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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 welldecomposed 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-organicmatter-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 tanaman
3. Mengubah agregasi tanah
4. 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.
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Ketersediaan Lengas Tanah



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
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
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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 humus
1. 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 mikroorganisme 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
1
0
1
0
0
1
1
Respirasi
C
N
P
S
2
0
0
Mineralisasi
C
2
0
2
2
N
P
S
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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
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:
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1/10
x
N=1/10 C
50/100 x 10-2 x
C/BOT
2.5x106 x
2x10-2
=
25 kg N
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.
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.
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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.
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
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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
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.
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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)
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).
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Tabel 1. Kapasitas retensi kation dari beberapa bahan organik tanah,
dengan larutan pencucian yang berbeda
Sumber bahan organik
Tanah hutan pinus
Lempung debu Honeoye
Lempung debu Ontario
Lempung debu Yates
Lempung
liat
berdebu
Dunkirk
Tanah hutan Sequoia
Kation yang ditahan dengan larutan pencucian
(me per 100 g)
BaCu-asetat
BaK-asetat
hidroksida
pH 5
asetat
pH 5
pH 5
533
410
155
139
295
306
146
60
309
278
125
54
301
278
124
43
275
270
135
64
286
181
Sumber: Broadbent, 1955, dalam Allison, 1973.
118
42
19
O
OH
O
C
OH
C
O
O
C
C – O-
+ Cu++
O
+
H+
OH
Cu
O
O
OH
O
C
O
Cu
C
O
O
C
C
+ Cu++
O
Gambar 1.
+ H+
O
Pembentukan khelate antara Cu dan gugusan karboksil /
fenolat; dan antara Cu dengan dua gugusan karboksilat
(Schnitzer dan Kahn, 1972),
20
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
actinomycetes
2. Mikroba menghasilkan produk metabolik, terutama karbohidrat yang
menjadi perekat yang mengikat bersama partikel tanah
3. 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.
21
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.
22
(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.
23
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.
24
Kehilangan BOT
Pengaruh pengolahan tanah
Most 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.
25
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 kandang
b. Sampah pemukiman, sudah atau belum dikomposkan
c. Material bumi, seperti lumpur gambut, lumpur selokan/parit, dll
d. 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.
Pentingnya pupuk organik sebagai
sumber hara ditentukan oleh
kandungan hara dan laju pelepasan hara tersebut. Laju pelepasan hara
26
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 kation
b. 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
manure crops are commonly associated with organic agriculture, and
are considered essential for annual cropping systems that wish to be
27
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.
2.
3.
4.
5.
6.
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;
Menyimpan karbon dalam tanah;
provide nutrients for soil biological activity and plant uptake;
capture the rainfall on the surface and thus increase infiltration and
the soil moisture content;
provide a cover to protect the soil from being eroded;
Mengurangi penguapan air dan “pengeringan” dari permukaan
tanah.
28
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.
2.
3.
4.
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.
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.
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.
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
29
5.
6.
7.
8.
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.
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.
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.
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.
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.
30
31
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