Download Soil Organic Matter

Soil Organic Matter
Energy and nutrient source
Aggregation
Water holding capacity
CEC, pH
It influences physical and chemical properties.
Sources of OM : 1. plant tissue, roots, leaves, grass…. 2. microorganisms 3. soil animals
-----------------------------------------------------------------------------------------------------------------------------Composition of Plant Residues:
Rabidly decompose: Carbohydrate, sugar, starch, 1-5% _____ 80% H2O +20% dry matter(DM=44%C, +
Cellulose, hemicellulose
55%
40%O2 + 8%H + 8% ash)
Very slow decomposed : Legnins
25%
Fats and wax
5%
Protein
10%
-----------------------------------------------------------------------------------------------------------------------------When organic tissues added to soil, 3 general reactions take place:
1. OM undergo enzymatic oxidation and hydrolysis . The major products are: CO2, H2O and Energy .
2. Release of nutrients through Mobilization process.
3. Compounds resistant to microbial action are formed either from compounds in the original plant tissue
or by microbial synthesis : Humus.
Results: 1. Energy for microorganisms. 2. simple products as CO2, CO3, NH4, NO3, H2PO4, H2O, O2.
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Factors affecting amount and distribution of OM in soils :
(1) Climate and vegetation.
(2) Cultivation.
(3) Soil texture.
(4) Aeration.
C/N Ratio and its effect on decomposition:
-
in soils C/N ratio ranges between 8:1 to 15:1. (in humid region ratio is higher, in arid region is lower).
In plant and microbes: 20:1 to 30:1. In straw 100:1.
C/N ratio in Humus 10:1, clover 20:1, straw 100:1.
C/N ratio is important because: 1) competition for available N when C/N is high . So N is needed.
2) this ratio is relatively constat.
Things used to prevent competition for N :
1. Crop residue and straw are burned : less OM.
2. N fertilization.. N Factor: Amount of inorganic N must be given to 100kg OM prevent
immobilization of available N ( 1 kg N for 100kg straw).
3. green manure.
4. shifting cultivation. .
If S is deficient in the soil then NO3-N is accumulated in plants which will be toxic to animals
feeding on these plants.