What is Soil Organic Matter? – Importance & Sources

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.

By the end of this article, you will be able to:

· Explain what organic matter is

· Outline the major sources of organic matter.

· Importance of soil organic matter

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. Aboveground organic matter includes plants and animal residues while the belowground matter comprises of living soil fauna and micro flora, partially decomposed plant and animal.

7 Importance of Organic Matter to the Soil

1.   Soil organic matter serves as a reservoir of nutrients for crops

2.   Provides soil aggregation

3.   Increases nutrient exchange

4.   Retains moisture

5.   Reduces compaction

6.   Reduces surface crusting

7.   Increases water infiltration into soil

Healthy soil is the foundation of the food system. It produces healthy crops that in turn nourish people. Maintaining a healthy soil demands care and effort from farmers because farming is not benign. By definition, farming disturbs the natural soil processes including that of nutrient cycling – the release and uptake of nutrients.

Plants obtain nutrients from two natural sources: organic matter and minerals. Organic matter includes any plant or animal material that returns to the soil and goes through the decomposition process. In addition to providing nutrients and habitat to organisms living in the soil, organic matter also binds soil particles into aggregates and improves the water holding capacity of soil. Most soils contain 2-10 percent organic matter. However, even in small amounts, organic matter is very important.

Soil is a living, dynamic ecosystem. Healthy soil is teeming with microscopic and larger organisms that perform many vital functions including converting dead and decaying matter as well as minerals to plant nutrients. Different soil organisms feed on different organic substrates. Their biological activity depends on the organic matter supply.

Nutrient exchanges between organic matter, water and soil are essential to soil fertility and need to be maintained for sustainable production purposes. Where the soil is exploited for crop production without restoring the organic matter and nutrient contents and maintaining a good structure, the nutrient cycles are broken, soil fertility declines and the balance in the agro-ecosystem is destroyed.

Soil organic matter – the product of on-site biological decomposition – affects the chemical and physical properties of the soil and its overall health. Its composition and breakdown rate affect: the soil structure and porosity; the water infiltration rate and moisture holding capacity of soils; the diversity and biological activity of soil organisms; and plant nutrient availability.

Many common agricultural practices, especially ploughing, disc-tillage and vegetation burning, accelerate the decomposition of soil organic matter and leave the soil susceptible to wind and water erosion. However, there are alternative management practices that enhance soil health and allow sustained agricultural productivity.

Conservation agriculture encompasses a range of such good practices through combining no tillage or minimum tillage with a protective crop cover and crop rotations. It maintains surface residues, roots and soil organic matter, helps control weeds, and enhances soil aggregation and intact large pores, in turn allowing water infiltration and reducing runoff and erosion.

In addition to making plant nutrients available, the diverse soil organisms that thrive in such conditions contribute to pest control and other vital ecological processes. Through combining pasture and fodder species and manuring with food and fibre crop production, mixed crop-livestock systems also enhance soil organic matter and soil health. This document recognizes the central role of organic matter in improving soil productivity and outlines promising technologies for improved organic matter management for productive and sustainable crop production in the tropics.

Soil organic matter content is a function of organic matter inputs (residues and roots) and litter decomposition. It is related to moisture, temperature and aeration, physical and chemical properties of the soils as well as bioturbation (mixing by soil macrofauna), leaching by water and humus stabilization (organomineral complexes and aggregates). Land use and management practices also affect soil organic matter.

Farming systems have tended to mine the soil for nutrients and to reduce soil organic matter levels through repetitive harvesting of crops and inadequate efforts to replenish nutrients and restore soil quality. This decline continues until management practices are improved or until a fallow period allows a gradual recovery through natural ecological processes. Only carefully selected diversified cropping systems or well-managed mixed crop-livestock systems are able to maintain a balance in nutrient and organic matter supply and removal.

Farmers can take many actions to maintain, improve and rebuild their soils, especially soils that have been under cultivation for a long time. A key to soil restoration is to maximize the retention and recycling of organic matter and plant nutrients, and to minimize the losses of these soil components caused by leaching, runoff and erosion.

However, rebuilding soil quality and health through appropriate farming practices may take several years, especially in dryland areas where limited moisture reduces biomass production and soil biological activity. Thus, the challenge is to identify soil management practices that promote soil organic matter formation and moisture retention and ensure productivity and profitability for farmers in the short term.

Sources of Organic Matter

Soil organic matter consists of diverse, heterogeneous 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. Organic matter is categorized into aboveground and belowground organic.

Aboveground organic matter includes plants and animal residues while the belowground matter comprises of living soil fauna and micro flora, partially decomposed plant and animal.

1. Crop Residues

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

i. Field residue is materials which are left in an agricultural field or orchard after the crop has been harvested.

This includes stalks, stems or leaves and seed pod. The residue can be ploughed directly into the ground or burned first.

ii. 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.

2. Green Manure and Cover Crops

Green manure can be defined as a practice of ploughing or turning into the soil under composed 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.

3. 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

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

Read: 6 Types of Farm Animals and Its Characteristic

Conclusion on what is Soil Organic Matter? – Importance & Sources

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. 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).

You have leant about soil organic matter as well as their sources.

In this article, you leant the following;

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).

Importance 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 include; crop residue, green manure, animal waste and compost.