BAB 1. SIFAT-CIRI TANAH
• Tanah adalah selapis tipis bahan yang menutup permukaan bumi.
• Tanah terdiri dari bahan anorganik & organik, air, udara, dan organisme hidup.
• Lapis tanah berupa horizon: topsoil, subsoil, dan bahan induk → secara ilmiah:
horizon A, B, & C (Gambar 1).
• Karena horizon A berada paling atas, disebut topsoil, dalam beberapa hal tidak
tepat.
• Topsoil atau permukaan tanah merupakan bagian yang diolah.
• Lahan olah yang kurang lebih kedalaman sama disebut horizon olah (Ap), juga
disebut topsoil.
Gambar 1. Profil tanah olah. Seluruh horizon tidak selalu tampak pd semua tanah.
Horizon transisi (A3, B1, dan B3) dll tidak digambar dan boleh jadi ada pada tanah
tertentu.
Pd beberapa tanah, horizon A tererosi memunculkan horizon B sebagai zona olah dan
topsoil.
Pd tanah hutan, lapis atas tertutup seresah organik , disebut horizon O atau horizon Ao.
Horizon A tipik adalah zona di mana materi tercuci dan terakumulasi di horizon B.
Bahan induk adalah asal terbentuknya tanah.
Bahan induk dapat berupa bahan anorganik: deposit batuan terbawa glasier, angin, air,
atau gaya gravitasi, dan bahan organik : sisa organisme hidup yg mati, atau gabungan
keduanya.
Petani dan pekebun memandang tanah sebagai habitat bagi tanaman.
Tanaman memperoleh air dan hara dari tanah. Semua nutrisi dijumpai di zona mulai 3
inci (5 cm) hingga 10 inci (15 cm) di bawah permukaan tanah (Gambar 2).
Air diserap di lapisan tersebut atau lebih dalam.
Tanah menyediakan udara dan jangkar akar agar tanaman dapat tegak.
Gambar 2. Ilustrasi pola perakaran dalam tanah. Tanaman semak memperoleh
semua nutrisi dari zona kedalaman tanah 3 hingga 10 inci (5 – 15 cm). Di bawah
kedalaman 10 inci (15 cm), fungsi utama akar adalah menyerap air.
Bahan Anorganik
Bahan anorganik tanah berasal dari berbagai fase pelapukan batuan mineral;
merupakan komponen utama hampir semua jenis tanah.
Tanah-tanah yg mengandung > 80% bahan anorganik adalah tanah mineral. Berdasar
berat kering, kebanyakan tanah upland berbahan anorganik > 95%.
Bila tanah-tanah terbentuk dari bahan induk atau mineral yang sama dan partikel
anorganik hanya berbeda ukuran, perbedaan prinsip di antara tanah yaitu sifat fisik.
Bila tanah-tanah terbentuk dari bahan induk atau mineral tidak sama, misalnya yang
satu dari batu pasir dan yang lain batu kapur,; ukuran partikelnya juga berbeda, maka
keduanya berbeda baik sifat fisik maupun kimia.
Sifat kimia dan fisik keduanya menentukan nilai kesuburan tanah.
Kesuburan tanah merupakan faktor pertimbangan terhadap sifat fisik: kapasitas
pegang air, aerasi, drainase, kedalaman dan struktur jelajah akar; dan sifat kimia:
kemasaman dan suplai unsur hara.
Partikel anorganik tanah adalah: pasir, debu, dan liat.
Ketiga grup partikel menentukan pemisahan atau gabungan kehalusan tanah.
Perbedaan fisik antara pasir, debu, dan liat adalah ukuran partikel. Selain ukuran,
partikel juga berbeda dalam bentuk (Gambar 3).
Di tanah pertanian (Tabel 1), diameter rata-rata partikel pasir berkisar antara 2 hingga
0.05 mm, lolos melallui ayakan berdiameter antara 0.05 hingga 2 mm.
Ukuran debu berkisar dari diameter 0.002 hingga 0.05 mm, dan liat lebih kecil dari
0.002 mm. Butir lebih besar dari 2 mm adalah kerikil (2 mm hingga 75 mm), koral,
stones, flags, dan bolders.
Tanah mengandung fragmen besar disebut sebagai gravelly sands, stoney clays, dan
lain-lain.
Tabel 1. Batas ukuran partikel dan beberapa fragmen mineral dalam tanah
Pemisahan pragmen
Kisaran, mm
Pragmen kasar
Boulders
>600
Stonesy
>250 to 600
Cobblesy
>75 to 250
Gravel
>2 to 75
Tanah halus
Pasir
Pasir sangat kasar
>1.0 to 2.0
Pasir kasar
>0.5 to 1.0
Pasir sedang
>0.25 to 0.5
Pasir halus
>0.10 to 0.25
Pasir sangat halus
>0.05 to 0.10
Debu
>0.002 to 0.05
Liat
<0.02
Gambar 3. Ilustrasi ukuran dan bentuk partikel tanah tidak
beraturan, penyelimutan partikel menjadi agregat (ped).
Tanah berbeda dalam ukuran dan imbangan relatif partikel fraksi anorganik
pada top soil (horizon A atau horizon mineral teratas) disebut pembeda
tekstur .
Tekstur adalah sifat dasar, mendekati sifat tanah permanen.
Ia tidak dapat diubah melalui praktek sehari-hari, seperti pengolahan; namun
demikian erosion, deposit, atau pencampuran lapisan dapat mengubah
tekstur di zona permukaan.
Karena tekstur berdasar hanya pada fraksi anorganik, jumlah bahan organik
dalam tanah tidak berpengaruh terhadap tekstur.
Struktur tanah berkaitan dengan penyusunan partikel tanah ke dalam grup
agregat-agregat , bongkah, atau gabungan partikel-partikel.
Struktur tanah tidak bersifat permanen dan dapat dipengaruhi oleh
pengelolaan dan oleh bahan organik.
Sifat tanah tidak hanya terdiri dari satu jenis partikel saja,
melainkan campuran antara pasir, debu, dan liat.
Berdasar pada perbandingan relatif fraksi pasir, debu, dan liat
yang ada pada top soil; tanah dikelaskan dalam tiga grup: tanah
berpasir, berlempung, dan berliat (Table 2).
Bila pasir dominan maka terbentuk tekstur kasar atau ringan.
Ringan artinya mudah diolah dan tidak berat.
Bila tanah didominasi debu, lait, atau keduanya maka tekstur
berat dan sukar diolah.
Tanah berlempung tekstur tanah bertekstur sedang, bila terasa
fraksi pasir, debu, dan liat imbang.
Tabel 2. Kelas tekstur tanah berdasar proporsi relatif antara pasir, debu, dan liat
Berpasir
Ringan, Kasar
Pasir
Pasir berlempung
Berlempung
Sedang, antara kasar hingga halus
Lempung berpasir
Lempung berpasir halus
Lempung berpasir sangat halus
Lempung
Lempung berdebu
Debu
Lempung berliat
Lempung liat berpasir
Lempung liat berpasir
Berliat
Berat, halus
Liat berpasir
Liat berdebu
Liat
Pasir, debu, atau liat tidak berpengaruh sama terhadap
penentuan kelas tekstur spesifik (Gambar 4).
Tanah masuk ke kelas tekstur spesifik pasir bila bobot fraksi
pasir sekitar 85%. Kelas tekstur modifikasi pasir adalah 50%
atau lebih fraksi pasir.
Tanah kelas tekstur spesifik debu bila fraksi debu sekitar 80%,
sedang dikatakan kelas debu atau berdebu bila fraksi debu 40%
atau lebih.
Di pihak lain, kelas tekstur spesifik liat, fraksi liat tidak lebih
dari 40%, dan pada beberapa tanah, sebagai contoh, lempung
liat berpasir, fraksi liat tidak lebih dari 20%.
Liat berpengaruh jauh lebih dominan terhadap kelas tekstur
tanah (sifat fisik tanah) daripada pasir dan debu.
Gambar 4. Petunjuk klasifikasi tekstur tanah (USDA), berdasar persentase bobot
masa fraksi pasir, debu, dan liat horizon-A
Di laboratorium, kelas tekstur tanah ditetapkan melalui
analisis mekanik, di mana pemisahan fraksi diukur secara
kuantitatif berdasar bobot fraksi.
Di lapangan, kelas tekstur tanah ditetapkan melalui perasaan
dengan jari; akurasinya perlu kemampuan dan pengalaman,
khusus bagi tanah-tanah bervariasi tekstur spesifik mencapai
14 kelas (Tabel 2).
Umumnya, kebanyakan orang tidak punya kemampuan
melakukan analisis mekanik dan jarang mampu menentukan
tekstur tanah di lapangan.
Definisi acuan tentang tekstur tanah berdasar perasaan (Soil
Survey Staff, USDA) disajikan pada Tabel 3.
Tabel 3. Penentuan kelas tekstur tanah dengan cara perasaan menggunakan jari tangan (Soil
Survey Staff. U.S. Dept. of Agriculture, l937. Soil Survey Manual. USDA Handbook No. l8,
Washington D. C.).
PETUNJUK UNTUK MENENTUKAN KELAS DASAR TEKSTUR
Tekstur lapangan ditentukan dengan cara piridan di antara telunjuk dan ibu jari tangan. Pernyataan berikut memberikan petunjuk grade
sifat dasar tekstur:
PASIR. Sand is loose and single-grained. The individual grains can readily be seen or felt. Single particles may appear shiny. Squeezed in
the hand when dry, sand will fall apart when the pressure is released. Squeezed when moist, it will form a cast, but will crumble when
touched.
LEMPUNG BERPASIR. A sandy loam much sand but has enough silt and clay to make it somewhat coherent. The individual sand grains
can be seen and felt. Squeezed when dry, a sandy loam will form a cast which will readily fall apart, but if squeezed when moist a cast can
be formed that will bear careful handling without breaking.
LEMPUNG. A loam feels to have a relatively even mixture of sand, silt, and clay. It is mellow with a somewhat gritty feel, yet fairly smooth
and slightly plastic. Squeezed when dry, loam will form a cast that will bear careful handling, while the cast formed by squeezing the
moist soil can be handled quite freely without breaking.
LEMPUNG BERDEBU. When dry, silt loam may appear cloddy, but the lumps break readily. When pulverized, it feels soft and floury. When
wet, the soil readily runs together. Either dry or wet it will form casts that can be handled without breaking, but when moistened and
squeezed between thumb and finger it will not "ribbon" but will give a broken appearance.
LEMPUNG BERLIAT. A clay loam breaks into clods or lumps that are hard when dry. When the moist soil is pinched between the thumb
and finger, it will form a thin ribbon which breaks readily, barely sustaining its own weight. The moist soil is plastic and will form a cast
that will bear much handling. When kneaded in the hand it does not crumble readily but tends to work into a heavy mass.
LIAT. A clay forms very hard lumps or clods when dry and is quite plastic and usually sticky when wet. When the moist soil is pinched out
between the thumb and fingers it will form a long, flexible ribbon.
Tabel 4. Sifat fisik tanah yang ditentukan oleh kelas tekstur
RESPON BERKAITAN DENGAN KELAS TEKSTUR TANAH
Perubahan tekstur dari:
Pasir --------------------------------------> Lempung -------------------------------> Liat
KASAR --------------------------------->
ke
-----------------------------------> HALUS
Berat jenis partikel ---------------------------------------------------------------> Sama
Luas permukaan dalam (internal) ----------------------------------------------------> Meningkat
Ruang pori total ------------------------------------------------------------> Meningkat
Berat jenis ------------------------------------------------------------------> Menurun
Kapasitas pegang air -------------------------------------------------------> Meningkat
Permeabilitas (aerasi, drainase) ----------------------------------------------> Menurun
Kehilangan pencucian --------------------------------------------------------------> Menurun
Pembentukan agregat -----------------------------------------------------> Meningkat
Kapasitas Tukar Kation ----------------------------------------------------> Meningkat
The pore space in fine-textured soils has many small pores or capillaries.
Movement of air and water is slow in these soils. Coating of the particles and filling of
pores with water restricts diffusion of air; consequently, clayey and silty soils often are
poorly drained and poorly aerated.
Poorly drained soils when wet will not provide adequate oxygen to roots.
On the other hand, fine-textured soils have good water-holding capacity because
water is retained in the capillaries.
Much of the water in fine-textured soils may not be available to plants because it is
held tightly to the surface of clay particles.
Partly because of the restricted downward movement of water, leaching of nutrients
from the root zone is a minor problem in fine-textured soils.
The surface area of soils increases as the texture becomes finer.
The total surface area of a unit weight of colloidal clay, depending on the type of clay,
may be about 10,000 times that of the finest sand and about 1,000 times that of the
finest silt.
Adsorption, the taking up of gases, liquids, or dissolved solids by surfaces, increases as
the surface area increases. Fine-textured soils are much more active in adsorbing water
and dissolved nutrients than coarse-textured soils.
This added retention of water and nutrients enhances the fertility of fine-textured soils
relative to that of coarse-textured soils.
As a guideline for the relative proportions of solid matter and pore space in soil, a
loamy soil is about 50% solid matter and about 50% pore space by volume.
At field capacity level of water retention, about half of the pore space is filled with
water, and half is filled with air (Figure 5).
Sandy soils will have more total volume occupied by solid matter, and clayey soils will
have more total volume occupied by pore space than loamy soils. In the sandy soil,
more of the pore space will be occupied by air than by water, whereas the converse will
occur in clayey soils.
25% Air-filled pores
50 % solid matter including:
25% Water-filled pores
Mineral matter and organic matter
Figure 5. Illustration of relative volume of solid particles (mineral and organic matter), air-filled pores, and water filled
pore space in a medium-textured soil. Fine-textured soils typically will have more total pore space and more waterfilled pores than medium-textured soils, whereas coarse-textured soils will have more space occupied by solid matter
and less total pore space and less water-filled pore space than either fine-textured soils or medium-textured soils.
Sand particles are fragments of rocks and minerals.
Quartz (silicon dioxide, SiO2) grains are the dominant fragments of sand.
Quartz provides no essential elements to plants.
Fragments of other primary minerals (such as feldspars and micas) that make up sand may contain essential
elements, but these minerals are so insoluble that their capacity to supply nutrients is insignificant in relation to
the total requirements of nutrients by plants.
Sandy soils, therefore, have a low capacity to supply plant nutrients from the inherent inorganic materials.
Silt is essentially microfine sand.
Silt is not much more active chemically than sand and has little more capacity to supply plant nutrients. Silt
does not impart a good physical structure to soil either.
Silty soils have tendencies to crust. Silts do not form aggregates unless the particles are coated with clay.
Large particles in the clay fraction may be fragments of quartz and other minerals, but the major fraction of
clays in temperate regions is the secondary silicate minerals. These minerals are formed from the
recrystallization of the decomposed products of primary minerals.
Silicate clays generally are laminated, being made up of mica-like layers of plates (Figure 6). The particles vary
greatly in sizes and shapes and in composition.
The different kinds of clays have differing effects on the physical and chemical properties of soils. Some
generalizations of the effects of clays are possible.
Figure 6. Exchangeable and nonexchangeable cations helt to negatively charged
(A) micaceous layer silicate clays and (B) kaolinitic layer silicate clay.
The more the particles are broken, the more the negative charges. Another source of charge is the substitution of
one ion for another in the structure of the plates, for example, the substitution of Mg+2 for Al+3, which results in
an unbalanced negative charge.
The unbalanced negative charges from the exposure of edges and ionic substitution attract positively charges
ions in a loosely held swarm around the clay particle. These ions that are attracted by the electronegativity of
clays are called exchangeable cations.
The capacity of colloids to hold exchangeable cations is called cation exchange capacity. Among the plant
nutrients, these cations are calcium, magnesium, potassium, and ammonium.
The platelike structures of some micaceous clay have the capacity to swell and shrink and have considerable
internal and external surface areas.
These clays are called expanding-lattice clays, in contrast to clays which do not have expanding lattices, such as
kaolinite which do not swell when wetted or shrink when dried.
The expanding types of clays dominate in mineral soils. Positively charged ions of nutrients such as calcium,
magnesium, potassium, and ammonium may enter into the internal spaces between the plates and become
trapped or fixed.
Fixation increases the nutrient content of clays. Fixed ions are held rather strongly in the plates and are not
removed ordinarily by exchange with other ions.
Fixed ions are often referred to as nonexchangeable cations.
Clays have negative charges. These charges come about generally from two sources. One source of charge is from
the unbalanced charges at the edges of the broken particles.
Generally, clay soils are more fertile with respect to capacity to supply nutrients because of the exchangeable and
nonexchangeable cations that are held on the surface or in the framework of the clays.
Expanding-lattice clays have greater cation exchange capacity than nonexpanding clays.
Sands and silts as such because of their chemical and physical constitution do not have much capacity to hold
cations in exchangeable or nonexchangeable forms.
In humid, temperate regions, sandy soils are likely to be infertile because they do not contain minerals that will
supply nutrients readily, they have only limited capacity to hold nutrients in exchangeable form, and they lose
nutrients rapidly with water that drains through the soil.
Application of fertilizers to maintain an adequate supply of nutrients will have to be much more frequently on
sandy soils than on loamy or clayey soils.
Organic matter
Most soils range from 1 to 6% organic matter by weight in the topsoil. A soil composed of greater than 20% organic matter is defined as an organic
soil. Soils of lowland, wet area may have 80% or more organic matter. These soils are called peats and mucks. The organic matter of soils is living
and dead matter. Living matter includes microorganisms of various kinds, but roots of living plants and macroorganisms are not included. Soil
organic matter includes any fresh and partly decomposed residues of plants and animals. Humus is a specific form of soil organic matter. It is a
relatively stable organic material, dark in color, and remains after the major portion of the original organic matter has decayed. Humus is an
amorphous, not crystalline, colloidal material, which has a very high adsorptive capacity. One cannot add humus directly to the soil. It must be
formed in the soil from additions of other forms of organic matter.
The cation exchange capacity and water-holding capacity of humus is several times, perhaps three or more, that of colloidal clays.
Soil organic matter is a major source of essential elements for plants. It contains some of every plant nutrient. Almost all of the nitrogen in the soil
is in the organic matter. About a third to a half of the phosphorus in the top horizon of soil is in the organic matter. Organic matter is an important
source of sulfur. These nutrients become available to plants after the organic matter has decomposed. Decomposition of soil organic matter,
including humus, is accomplished by the microbiological and animal life in the soil. Essential elements are held in exchangeable sites of humus or
in other complexes with organic matter and may be released by exchange with other ions or by decomposition of the organic matter. The
decomposition of organic matter acidifies the soil and facilitates the release of nutrients from inorganic soil minerals. Another function of soil
organic matter is in the binding of mineral particles into aggregates. Formation of aggregates improves the structure of soils, particularly those that
have a substantial clay content. Additions of sand to improve the structure of clay is not a productive process. Mixing of sand and clay often leads
to a concrete-like material of poor structure.
Also, building the humus content of soils increases their abilities to retain nutrients and water (Table 5). A good organic matter content is essential
to the maintenance of fertility in sandy soils. Fresh organic matter added to the soil usually decomposes away in a year or two. The rate of
decomposition depends on the physical, chemical, and biological properties of the soil and of the organic matter and on the amount of material
added.
Decomposition of organic matter is a biological, largely microbiological, process. The better that a soil is aerated, the
faster organic matter will decompose. For this reason, organic matter decomposes faster in sandy soils than in loamy or
clayey soils, which are not as well aerated as sandy soils. Sandy soils generally have lower organic matter contents than
loamy or clayey soils. Maintenance of organic matter in sandy soils requires more frequent additions of fresh organic
matter than loamy or clayey soils.
Table 5. Effects of organic matter on soil physical properties.
Response to Increase in Soil Organic Matter
Bulk density -------------------------------------------------------------------> Decrease
Moisture retention -------------------------------------------------------------> Increase
Field capacity -----------------------------------------------------------------> Increase
Wilting percentage ------------------------------------------------------------> Increase
Available water ---------------------------------------------------------------> Increase
Cation exchange capacity -----------------------------------------------------> Increase
Aggregation (structure) -------------------------------------------------------> Increase
Infiltration rate ----------------------------------------------------------------> Increase
Leaching ---------------------------------------------------------------------->Decrease
The composition of the organic matter also governs its rate of decomposition. The
most important factor of composition is the relative amount of carbon and nitrogen in
the organic matter, the carbon to nitrogen ratio (C:N). In general, but with many
exceptions, materials with wide ratios are slower to decompose than those with
narrow ratios (Table 6). All of these materials are of biological origin, and although a
definite C:N value is given, the actual value will vary depending on the origin and
handling of the specific materials.
Table 6. Approximate carbon to nitrogen ratios of some common organic substances
---------Material---------------
---------------C:N---------------- ------------Material------------
Newspaper
1,000
Farm manures, large animalsz
30
Sawdust, wood chips
500
Green plants
20
Straw (small grain)
180
Poultry manure
17
Pine needles, dead
180
Soil
12
Broad leaves, dead
180
Seed meals
8
Peatmoss
100
Soil microorganisms
7
Composts
30
Dried blood
5
zLarge
farm animals no bedding (cattle, hogs, horses, sheep
---------------C:N----------------
Fresh sawdust may require three or more years to decay in the soil. Paper, which has a wider C:N, may decay more quickly
because the cellulose of paper will be more prone to decay than the complex mixture of carbohydrates, cellulose, lignin, and
other substances of sawdust. Conversely, peatmoss decomposes slowly in the soil, because the easily decomposable
substances have been rotted away leaving behind skeletal material, mostly lignin, which is resistant to decay. Manures and
composts will require two or more years to break down in the soil. A green manure crop decomposes about 75% in the first
year. Seed meals and dried blood are fertilizer-grade organic matter and will be decomposed almost entirely in one year.
Decomposition of organic matter may release nitrogen that is in the organic matter into a form that plants can use. This
process is called nitrogen mineralization (Figure 7).
Figure 7. Partial nitrogen cycle in soils, illustrating mineralization of soil organic matter (or organic fertilizers), oxidation of ammonium
to nitrate (nitrification), losses of ammonia by volatilization, losses of gaseous nitrogen products by denitrification of nitrate, absorption
of plant nutrients, and microbial immobilization.
The forms of nitrogen that are available to plants are ammonium (NH4+) and nitrate (NO3-). In well-aerated soils, nitrate is the
dominant form, because nitrification proceeds rapidly in these soils. Decomposition takes place over a period of time, and the
organic matter changes chemically and physically as decomposition occurs. Different kinds of organisms participate in the
decomposition as the organic matter changes with time. The first group of microorganisms live on the easily decomposable sugars,
starches, and cellulose. As that energy supply becomes depleted, these organisms die and become part of the organic matter,
effectively narrowing its C:N ratio. Another group of fungi and bacteria then will dominate in the decomposition, using the dead
fungal tissue and the remaining organic residue. Bacterial decomposition narrows the C:N ratio toward a limiting value of 4:1 to 9:1,
but the C:N ratio seldom reaches this value. The organic matter will stabilize in humus at a C:N ratio that is characteristic of the
particular soil, but a ratio of 10:1 to 15:1 is common in soils.
If environmental factors are constant, humus is built and decomposed at about the same rate. Tillage accelerates the rate of
decomposition by loosening the soil thereby increasing aeration. Rate of decomposition of humus declines with increasing time of
cultivation, eventually reaching a constant rate. In a newly cultivated field in temperate regions, the decomposition of humus may
be about 4% in the first year. This rate falls 1 to 2% per year in fields under prolonged cultivation.
A soil that has not been cultivated for a long period of time may have 100,000 pounds of humus per acre (tilled zone of the soil),
which holds about 5,000 pounds of nitrogen--a ratio of total humus to nitrogen of about 20 to 1 is common (this is not the C:N
ratio). In the first year, the release of nitrogen will be about 200 pounds per acre (4% of 5,000 lb N). This amount of nitrogen will
support a productive crop, which will be able to recover about half of the released nitrogen. After 30 or 40 years of cultivation with
no deliberate effort made to maintain soil organic matter, the humus in this soil will fall to 60,000 or few pounds per acre (3,000 lb
N/acre), and the decomposition will be at the slower rates. Now the release of nitrogen from humus will be only 30 to 60 pounds
per acre (1% or 2% of 3,000 lb N). If the crop recovers half of the nitrogen (15 to 30 lb/acre), insufficient nitrogen will be available to
support a productive crop, and nitrogen from fertilizers will be necessary.
Humus in a soil can be maintained by large additions of manures or composts or by a green manure crop in a rotation. The
management of green manures, farm manures, and composts is discussed in chapters that follow this one. On small plots of land,
the use of manures and composts is better, for the green manure crop may take the land out of production for a year. On large
acreages, green manures are better, for the quantities of manures and composts and the labor required to obtain and spread them
may exceed the supplies of material and labor. Materials with a wide C:N ratio will build up the humus content of soil faster than
ones with a narrow C:N ratio due to differences in rate of decomposition of the organic matter and the amount of stable organic
matter that remains after the easily decomposable materials are gone.
.
Most of the nitrogen in the added organic matter becomes part of the bodies of the microorganisms that are
living on the organic matter. The carbon goes into the microbial bodies, but a lot of it is lost to the air as carbon
dioxide. Thus, with time the C:N ratio of the organic matter narrows. If the C:N ratio is very wide as in sawdust,
straw, dead leaves and garden residues, and manures with bedding, the organic matter may not supply enough
nitrogen to meet the needs of the microorganisms.
In this case, the soil is depleted of available nitrogen by the microorganisms. In other words, the deficiency of
nitrogen in the organic matter is made up by the microorganisms using nitrogen from the soil. This process is
called nitrogen immobilization.
Microorganisms are better competitors for soil nitrogen than plant roots; hence, nitrogen deficiency will develop
on plants grown in soils to which organic matter with a wide C:N ratio is added. The solutions to nitrogen
immobilization are to delay planting of crops until the C:N ratio has narrowed enough so that nitrogen is released
by mineralization or to add nitrogen as fertilizer to compensate for the nitrogen that is consumed by the
microorganisms. Generally, the amount that needs to be added is one pound of nitrogen for each 100 pounds of
organic matter with a wide C:N ratio.
These applications will need to be continued until the organic matter had decomposed sufficiently to lower the
C:N ratio to 35:1 or lower. Generally, this amount of time is 3 years in the case of sawdust, 2 years in the case of
straws and coarse dead residues, and 1 year in the case of manures with bedding. Trying to accelerate the rate of
decomposition of highly carbonaceous organic matter by additions of nitrogen in excess of 1 pound per 100
pounds per year usually leads to problems of salinity or ammonium toxicity.
A C:N ratio of 35:1 is about the breaking point between mineralization and immobilization. Materials well below
this value will begin to release nitrogen soon after their addition to the soil, whereas materials well above this
value will immobilize nitrogen until the C:N ratio is narrowed to 35:1. Materials that are near a C:N ratio of 35:1
may have a short period in which nitrogen is immobilized before it is released. The major purpose of composting
is to narrow the C:N of organic matter to a value at which immobilization will be short lived after the addition of
the organic matter to the soil.