergantung pH.

Kation juga dapat diikat oleh bahan organik ke dalam struktur cincin membentuk “khelate” dengan ligand organik. Stabilitas bentuk kompleks ini tergantung pada tipe kation (Tabel 1).

Tabel 1. Kapasitas retensi kation dari beberapa bahan organik tanah, dengan larutan pencucian yang berbeda

Sumber bahan organik Kation yang ditahan dengan larutan pencucian (me per 100 g)

Ba-hidroksida

Cu-asetat pH 5

Ba-asetat pH 5

K-asetat pH 5

Tanah hutan pinus 533 410 155 139Lempung debu Honeoye 295 306 146 60Lempung debu Ontario 309 278 125 54Lempung debu Yates 301 278 124 43Lempung liat berdebu Dunkirk

275 270 135 64

Tanah hutan Sequoia 286 181 118 42Sumber: Broadbent, 1955, dalam Allison, 1973.

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O OH O OH

C C O O C C – O- + Cu++ O

+ H+

OH Cu

O

O O H O O Cu C C O O

C C + Cu++ + H+

O O

Gambar 1. Pembentukan khelate antara Cu dan gugusan karboksil / fenolat; dan antara Cu dengan dua gugusan karboksilat (Schnitzer dan Kahn, 1972),

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BOT DAN DAYA SIMPAN LENGAS TANAH

BOT mampu menahan 3 g air per satu gram bahan organik, berarti tambahan 1% bahan organik dalam topsoil 0-25 cm akan meningkatkan WHC sebesar 3% volume.

Bahan organik tanah mampu memperbaiki stabilitas agregat tanah melalui cara-cara berikut:1. Partikel-partikel tanah diikat bersama-sama oleh hifa jamur dan

actinomycetes2. Mikroba menghasilkan produk metabolik, terutama karbohidrat yang

menjadi perekat yang mengikat bersama partikel tanah3. Di antara lempengan liat dengan asam humat dapat terbentuk

semacam “jembatan kimiawi”4. Melalui stimulasi pertumbuhan akar tanaman, stabilitas struktur tanah

diperbaiki karena akar dapat berfungsi sebagai “tali” di seputar partikel tanah, dan karena mikroba dalam rizosfer menghasilkan material perekat

5. Bahan organik menjadi makanan cacing tanah dan cacing ini mampu memperbaiki stabilitas agregat tanah dan porositas tanah.

Kalau agregat tanah tidak stabil dan bercerai-berai, maka bagian-bagian yang kecil akan mengisi pori tanah sehingga akan merusak aerasi/porositas tanah.Dengan demikian dapat disimpulkan bahwa efek utama bahan organik terhadap struktur tanah adalah melalui perbaikan aerasi tanah untuk tanah-tanah berat dan perbaikan WHC untuk tanah-tanah berpasir.

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Berman D. Hudson. 1994. Soil organic matter and available water capacity. Journal of Soil and Water Conservation March/April 1994 vol. 49 no. 2 189-194.For the last 50 years, the consensus view among researchers has been

that organic matter (OM) has little or no effect on the available water capacity (AWC) of soil. The historical development of this viewpoint is traced. It is argued that the the literature on this subject has been misconstrued and that the consensus view is wrong. In addition to a critical review of the literature, published data were evaluated to assess the effect of OM content on the AWC of surface soil within three textural groups. Within each group, as OM content increased, the volume of water held at field capacity increased at a much greater rate (average slope = 3.6) than that held at the permanent wilting point (average slope = 0.72). As a result, highly significant positive correlations were found between OM content and AWC for sand (r2 = 0.79***), silt loam (r2 = 0.58***) and silty clay loam (r2 = 0.7G***) texture groups. In all texture groups, as OM content increased from 0.5 to 3%, AWC of the soil more than doubled. Soil OM is an important determinant of AWC because, on a volume basis, it is a significant soil component. In this study, 1 - 6% OM by weight was equivalent to approximately 5 to 25% by volume.

BOT dan Hasil Tanaman

“BOT melebihi Pupuk”Organic matter is not just N, P, K, and carbon. Two sources of organic

matter with the same nutrient content or total organic matter content might not have equal effects on your crops and soils. In one research trial, fields treated with animal manure had different microorganisms and enzymes than fields where green manure or mineral fertilizers were used. The importance of these differences are not well studied, but they probably affect nutrient cycling and pests. In your system, manure may mean positive effects such as reducing some diseases, or negative effects such as increasing weed growth. Plant residues also differ greatly as a source of organic matter. Above-ground growth has a different action in soil than roots, even when it is tilled into the soil. All roots do not act the same. For example, tap-rooted plants such as alfalfa create vertical pores in the soil, whereas the finely branched roots of grasses enhance soil aggregation.

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(http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html#do).

Pengaruh BOT terhadap hasil tanaman terutama melalui suplai unsur hara kepada tanaman. Dalam tanah-tanah berpasir juga peningkatan WHC dapat meningkatkan hasil tanaman, terutama selama musim kering. Pada tanah liat berat, penggemburan tanah sangat penting, terutama bagi akar dan umbi-umbian. Dalam banyak kasus untuk tanaman seperti ini peningkatan hasil dapat mencapai 5% atau lebih.

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Pengelolaan bahan organik tanah

Cara-cara untuk meningkatkan kandungan BOT: Aplikasi kompos Tanaman penutup tanah / pupuk hijau Rotasi tanaman perennial forage crops zero or reduced tillage Agroforestry.

Sepanjang tahun, sebagian bahan organik dalam tanah mengalami dekomposisi. Laju relatif proses dekomposisi ini biasanya diberi simbol k; nilainya sekitar 2% di daerah iklim dengan rataan suhu tahunan 9oC dan akan meningkat dua kali setiap kenaikan suhu 9oC; sebagai teladan nilai k = 0.08 untuk tanah berpasir di Malang selatan.

Untuk mengimbangi kehilangan ini, harus ditambahklan bahan organik baru. Bahan organik segar seperti jerami, dedaunan, pupuk kandang mempunyai laju dekomposisi yang cukup tinggi daripada BOT. Dalam waktu 3-4 bulan bahan organik segar ini sudah berubah menjadi seperti BOT.

Rasio antara jumlah bahan organik yang tertinggal (masih ada) setelah periode waktu tersebut dengan jumlah bahan organik pada saat awal ditambahkan ke tanah disebut koefisien humifikasi. Bahan organik yang masih tertinggal tersebut dinamakan “bahan organik efektif”.

Kalau penambahan bahan organik efektif sama dengan dekomposisi bahan organik yang telah ada dalam tanah, maka kondisi setimbang telah tercapai, dimana:

h.X = k.Y

h = koefisien humifikasi, X = jumlah bahan organik segar yang ditambahkan, k = laju dekomposisi relatif, Y = jumlah bahan organik dalam tanah.

Dengan demikian dapat dihitung jumlah bahan organik yang diperlukan untuk mempertahankan kandungan bahan organik tanah pada tingkat tertentu.

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Kehilangan BOT

Pengaruh pengolahan tanahMost organic matter losses in soil occurred in the first decade or two after land was cultivated. Native levels of organic matter may not be possible under agriculture, but many farmers can increase the amount of active organic matter by reducing tillage and increasing organic inputs.

Sumber: http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html#do

Perubahan kandungan bahan organic tanah (MT/Ha) jangka panjang sangat dipenmgaruhi oleh pola pengelolaan tanahnya, aplikasi bahan organic dalam bentuk kompos, pupuk hijau, pupuk kandang atau mulsa seresah sisa panen ternyata dapat memperbaiki / memelihara kandungan bahan organic tanah.

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Perubahan kandungan BOT jangka panjang pada berbagai pola pengelolaan lahan (Sumber: http://www.agnet.org/library.php?func=view&id=20110808172707&type_id=4)

Pupuk Organik

Klasifikasi, sifat dan efek pupuk organik

Salah satu klasifikasi pupuk organik adalah:a. Limbah manusia dan pupuk kandangb. Sampah pemukiman, sudah atau belum dikomposkanc. Material bumi, seperti lumpur gambut, lumpur selokan/parit, dlld. Residu tumbuhan, bahan segar seperti pupuk hijau, limbah sayuran,

buah-buahan dan garden; bahan tua seperti kulit kakao, polong kacangtanah, gambut dll. Bahan mulsa dapat berupa material tumbuhan muda, tua, kering, tidak dibenamkan ke dalam tanah.

Pada umumnya penggunaan pupuk organik mempunyai dua tujuan pokok, yaitu (1) suplai unsur hara, dan (2) meningkatkan kandungan BOT.

Hanya denga

n pupuk kimia

Pengolahan tanah

konvensional tanpa pupuk

Dengan aplikasi

kompos dan pupuk

Sejarah

Pengelolaan

Prediksi masa depan

Baha

n or

gani

k ta

nah,

mt/

ha

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Pentingnya pupuk organik sebagai sumber hara ditentukan oleh kandungan hara dan laju pelepasan hara tersebut. Laju pelepasan hara ini tergantung pada resistensi bahan organik terhadap mikroba. Bahan tumbuhan yang masih hijau banyak mengandung sakarida dan protein, yang mudah didekomposisikan oleh mikroba. Bagian tanaman yang berkayu banyak mengandung selulose, hemi-selulose dan lignin, yang sukar didekomposisi. Faktor lain yang mempengaruhi laju mineralisasi bahan organik adalah kandungan nitrogennya.

Peningkatan kandungan BOT juga tergantung pada daya dekomposisi pupuk organik. Semakin mudah pupuk organik mengalami dekomposisi, maka yang tertinggal dalam tanah semakin sedikit. Dengan kata lain, dua macam tujuan utama penggunaan ppuk organik seperti tersebut di atas tidak mungkin dapat dicapai pada waktu yang bersamaan.

Peningkatan kandungan BOT akan berpengaruh terhadap:a. Peningkatan KTK, sehingga menurunkan laju pencucian kationb. Perbaikan struktur tanah. Individu agregat tanah menjadi lebih stabil

dan kohesi di antara partikel fraksi tanah menjadi lebih kuat. Sehingga kepekaan tanah terhadap erosi menjadi rendah, aerasi tanah menjadi lebih baik, dan akhirnya akar tanaman dapat menyerap ion lebih mudah.

c. Peningkatan WHC tanah. Ketersediaan air tanah menjadi lebih bagus, mobilitas hara lebih tinggi dan kadangkala lebih sedikit pencucian, karena tanah mampu menampung lebih banyak air sebelum terjadi perkolasi ke dalam subsoil.

d. Perbaikan kondisi pertumbuhan bagi mikroba tanah. e. Pengembangan cadangan hara, terutama N, P dan S.

Beberapa hasil penelitian membuktikan bahwa pupuk organik alami juga mengandung “senyawa aktif fisiologis” yang mampu merangsang pertumbuhan.

Pupuk hijau (green manure) adalah sejenis tanaman penutup tanah yang ditanam dengan tujuan utamanya untuk menambahkan unsure hara dan bahan organic ke tanah. Typically, a green manure crop is grown for a specific period of time , and then plowed under and incorporated into the soil while green or shortly after flowering. Green

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manure crops are commonly associated with organic agriculture, and are considered essential for annual cropping systems that wish to be sustainable. Traditionally, the practice of green manuring can be traced back to the fallow cycle of crop rotation, which was used to allow soils to recover. (http://en.wikipedia.org/wiki/Green_manure)

Pupuk hijau biasanya berfungsi ganda, yaitu perbaikan kualitas tanah dan perlindungan tanah:

Leguminous green manures such as clover and vetch contain nitrogen-fixing symbiotic bacteria in root nodules that fix atmospheric nitrogen in a form that plants can use.

Green manures increase the percentage of organic matter (biomass) in the soil, thereby improving water retention, aeration, and other soil characteristics.

The root systems of some varieties of green manure grow deep in the soil and bring up nutrient resources unavailable to shallower-rooted crops.

Fungsi tanaman penutup tanah untuk mengendalikan gulma, mencegah erosi tanah, dan meminimumkan pemadatan tanah, juga menjadi pertimbangan dalam memilih dan menggunakan pupuk hijau.

Some green manure crops, when allowed to flower, provide forage for pollinating insects.

Pengelolaan Seresah sisa panen

Seresah sisa panen tanaman kalau dikelola dengan baik akan mendatangkan manfaat ganda a.l.:

1. Menambah bahan organic tanah, which improves the quality of the seedbed and increases the water infiltration and retention capacity of the soil, buffers the pH and facilitates the availability of nutrients;

2. Menyimpan karbon dalam tanah;3. provide nutrients for soil biological activity and plant uptake;4. capture the rainfall on the surface and thus increase infiltration and

the soil moisture content;5. provide a cover to protect the soil from being eroded;6. Mengurangi penguapan air dan “pengeringan” dari permukaan

tanah.

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MULSA ORGANIK

Mulsa adalah penutup protektif yang diletakkan di permukaan tanah untuk menahan lengas-tanah, mereduksi erosi, menyediakan hara, dan menekan pertumbuhan gulma serta perkecambahan biji.

Mulsa organic mengalami dekomposisi dengan waktu, sehingga sifat kemulsaan-nya "sementara”. Ada kepercayaan bahwa mulsa organik berpengaruh negative terhadap pertumbuhan tanaman, pada saat ia mengalami dekomposisiyang cepat oleh bakteri dan fungi, kebutuhan nitroghennya diambil dari N-tanah yang tersedia di sekitarnya.

Mulsa organic biasanya berupa (Louise; Bush-Brown, James (1996), America's garden book, New York: Macmillan USA, pp. 768):

1. Leaves from deciduous trees, which drop their foliage in the fall. They tend to be dry and blow around in the wind, so are often chopped or shredded before application. As they decompose they adhere to each other but also allow water and moisture to seep down to the soil surface. Thick layers of entire leaves, especially of Maples and Oaks, can form a soggy mat in winter and spring which can impede the new growth lawn grass and other plants. Dry leaves are used as winter mulches to protect plants from freezing and thawing in areas with cold winters, they are normally removed during spring.

2. Grass clippings, from mowed lawns are sometimes collected and used elsewhere as mulch. Grass clippings are dense and tend to mat down, so are mixed with tree leaves or rough compost to provide aeration and to facilitate their decomposition without smelly putrefaction. Rotting fresh grass clippings can damage plants; their rotting often produces a damaging buildup of trapped heat. Grass clippings are often dried thoroughly before application, which mediates against rapid decomposition and excessive heat generation. Fresh green grass clippings are relatively high in nitrate content, and when used as a mulch, much of the nitrate is returned to the soil, but the routine removal of grass clippings from the lawn results in nitrogen deficiency for the lawn.

3. Peat moss, or sphagnum peat, is long lasting and packaged, making it convenient and popular as a mulch. When wetted and dried, it can form a dense crust that does not allow water to soak in. When dry it can also burn, producing a smoldering fire. It is sometimes mixed with pine needles to produce a mulch that is friable. It can also lower the pH of the soil surface, making it useful as a mulch under acid loving plants.

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4. Wood chips are a byproduct of the pruning of trees by arborists, utilities and parks; they are used to dispose of bulky waste. Tree branches and large stems are rather coarse after chipping and tend to be used as a mulch at least three inches thick. The chips are used to conserve soil moisture, moderate soil temperature and suppress weed growth. The decay of freshly produced chips from recently living woody plants, consumes nitrate; this is often off set with a light application of a high-nitrate fertilizer. Wood chips are most often used under trees and shrubs. When used around soft stemmed plants, an unmulched zone is left around the plant stems to prevent stem rot or other possible diseases. They are often used to mulch trails, because they are readily produced with little additional cost outside of the normal disposal cost of tree maintenance.

5. Woodchip Mulch is a byproduct of reprocessing used (untreated) timber (usually packaging pallets), to dispose of wood waste by creating Woodchip Mulch. The chips are used to conserve soil moisture, moderate soil temperature and suppress weed growth. Woodchip Mulch is often used under trees, shrubs or large planting areas and can last much longer than arborist mulch. Woodchips can also be reprocessed into playground woodchip to be used as a impact-attenuating playground surfacing.

6. Bark chips, of various grades are produced from the outer corky bark layer of timber trees. Sizes vary from thin shredded strands to large coarse blocks. The finer types are very attractive but have a large exposed surface area that leads to quicker decay. Layers two or three inches deep are usually used, bark is relativity inert and its decay does not demand soil nitrates.

7. Straw mulch or field hay or salt hay are lightweight and normally sold in compressed bales. They have an unkempt look and are used in vegetable gardens and as a winter covering. They are biodegradable and neutral in pH. They have good moisture retention and weed controlling properties but also are more likely to be contaminated with weed seeds. Salt hay is less likely to have weed seeds than field hay.

8. Cardboard or newspaper can be used as mulches. These are best used as a base layer upon which a heavier mulch such as compost is placed to prevent the lighter cardboard/newspaper layer from blowing away. By incorporating a layer of cardboard/newspaper into a mulch, the quantity of heavier mulch can be reduced, whilst improving the weed suppressant and moisture retaining properties of the mulch (Patrick Whitefield, 2004, The Earth Care Manual, Permanent Publications). However, additional labour is expended when planting through a mulch containing a cardboard/newspaper layer, as holes must be cut for each plant. Sowing seed through mulches containing a cardboard/newspaper layer is impractical. Application of newspaper mulch in windy weather can be facilitated by briefly pre-soaking the newspaper in water to increase its weight.

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BAHAN BACAAN

Alexandra Bot and José Benites. 2005. The importance of soil organic matter. Key to drought-resistant soil and sustained food and production. FAO SOILS BULLETIN 80. FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS, Rome, 2005.

Amado, T.J.C., S.B.Fernandez and J. Mielniczuk. 1998. Nitrogen availability as affected by ten years of cover crop and tillage systems in southern Brazil. J. Soil Wat. Con., 53(3): 268–271.

Anderson, J.M. and P. Flanagan. 1989. Biological processes regulating organic matter dynamics in tropical soils. In D.C. Coleman, J.M. Oades & G. Uheara, eds. Dynamics of soil organic matter in tropical ecosystems, pp. 97–125. Niftal Project. University of Hawaii, USA.

Baigorri R, Fuentes M, González-Gaitano G, García-Mina JM, Almendros G, F.J. González-Vila. 2009. "Complementary Multianalytical Approach To Study the Distinctive Structural Features of the Main Humic Fractions in Solution: Gray Humic Acid, Brown Humic Acid, and Fulvic Acid". J Agric Food Chem. 57 (8): 3266–72. doi:10.1021/jf8035353. PMID 19281175.

Bauer, A. and A.L. Black. 1994. Quantification of the effect of soil organic matter content on soil productivity. Am. J. Soil Sci. Soc., 5: 185-193.

Bell, M.J., Moody, P.W., Connolly, R.D. and B.J. Bridge. 1998. The role of active fractions of soil organic matter in physical and chemical fertility of Ferrosols. Aust. J. Soil Res., 36: 809-819.

Bell, M.J., Moody, P.W., Yo, S.A. and R.D.Connolly. 1999. Using active fractions of soil organic matter as indicators of the sustainability of Ferrosol farming systems. Aust. J. Soil Res., 37: 279-287.

Berg, B., McClaugherty, C., 2007. Plant litter: decomposition, humus formation, carbon sequestration, 2nd ed. Springer, 338 pp., ISBN 3540749225

Bessam, F. and R. Mrabet. 2003. Long-term changes in particulate organic matter under no-tillage systems in a semiarid soil of Morocco. In: Proc. 16th ISTRO Conference, pp. 144-149. 13-18 July 2003, Brisbane, Australia.

Blair, G.J., Lefroy, R.D. and L. Lisle. 1995. Soil carbon fractions based on their degree of oxidation, and the development of a carbon management index for agricultural systems. Aust. J. Soil Res., 46: 1459-1466.

Blair, N. and G.J.Crocker. 2000. Crop rotation effects on soil carbon and physical fertility of two Australian soils. Aust. J. Soil Res., 38: 71-84.

Blair, N., Faulkner, R.D., Till, A.R. and K.E. Prince. 2003. The impact of plant residues with different breakdown rates on soil carbon and soil

32

structure. In: Proc. 16th ISTRO Conference, pp. 175-181. 13-18 July 2003, Brisbane, Australia.

Brammer, H. 2000. Ploughpans and tillage problems. In: Agroecological aspects of agricultural research in Bangladesh, pp. 151-158. Dhaka, UPL.

Brussaard, L. 1994. Interrelationships between biological activities, soil properties and soil management. In D.J. Greenland & I. Szabolcs, eds. Soil resilience and sustainable land use, pp. 309-329. Wallingford, UK, CAB International.

Brussaard, L. and N.G. Juma. 1995. Organisms and humus in soils. In A. Piccolo, ed. Humic substances in terrestrial ecosystems, pp. 329-359. Amsterdam, The Netherlands, Elsevier.

Caesar-Tonthat, T.C., 2002. Soil binding properties of mucilage produced by a basidiomycete fungus in a model system. Mycological Research 106:930–937.

Calegari, A. 1998. The effects of winter green manure and no-tillage on soil chemical properties and maize yield.

Calegari, A. and I. Alexander. 1998. The effects of tillage and cover crops on some chemical properties of an oxisol and summer crop yields in south-western Paraná, Brazil. Adv. Geo. Ecol., 31: 1239–1246.

Calegari, A., Darolt, M.R. and M.Ferro. 1998. Towards sustainable agriculture with a no-tillage system. Adv. Geo. Ecol., 31: 1205-1209.

Chan, K.Y., Heenan, D.P., Oates, A. & Munro, K. 2003. Hydraulic properties of an Alfisol with pre-existing pan under conservation tillage - effect of earthworms. In: Proc. 16th ISTRO Conference, pp. 300-304. 13-18 July 2003, Brisbane, Australia.

Coulombe, C.E., Dixon, J.B. & Wilding, L.P. 1996. Mineralogy and chemistry of Vertisols. In N. Ahmad & A. Mermut, eds. Vertisols and technologies for their management, pp. 115-200. Developments in Soil Science 24. Amsterdam, The Netherlands, Elsevier.

Curry, J.P. and J.A. Good. 1992. Soil faunal degradation and restoration. Adv. Soil Sci., 17: 171-215.

Dalal, R.C. and R.J. Mayer. 1986. Long-term trends in fertility of soils under continuous cultivation and cereal cropping in Southern Queensland. II. Total organic carbon and its rate of loss from the soil profile. Aust. J. Soil Res., 24, 281-292.

Davidson, E.A. and I.L. Ackerman. 1993. Changes in soil carbon inventories following cultivation of previously untilled soil. Biogeochemistry, 20: 161-193.

33

Debarba, L. and T.J.C.Amado. 1997. Desenvolvimento de sistemas de produção de milho no sul do brasil com características de sustentabilidade. Rev. Bras. Ciên. Solo, 21: 473-480.

Derpsch, R. 1993. Sistema de Plantio Direto em Residuos de Adubos Verdes em Pequenas Propriedades no Paraguai - Desenvolvimento e Difusão. In: I Encontro Latino Americano sobre Plantio Direto na Pequena Propriedade, pp. 375-386. Ponta Grossa. Anais.

Derpsch, R. 1997. Importancia de la siembra directa para obtener la sustentabilidad de la producción agricola. In: Agricultura sustentable de alta producción, ya! pp. 153-176. 5o Congreso Nacional de AAPRESID, Mar del Plata, Argentina.

Douglas, M.G., Mughogho, S.K., Saka, A.R., Shaxson, T.F. & Evers, G. 1999. Report on an investigation into the presence of a cultivation hoe pan under smallholder farming conditions in Malawi. Investment Centre Division FAO/World Bank Cooperative Programme. Washington, DC, World Bank.

Duxbury, J.M., Smith, M.S. & Doran, J.W. 1989. Soil organic matter as a source and sink of plant nutrients. In D.C. Coleman, J.M. Oades & G. Uehara, eds. Dynamics of soil organic matter in tropical ecosystem, pp. 33-67. USA, University of Hawaii Press.

Duxbury, J.M., Smith, M.S. and J.W. Doran. 1989. Soil organic matter as a source and sink of plant nutrients. In D.C. Coleman, J.M. Oades & G. Uehara, eds. Dynamics of soil organic matter in tropical ecosystem, pp. 33–67. USA, University of Hawaii Press.

Elliot, L.F. and J.M.Lynch. 1984. The effect of available carbon and nitrogen in straw on soil and ash aggregation and acetic acid production. Plant Soil, 78: 335-343.

Elliot, L.F. and J.M.Lynch. 1994. Biodiversity and soil resilience. In J. Greenland & I. Szabolcs, eds. Soil resilience and sustainable land use, pp. 353-364. Wallingford, UK, CAB International.

Elo, S., Maunuksela, L., Salkinoja-Salonen, M., Smolander, A., Haahtela, K., 2006. Humus bacteria of Norway spruce stands: plant growth promoting properties and birch, red fescue and alder colonizing capacity. FEMS Microbiology Ecology 31:143–152.

Eyheraguibel, B., Silvestrea, J. Morard, P., 2008. Effects of humic substances derived from organic waste enhancement on the growth and mineral nutrition of maize. Bioresource Technology 99:4206–4212.

FAO. 1984. Tillage systems for soil and water conservation. FAO Soils Bulletin No. 54. Rome.

34

FAO. 1989. Soil conservation for small farmers in the humid tropics. FAO Soils Bulletin No. 60. Rome.

FAO. 1993. Soil tillage in Africa: needs and challenges. FAO Soils Bulletin No. 69. Rome.

FAO. 1994. Land husbandry. Components and strategy. FAO Soils Bulletin No. 70. Rome.

FAO. 1995. Tillage systems in the tropics. Management options and sustainability implications. FAO Soils Bulletin No. 71. Rome.

FAO. 2000. Fertilizers and their use. A pocket guide for extension officers. Fourth edition. FAO/IFA. 70 pp.

FAO. 2001. Conservation agriculture. Case studies in Latin America and Africa. FAO Soils Bulletin No. 78. Rome. 69 pp.

Ferreira, M.C., Andrade, D.S., Chueire, L.M.O., Takemura, M. & Hungria, M. 2000. Tillage method and crop rotation effects on the population sizes and diversity of bradyrhizobia nodulating soybean. Soil Biol. Biochem., 32: 627-637.

Fiorentino G., Spaccini R., A.Piccolo. 2006. "Separation of molecular constituents from a humic acid by solid-phase extraction following a transesterification reaction". Talanta 68 (4): 1135–1142.

Ghabbour, E.A. dan G.Davies. 2001. Humic Substances: Structures, Models and Functions. Cambridge, U.K.: RSC publishing. ISBN 9780854048113..

Gregorich, E.G., Greer, K.J., Anderson, D.W. & Liang, D.C. 1998. Carbon distribution and losses: erosion and deposition effects. Soil Till. Res., 47: 291-302.

Gupta, V.V.S.R. and J.J. Germida. 1988. Distribution of microbial biomass and its activity in different soil aggregate size classes as affected by cultivation. Soil Biol. Biochem., 20: 777-786.

Hargitai, L., 1993. The soil of organic matter content and humus quality in the maintenance of soil fertility and in environmental protection. Landscape and Urban Planning 27:161–167.

Hessen, D.O. dan L.J. Tranvik. 1998. Aquatic humic substances : ecology and biogeochemistry. Berlin: Springer. ISBN 3540639101.

Holland, E.A. and D.C.Coleman. 1987. Litter placement effects on microbial and organic matter dynamics in an agro-ecosystem. Ecology, 68: 425-433.

Huang, D.L., Zeng, G.M., Feng, C.L., Hu, S., Jiang, X.Y., Tang, L., Su, F.F., Zhang, Y., Zeng, W., Liu, H.L., 2008. Degradation of lead-contaminated lignocellulosic waste by Phanerochaete chrysosporium and the reduction of lead toxicity. Environmental Science and Technology 42:4946–4951.

35

Hudson, B.D. 1994. Soil organic matter and available water capacity. J. Soil Wat. Con., 49(2): 189-194.

Izaurralde, R.C. and C.C.Cerri. 2002. Organic matter management. In: Encyclopedia of soil science, pp. 910-916. New York, USA, Marcel Dekker Inc.

Juo, A.S.R. and R.Lal. 1977. The effect of fallow and continuous cultivation on physical and chemical properties of an Alfisol in western Nigeria. Plant & Soil, 47: 507-584.

Kikuchi, R., 2004. Deacidification effect of the litter layer on forest soil during snowmelt runoff: laboratory experiment and its basic formularization for simulation modeling. Chemosphere 54:1163–1169.

Ladd, J.N. and M. Amato. 1985. Nitrogen cycling in legume cereal rotations. In B.T. Kang & J. Van der Heide, eds. Nitrogen management in farming systems in humid and sub-humid tropics, pp. 105-127. Haren, The Netherlands, Institute for Soil Fertility (IB), and Ibadan, Nigeria, International Institute for Tropical Agriculture.

Lavelle, P. and A. Spain. 2001. Soil ecology. Dordrecht, The Netherlands, Kluwer Academic Publishers.

Lavelle, P. and B. Pashanasi. 1989. Soil macrofauna and land management in Peruvian Amazonia (Yurimaguas, Loreto). Pedobiologia, 33: 283-291.

Lavelle, P., Blanchart, E., Martin, A., Martin, S., Spain, A.V., Toutain, F., Barois, I. and R. Schaefer. 1993. A hierarchical model for decomposition in terrestrial ecosystems: application to soils of the humid tropics. Biotropica, 25(2): 130–150.

Lefroy, R.D.B., Blair, G.J. and W.M. Strong. 1993. Changes in soil organic matter with cropping as measured by organic matter fractions and 13 C natural isotope abundance. Plant Soil, 155/156: 399-402.

Linn, D.M. and J.W. Doran. 1984. Effect of water-filled pore space carbon dioxide and nitrous oxide production in tilled and nontilled soils. Soil Sci. Soc. Am. J., 48: 1267-1272.

Matson, P.A., Parton, W.J., Power, A.G. and M.J.Swift. 1997. Agricultural intensification and ecosystem properties. Science, 277: 504-509.

Mrabet, R. 2000. Long-term no tillage influence on soil quality and wheat production in semiarid Morocco. Paper presented at the 15th ISTRO Conference, USA, 2-7 July 2000.

Olness, A., Archer, D., 2005. Effect of organic carbon on available water in soil. Soil Science 170:90–101

Palm, C.A. and P.A.Sanchez. 1990. Decomposition and nutrient release patterns of the leaves of three tropical legumes. Biotropica, 22: 330-338.

36

Paustian, K. 2002. Organic matter and global cycle. In: Encyclopedia of soil science, pp. 895-898. New York, USA, Marcel Dekker Inc.

Piccolo, A. 2002. "The Supramolecular structure of humic substances. A novel understanding of humus chemistry and implications in soil science". Advances in Agronomy. Advances in Agronomy 75: 57–134.

Pieri, C., Evers, G., Landers, J., O’Connel, P. & Terry, E. 2002. No till farming for sustainable rural development. Agriculture and Rural Development Working Paper. Washington, DC, World Bank. 65 pp.

Reicosky, D.C. 2005. Alternatives to mitigate the greenhouse effect: emission control by carbon sequestration. In: Simpósio sobre Plantio direto e Meio ambiente; Seqüestro de carbono e qualidade da agua, pp. 20-28. Anais. Foz do Iguaçu, 18-20 de Maio 2005.

Rice, C.W. 2002. Organic matter and nutrient dynamics. In: Encyclopedia of soil science, pp. 925-928. New York, USA, Marcel Dekker Inc.

Satchell, J.E. 1971. Feasibility study of an energy budget for Meathop Wood. In P. Duvigneaud, ed. Productivité des Ecosystèmes Forestiers, pp. 619–630. Paris, UNESCO.

Schnitzer, M. 1986. The synthesis, chemical structure, reactions and functions of humic substances. In R.G. Burns, G. dell’Agnola, S. Miele, S. Nardi, G. Savoini, M. Schnitzer, P. Sequi, D. Vaughan & S.A. Visser, eds. Humic substances: effect on soil and plants. Congress on Humic Substances. March 1986, Milan, Italy.

Stevenson, F.J. 1994. Humus chemistry. genesis, composition, reactions. 2nd edition. New York, USA, Wiley Interscience. 512 pp.

Stevenson, F.J. 1994. Humus Chemistry: Genesis, Composition, Reactions. John Wiley & Sons, New York.

Swift, M.J., Heal, O.W. & Anderson, J.M. 1979. Decomposition in terrestrial ecosystems. Oxford, UK, Blackwell Scientific Publications.

Swift, M.J., Heal, O.W. and J.M. Anderson. 1979. Decomposition in terrestrial ecosystems. Oxford, UK, Blackwell Scientific Publications.

Szalay, A., 1964. Cation exchange properties of humic acids and their importance in the geochemical enrichment of UO2++ and other cations. Geochimica et Cosmochimica Acta 28:1605–1614.

Tan, K.H. and A. Binger. 1986. Effect of humic acid on aluminium toxicity in corn plants. Soil Sci., 14: 20-25.

Tate, R.L. 1987. Soil organic matter: biological and ecological effects. New York, USA, John Wiley & Sons. 291 pp.

37

Theng, B.K.G. 1987. Clay-humic interactions and soil aggregate stability. In P. Rengasamy, ed. Soil structure and aggregate stability, pp. 32-73. Proc. Institute of Irrigation and Salinity Research. Tatura, Australia.

Tipping, E. 1994. "'WHAM - a chemical equilibrium model and computer code for waters, sediments, and soils incorporating a discrete site/electrostatic model of ion-binding by humic substances". Computers and Geosciences 20 (6): 973–1023.

Unger, P.W. 1978. Straw-mulch rate effect on soil water storage and sorghum yield. Soil Sci. Soc. Am. J., 42: 486-491.

Vreeken-Buijs, M.J., Hassink, J., Brussaard, L., 1998. Relationships of soil microarthropod biomass with organic matter and pore size distribution in soils under different land use. Soil Biology and Biochemistry 30:97–106.