THE ROLES AND ACTIVITIES OF SOIL ORGANIC MATTER IN CROP PRODUCTIVITY

The roles and activities of soil organic matter can be classified into three major categories which are biological, physical, as well as chemical. There are many and varied interactions that occur between these aspects of soil organic matter. Additionally, the active and stable fractions of the soil will also play different roles in specific soil organic matter functions.

• Cation exchange capacity
• Nutrient retention and release
• Soil structure and bulk density
• Water-holding and snow/drain catchment
• Biological activity

Cation exchange capacity of soil

Cation exchange capacity is the total sum of exchangeable cations which are the positively charged ions that a soil can hold. Cation exchange capacity determines a soil’s ability to retain positively charged plant nutrients, such as NH4⁺, K⁺, Ca2⁺, Mg²⁺, and Na⁺. As the cation exchange capacity increases for a soil, it is able to retain more of these plant nutrients and reduces their potential for leaching. The soil cation exchange capacity also influences the application rates of liming as well as herbicides required for optimum effectiveness on the field. The stable humus of soil organic matter is the most important fraction for contributing to the cation exchange capacity of a soil.

Plants nutrient retention and release

Humus plays an important role in the regulating of the retention and release of plant nutrients. Humus has a highly negatively charged soil component, and it is also 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 on the soil. As nutrients are removed from the soil cation exchange sites, they become available for plant uptake.

The estimation of the amount of nutrients released from the soil organic matter are complicated and there are no widely agreed upon methods in use or adopted. Prediction of Nitrogen release to the soil from soil organic matter is difficult but can be estimated by the pre-plant soil profile nitrate or pre-sidedress nitrate examination.

Soil Structure and Bulk Density

The structure of the soil refers to the way that individual soil mineral particles such as 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 one of the the primary factor in the development and modification of soil structure.

However, while the binding forces may be of organic or inorganic origins, the organic forces are more significant for building large, stable aggregates in most soils. For instance the 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 organic matter and freezing and wetting as well as drying cycles within the soil as well as compression and deformation forces.

Soil water-holding and snow/drain Catchment

The increasing soil organic matter is an effective method for increasing drought-resistance in arid regions. The challenge that drought has to reduce crop yields is not only due to the cause of irregular or insufficient rainfall, but also because a large proportion of rainfall is lost from the fields as a result of runoff which has acted on it. Some factors, especially those that reduce soil organic matter, such as burning crop residues, excessive tillage, and eliminating windrows reduce water infiltration and increase water runoff. Soil organic matter affects the amount of water in a soil by influencing water infiltration and percolation, evaporation rates, as well as increasing the soil water holding capacity.

The 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 in the soil. If water is running off of a field at a high volume, it cannot overcome the lateral force of water movement and thus will not move vertically down into the soil profile. Erosion of the valuable top soil is a common result 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 manner, increasing soil organic matter and leaving the residue on the soil surface can increase water infiltration to the soil.

Biological Activity

A natural body of degraded mineral or organic material cannot be considered a soil without soil organisms. This emphasizes the significant of soil organisms in the study of soil science.

When examining the life in soils, you should evaluate soil microorganisms such as bacteria and fungi, plants, and fauna which are nematodes, springtails, mites, earthworms, and insects. While microorganisms only make up a small portion of the soil organic matter which is 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, Nitrogen fixation, symbiotic plant relationships, and pathogen control.

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. Soil fauna plays a very 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 the soil.