Soil organic matter (SOM) is the organic component of soil, consisting of three primary parts including small (fresh) plant residues and small living soil organisms, decomposing (active) organic matter, and stable organic matter (humus).

Soil organic matter serves as a reservoir of nutrients for crops, provides soil aggregation, increases nutrient exchange, retains moisture, reduces compaction, reduces surface crusting, and increases water infiltration into soil.

Components vary in proportion and have many intermediate stages. Plant residues on the soil surface such as leaves, manure, or crop residue are not considered SOM and are usually removed from soil samples by sieving through a 2 mm wire mesh before analysis.

Soil organic matter content can be estimated in the field and tested in a lab to provide estimates for Nitrogen, Phosphorus and Sulfur mineralized available for crop production and adjust fertilizer recommendations.

Soil organic matter impacts the rate of surface applied herbicides along with soil pH necessary to effectively control weeds. Soil organic matter impacts the potential for herbicide carryover for future crops, and amount of lime necessary to raise pH.

What is Soil Organic Matter?

The term soil organic matter (SOM) has been used in different ways to describe the organic constituents of soil. Baldock and Skjemstad (1999) defined SOM as all organic materials found in soils irrespective of origin or state of decomposition.

SOM consists of C, H, O, N, P and S. Included are living organic matter (plants, microbial biomass and faunal biomass), dissolved organic matter, particulate organic matter, humus and inert or highly carbonized organic matter.

Part of soil organic matter consists of carbohydrates, lipids and proteins that are abundant in fresh plant residues. These are rapidly metabolized, immobilized or decomposed.

Organic matter is categorized into aboveground and belowground organic. Above ground organic matter includes plants and animal residues while the belowground matter comprises of living soil fauna and micro flora, partially decomposed plant and animal.

Soil Organic Matter and Sources of Organic Matter

Figure: Major Soil Organic Matter Components (Source: The SoilFoodWeb,USDA-NRCS)

Benefits of Organic Matter to the Soil

Soil organic matter serves as a reservoir of nutrients for crops,

Provides soil aggregation,

Increases nutrient exchange,

Retains moisture,

Reduces compaction,

Reduces surface crusting, and increases water infiltration into soil.

Sources of Organic Matter

Soil organic matter consists of diverse, heterogenous components. It was reported that living C rarely exceeds 4% of total soil organic C and is present as roots, microorganisms and soil fauna.

Non-living C represents the major portion of organic C, consisting of surface litter, root litter, microbial metabolites and humic substances.

The living and non-living C component constantly interacts, as do the saprophytic organisms that acquire metabolites from non-living C in the soil and then die.

The term “soil organic matter” (SOM) has been used in different ways to describe the organic constituents of soil. Baldock and Skjemstad (1999) defined SOM as “all organic materials found in soils irrespective of origin or state of decomposition”. SOM consists of C, H, O, N, P and S.

Included are living organic matter (plants, microbial biomass and faunal biomass), dissolved organic matter, particulate organic matter, humus and inert or highly carbonized organic matter.

Part of soil organic matter consists of carbohydrates, lipids and proteins that are abundant in fresh plant residues. These are rapidly metabolized, immobilized or decomposed.

Crop Residues

Crop residue is divided into two types- field and process residue.

Field residue is materials which are left in an agricultural field or orchard after the crop has been harvested. This include stalks, stems or leaves and seed pod. The residue can be ploughed directly into the ground or burned first.

Process residue: They are materials which are left after the crop is processed into useable resource. It includes husks, seed and root which can be used as soil amendment, fertilizer and in manufacturing.

They have high CN ratio with low N content but fairly high Potassium and silica content which help to improve the resistance of crops to disease and lodging. Fibrous materials provide an energy source for soil microorganisms which improve soil physical properties.

Green Manure and Cover Crops

Green manure can be defined as a practice of ploughing or turning into the soil undecomposed green plant tissues for improving physical structure and soil fertility. The green manure crops supplies organic matter as well as nitrogen, particularly if it is a legume crop.

Animal Waste

Amending soil with animal waste has been old practice. Animal waste can supply nutrients, OM and enriched soil with beneficial organisms. Dung’s comes mostly as undigested material and the urine from the digested material.

More than 50% of the organic matter that is present in dung is the form of complex product consists of lignin and protein which are resistant to further decomposition and therefore the nutrients present in dung are released very slowly

Compost

Compost is any organic material that undergoes decomposition under controlled conditions. Any organic material can be converted to compost, but there are rules regarding what material can and cannot be used.

Compared to some uncomposted animals waste, it may have low nutrient levels. Nutrients from compost are often less available to the crop; thus, compost may be more useful for building SOM. Compost causes less water pollution.

Nyamangara etal.,(2003) reported that management of soil organic matter by using composted organic waste is the key for sustainable agriculture.

In summary, soil organic matter may be defined as “all organic materials found in soils irrespective of origin or state of decomposition”. SOM consists of C, H, O, N, P and S.

This article has covered the following areas;

That soil organic matter (SOM) is the organic component of soil, consisting of three primary parts including small (fresh) plant residues and small living soil organisms, decomposing (active) organic matter, and stable organic matter (humus).

Benefits of organic matter to the soil includes; organic matter serves as a reservoir of nutrients for crops, provides soil aggregation, increases nutrient exchange, retains moisture, reduces compaction, reduces surface crusting, and increases water infiltration into soil.

Sources of organic matter includes; crop residue, green manure, animal waste and compost.