Role of Organic Matter in Soil Fertility

Organic matter affects soil properties of the soil and its overall health. Properties influenced by organic matter include: soil structure; moisture holding capacity; diversity and activity of soil organisms, both those that are beneficial and harmful to crop production; and nutrient availability. It also influences the effects of chemical amendments, fertilizers, pesticides ,and herbicides.

When crops are harvested or residues bare urned, organic matter is removed from the system. However, the loss can be minimized by retaining plant roots in the soil and leaving crop residues on the surface. Organic matter can also be restored to the soil through growing green manures, cuttings from agroforestry species and the addition of manures and compost.

1. Cation Exchange Capacity (CEC)

The stable fraction of soil organic matter (humus) is the most important fraction contributing to the CEC of a soil. Cation exchange capacity is the total sum of exchangeable cations that a soil can hold (Brady and Weil, 2004).

Cation exchange capacity determines a soil’s ability to retain positively charged plant nutrients, such as NH4+, K+, Ca2+, Mg2+, and Na+.

As CEC increases for a soil, it is able to retain more of these plant nutrients and reduces the potential for leaching.

Soil CEC also influences the application rates of lime and herbicides required for optimum effectiveness.

2. Nutrient Retention and Release

Humus plays an important role in regulating the retention and release of plant nutrients. Humus has a highly negatively charged soil component, and is thus capable of holding a large amount of cations.

The highly charged humic fraction gives the soil organic matter the ability to act similarly to a slow release fertilizer. Over time, as nutrients are removed from the soil cation exchange sites, they become available for plant uptake.

Read Also: Classification of Soil Organism

3. Soil Structure and Bulk Density

Soil structure refers to the way that individual soil mineral particles (sand, silt, and clay) are arranged and grouped in space. Soil structure is stabilized by a variety of different binding agents.

Soil organic matter is a primary factor in the development and modification of soil structure (Coleman et al., 2004). While binding forces may be of organic or inorganic origins, the organic forces are more significant for building large, stable aggregates in most soils.

Examples of organic binding agents include plant- and microbially derived polysaccharides, fungal hyphae, and plant roots. Inorganic binding agents and forces include charge attractions between mineral particles and/or organic matter and freezing/thawing and wetting/drying cycles within the soil as well as compression and deformation forces.

Both the stable and the active fraction of SOM contribute to and maintain soil structure and resist compaction.

4. Water Holding Capacity

Soil organic matter affects the amount of water in a soil by influencing: water infiltration and percolation; evaporation rates; and increasing the soil water holding capacity.

Factors that reduce water infiltration and percolation are compaction in surface soils, lack of surface residue, poor soil structure, surface crusting due to salinity, and steep slopes that facilitate high volumes of water runoff.

Surface residues physically impede water runoff, resulting in reduced velocity of water movement. As water movement across the soil surface slows down, water has more time to move downward into the soil profile, rather than across the soil surface.

In this way, increasing soil organic matter and leaving residue on the soil surface can increase water infiltration.

Surface residues also slow the rate of water evaporation from the soil and improve soil structure, which helps prevent soil crusting. Crusting can result in significant losses to crop stand.

The result of increasing soil organic matter is greater in soil pore space, which provides an area for water to be stored during times of drought.

A unique characteristic of the pore space in soil organic matter is that the pores are found in many different sizes. The large pores do not hold water as tightly and thus will drain more readily.

The medium and small-sized pores will hold water more tightly and for a longer period of time, so that during a dry period the soil retains moisture and a percentage of that water is made available over time for plant uptake.

The benefit of leaving residue on the soil surface and increasing soil organic matter is that water infiltration is increased, soil crusting is decreased and the soil can hold more of the water that infiltrates and will eventually make it available for plant use.

5. Biological Activity

While microorganisms make up a small portion of the SOM (less than 5%) they are imperative to the formation, transformation, and functioning of the soil.

In the soil, they conduct indispensable processes such as decomposition, nutrient cycling, and degradation of toxic materials, N fixation, symbiotic plant relationships, and pathogen control.

About soil fauna, Jenny said, “They break up plant material, expose organic surface areas to microbes, move fragments and bacteria-rich excrement around, up, and down, and function as homogenizers of soil strata” (Jeny, 1980).

Soil fauna pla an important role in the initial breakdown of complex and large pieces of organic matter, making it easier for soil microorganisms to release carbon and plant nutrients from the material as they continue the process of decomposition.

In conclusion, soil fertility is very important in agriculture and has to be maintained in order to have continuous crop production.

Soil organic matter is the key to soil life and the diverse functions provided by the range of soil organisms.

Soil organic matter improves the fertility of the soil in many ways such as improving CEC, water and nutrient retention, stabilizing soil structure ,and improving microbial activities.

Read Also: Beneficial Vs Harmful Organisms in Agricultural Soils