Soil Organic Matter: Meaning, Factors and Management

After reading this article you will learn about:- 1. Meaning Soil Organic Matter (SOM) 2. Structure of SOM 3. Management.

Meaning Soil Organic Matter (SOM):

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 crops 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 estimates in the field and tested in a laboratory to provide estimates for nitrogen, phosphorus and sulfur mineralized available for crop production and adjust fertilizer recommendations.

Inherent factors affecting soil organic matter such as climate and soil texture cannot be changes. Climatic conditions, such as rainfall, temperatures, moisture and soil aeration (oxygen levels) affect the rate of organic matter decomposition. Organic matter decomposes faster in climates that are warm and humid and slower in cool, dry climates.

Organic matter also decomposes faster when soil is well aerated (higher oxygen levels) and much slower on saturated wet soils. Soils formed under grass (prairie) vegetation usually have organic matter levels at least twice as high as those formed under forests because organic material is added to topsoil from both top growth and roots that die back every year.

Soils formed under forests usually have comparably low organic-matter levels because of two main factors listed below:

1. Trees produce a much smaller root mass per acre than grass plants, and

2. Trees do not die back annually and decompose every year. Instead, much of the organic material in a forest is tied up in the tree’s wood rather than being returned to the soil annually.

Important factors controlling soil organic matter (SOM) levels include climate, hydrology, parent material, soil fertility, biological activity, pattern of vegetation and land use.

Soil organic matter consists of diverse heterogeneous components in continuum ranging from labile components that mineralize very rapidly during the first and initial stage of decomposition to more recalcitrant residues that accumulate as they are deposited during advanced stages of decomposition as microbial byproducts.

It refers to the non-living heterogeneous mixture of organic components arising from the microbial and chemical transformation of organic debris. A large portion of SOM consists of humic substances which are amorphous, dark coloured, partly aromatic, mainly hydrophilic, chemically complex polyelectrolyte materials with molecular weight ranging from a few hundred to several thousands.

In fact, SOM is that part of organic matter which has become of the soil and is not merely in the soil. Soil organic matter is known to serve as soil conditions, nutrient reservoir, energy sources for micro-organisms, preserver of the environment and major determinant for sustaining or enhancing agricultural productivity.

Energy capture and carbon fixation via photosynthesis are the first steps in organic matter management. Water availability in the soil is a principal determinant of primary production. In general to maintain SOM levels the input rate should be equal to the rate of organic matter decomposition, but to build up SOM the input rate must be higher to the of the rate of decomposition.

Soil organic matter is sensitive to human activities viz. deforestation, biomass burning, land use pattern and changes, and environmental pollution. The unfavourable climatic condition in the Indian Peninsula, have increased the rate of decomposition of SOM and consequent depletion of SOM.

A stock taking of SOM will indicate the factors associated with the accumulation (gain) or decomposition (loss) of organic carbon as well as help in deciding methods and measures for sequestration of organic carbon in soils of India.

Importance of Soil:

(i) It is the store house of nutrients including major, secondary and micro-nutrients.

(ii) It acts as a source of food and energy for the soil micro-organisms.

(iii) It improves physical, chemical and biological properties of soils.

Physical:

Improves soil structure, water holding capacity, porosity and aeration, maintains thermal regimes and other physical constraints etc. Release of nutrients from soil minerals by weathering.

Chemical:

Improves ion exchange capacity, optimize buffering capacity of the soil, enhanced ligancy help in trapping nutrients, chelation reactions with different plant nutrients in soils.

Biological:

Enhances the proliferation of various beneficial soil micro-organisms, enhances different biological processes like mineralization, fixation of nutrients etc.

Structure of SOM:

Management of SOM:

It is evident that the adverse effect of climate on SOM of Indian soils has been observed and the depletion of organic carbon has been reported to the level of 60-70% due to prevailing higher temperature in India. The quality of SOM is related with its functional pools vis-a-vis its turnover time (Table 19.22).

The quality of SOM may be assessed by the presence of decomposable (labile) or resistant fractions. The labile fractions are further into an active pool composed of microbial biomass, soluble carbohydrates and exo-cellular enzymes (turnover time < 1 year) and a recalcitrant or resistant pool.

This resistant pool composed of particulate organic matter (turnover time 8-50 years) and passive pool comprising humic acid, fulvic acid, organo- mineral complex (turn over time 400-2200 years).

The quality of SOM according to its functional groups indicates that plant and animal residues with average turnover period of < 1 year could be the ideal source of organic matter for rapid sequestration of organic carbon. Soil microbial biomass form an important component of SOM which is the seat of mineralization-immobilisation turnover (MIT) and forms “organic pool” for isotopic exchange of C and N.

Tillage and SOM:

Tillage increases oxidation of organic matter by breaking soil aggregates, exposing new surfaces to microbial attack and changing redox conditions of the soil. It is reported that the counts of aerobic micro-organisms, anaerobes and denitrifies were much higher in the no-till plots.

Adoption of management practices, which enhance production such as no-till practices, application of amendments, fertilization and irrigation also affect C cycle in the soil.

Soil organic matter generally increases where biomass production is higher and where organic material additions occur. Plant residue with a low C/N ratio (high nitrogen content) decompose more quickly than those with a high C/N ratio and do not increase soil organic matter levels as quickly. Excessive tillage destroys soil aggregates increasing the rate of soil organic matter decomposition.

Stable soil aggregates increase active organic matter and protect stable organic matter from rapid microbial decomposition. Measures that increase soil moisture, soil temperature, and optimal aeration accelerate SOM decomposition; Management measures utilized on the field you are evaluating can either degrade or increase SOM.

Some key management measures that can increase SOM are described here:

i. Use of cropping systems that incorporate continuous no-till, cover crops, solid manure or other organic materials, diverse rotations with high residue crops and perennial legumes or grass used in rotation. Reducing or eliminating tillage that causes a flush of microbial action that speeds up organic matter decomposition and increases erosion.

ii. Reduce erosion using appropriate measures. Most SOM is in the topsoil. When soil erodes, organic matter goes with it.

iii. Soil-test and fertilize properly. Proper fertilization encourages growth of plants, which increases root and top growth. Increased root growth can help build or maintain SOM.

iv. Use of perennial forages provides for annual die back and re-growth of perennial grasses and their extensive root systems and after math contributing organic matter to soil. Fibrous root systems of perennial grasses are particularly effective as a binding agent in soil aggregation.

Soil Organic Matter Relationship to Soil Function:

Under average conditions in temperate regions approximately 1.5 per cent of SOM mineralizes yearly for most crops, while maintaining current organic matter levels on soils with 2 to 5% SOM. Depending on site conditions, management, and climate mineralization rates and loss of SOM can increase dramatically if temperature, aeration, and moisture conditions are favourable. Key soil functions SOM provide for include:

Nutrient Supply:

Upon decomposition, nutrients are released in a plant-available form maintaining current levels.

Water-Holding Capacity:

Organic matter behaves somewhat like a sponge. It has the ability to absorb and hold up to 90 per cent of its weight in water. Another great advantage of organic matter is that it releases nearly all of the water it holds for use by plants. In contrast, clay holds large quantities of water, but much of it is unavailable to plants.

Soil Aggregation:

Organic matter improves soil aggregation, which improves soil aggregation, which improves soil structure. With better soil structure, water infiltration through the soil improves, which improves soil’s ability to take up and hold water.

Erosion Prevention:

Because of increased water infiltration and stable soil aggregates erosion is reduced with increased organic matter.

Estimating Organic Material Needed to Increase:

Soil Organic Matter:

The term steady state is where the rate of organic matter addition from crop residues, roots and manure or other organic materials equals the rate of decomposition. If the rate of organic matter addition is less than the rate of decomposition, SOM will decline and, conversely if the rate of organic matter addition is greater than the rate of decomposition, SOM will increase.

Active Organic Matter:

Microorganisms and other organic compounds are used as food by microorganisms. Active soil organic matter decomposes faster than other components of soil organic matter in response to management changes.

Fresh Plant Residues:

Plant residue, animal, or other organic substances that have recently been added to the soil and have only begun to show signs of decay. Does not include surface residue cover.

Humus or Stable Organic Matter:

Complex organic compounds that remain after many organisms have used and transformed the original organic material leaving a stable form. Humus is not readily decomposed because it is either physically protected inside soil aggregates or chemically too complex to be used by most organisms. Humus is important in binding tiny soil aggregates, and improves water and nutrient holding capacity.

Mineralize:

Organic matter decomposition which releases nutrients in plant available form (e.g. phosphorus, nitrogen, and sulfur).

Small Living Organisms or Soil Microorganisms:

Bacteria, fungi, nematodes, protozoa, arthropods, etc.

Carbon Sequestration in Soils:

(i) C sequestration in Degraded Soils:

There is a scope for sequestration of organic carbon (OC) in degraded soils through suitable land use planning such as agriculture, agroforestry and silviculture etc. Plantation of forest trees not only improves the physical conditions of sodic soils but also help in increasing OC content.

(ii) Application of Crop Residues, Fertilizers and Manures:

It is suggested that organic carbon could be sequestered in soils through green manuring and application of farmyard manure (FYM). However, continuous application of FYM and green manure improves the organic carbon content of the soil substantially under different soils and cropping systems.

Land use Management:

In view of high cost of fertilizers and increasing concern of ground water pollution through leaching of high analysing chemical fertilizers, new ways of site specific nutrient and site specific integrated use of fertilizers, organic and green manures, vermin-composts, bio-fertilizers etc.. are considered to be the best management options for future rational land use founds productivity without any environmental hazard.

Cropping System and Cultural Practices:

Soil productivity is increased when crops are changed over seasons or years. Where legume crops are to be included in the system. The adoption of suitable cultural practices can increase crop production with the simultaneous increase in the OC content in the soil.

Soil Carbon Sequestration:

World soils constitute the third largest global C pool, comprising of two distinct components:

(i) Soil organic C (SOC) estimated at 1550 Pg, and

(ii) Soil inorganic C (SIC) pool estimated at 950 Pg, both to 1-m depth. Other pools include the oceanic (38,400 Pg), geologic/fossil fuel (4500 Pg), biotic (620 Pg), and atmospheric (750 Pg).

Thus the soil C pool of 2500 Pg is 3.3 times the atmospheric pool and 4.0 times the biotic pool. However, soils of the managed ecosystems have lost 50 to 75% of the original SOC pool. Conversion of natural to managed ecosystems depletes SOC pool because C input into the agricultural ecosystems is lower, and losses due to erosion, mineralization and leaching are higher than those in the natural ecosystems.

The magnitude of SOC depletion is high in soils prone to erosion and those managed by low-input or extractive farming practices. The loss of SOC pool is also high in soils of coarse texture and those with a high initial pool. Most agricultural soils have lost 20 to 40 Mg C/ha due to historic land use and management.

The maximum soil C sink capacity, amount of C that can be stored in it, approximately equals the historic C loss. In other words, most agricultural soils now contain lower SOC pool than their capacity because of the historic loss.

The maximum soil C pool is determined by the climate, parent material, physiography, drainage and soil properties including clay content, clay minerals, and nutrient reserves. Soil drainage and moisture regime, along with soil aspect and landscape position, are important controls of soil C pool.

The SOC pool is at a dynamic equilibrium under a specific land use and management system. At equilibrium, the C_input into a system equals C_output. Upon conversion to another land use and management, depletion of SOC pool occurs if C_input< C_output, and sequestration if C_input> C_output (Eq. 1 to Eq. 3).  

Land use and soil management techniques which lead to C sequestration are retention of crop residues, NT farming and incorporation of cover crops in a diversified rotation cycle (together also referred to as CA), INM techniques of using compost and other bio-solids, erosion control, water conservation, contour hedges with perennials, controlled grazing, etc.

An average long-term rate of SOC sequestration with these techniques is 200 to 1000 kg/ha/yr. for humid temperate regions and 50 to 250 kg/ha/yr. for dry tropical regions. In addition, the rate of SIC sequestration as secondary carbonates is about 5 to 25 kg/ha/yr.

in arid and semi-arid regions. In contrast, depletion of SOC pool occurs with the use of excessive plowing, residue removal and biomass burning, and extractive farming practices where nutrient balance is often negative.