On the basis of organic matter present in the soil, soils are classified as mineral or organic soils. Most of the cultivated land of the world is in the form of mineral soils ranging from 1 % to up to 30 % in organic matter content depending upon their location. Soil organic matter serves as the reservoir of nutrients and water for the crops and Improves soil quality and makes it favourable for crop growth by reducing soil compaction, increasing water infiltration. Yet having all these beneficial effects, it is often ignored and neglected by majority of the farmers.
SOIL ORGANIC MATTER: Soil organic matter is a material that has been originated from living organisms i.e. plants and animals, and then it is buried or incorporated in the soil and as a result its decomposition takes place. Soil organic matter is the main organic component of soil which consists of different processes like animal and plant residue decomposition by soil organisms. Organic matter has been classified into three main categories as
- Organic residues and microorganism
- Humus referred as stable OM
- Active soil OM
The living microbial biomass also known as microorganisms, are the main source of decomposing the plant residues and active soil organic matter by breaking them down. While humus is the decomposed part of the organic matter, which is formed through decomposition of plant and animal residues and it is a stable fraction. The first two fractions contribute to soil fertility as they provide N, P, K etc. as their breakdown occurs. On the other hand, humus as not any effect on soil fertility as it does not decompose any further (stable fraction), so there is no release of nutrients in the soil by humus. But it still helps in soil fertility as it improves soil structure, soil tilth and cation exchange capacity (CEC). The dark colour of the soil is also due to humus.
BENEFITS OF SOIL ORGANIC MATTER:
Scientists have divided benefits of Soil organic matter into physical, chemical and biological benefits. These are as follows:
It improves water infiltration, enhances soil aeration and reduces runoff. It enhances water holding capacity of the soil. It reduces the compactness of clay soils and makes it easier to perform tillage operations. It improves CEC which in turn increases its ability to supply essential nutrients such as, calcium, magnesium and potassium, for a longer period of time and also enhances the buffering capacity of soil. It ensures availability of nutrients to plants over time by decomposing soil minerals. It is essential as it provides food for living organisms in the soil. These organisms are essential for the decomposition of animal and plant residues. Due to this soil microbial biodiversity and activity is increased which in return helps to supress diseases and pests. Due to the action of soil microorganisms, it enhances to increase infiltration and reduce runoff.
FACTORS AFFECTING THE AMOUNT OF ORGANIC MATTER:
Scientists have divided these factors which influence the amount of organic matter in a soil into two main categories as:
Natural Factors: The mechanism of conversion of soil organic matter to different nutrients and then the movement and distribution or uptake of these materials in soil organic pool is influenced by soil type, climate, vegetation and soil organisms. All these factors control the mechanism of soil organic matter decomposition and its availability to plants. Soil microorganisms are the major source for the decay and then cycling of macronutrients and micronutrients, and the soil structure, soil tilth and productivity of the soil is dependent upon the activity of these organisms. These factors are temperature, soil moisture and water saturation, soil texture, topography, salinity and acidity.
It has been observed in several field studies that temperature is an important factor as for as the rate of decomposition of plant residue is concerned. Decomposition of plant residues occurs rapidly in the tropics areas as compared to temperate areas.
Soil organic matter level increases with the increase an in annual precipitation which means more the soil moisture more will be the soil organic matter. Soil microbes perform better at optimal field capacity which means 60 percent pore spaces filled with water. Soil aeration is also needed for the activity of microorganism which is necessary for the provision of nutrients to the plants.
Soil organic matter improves soil texture as it increases soil aeration, water holding capacity etc. Scientist observed that soil organic matter tends to increase as soil clay particles increase and it is because of two factors, first one is that the bonds between clay and OM slow down the decomposition. And secondly, due to more clay particles the formation of aggregates occur which also slow down the decomposition of OM. Scientists have observed that Under similar climate conditions, the OM content in fine textured (clayey) soils is 2-4 times that of coarse textured (sandy) soils. It has been noted that organic matter contents increase with the increase in topography. It is due to the conditions of that area, as it is wetter than the other areas and also has lower temperature. So these two factors affect the content of organic matter in high areas. Salinity (toxicity) and acidity adversely affect the content of OM in a soil. In saline and acidic soils, the environment is not favourable for the microbial activity, so as a result of this adverse effect the OM of these soils is lower.
There is a range of actions of human beings which cause a decrease or an increase in soil organic matter. Though, for increasing or maintaining the soil organic matter required sustainable agricultural practices which include growing green manure crops, effective crop rotations etc. however it is very difficult to increase or raise the OM content of soils which are well aerated e.g., coarse sands, and soils in warm and arid regions because there the added material is decomposed quickly. Soil organic matter content can be raised or increased in soils having cold or temperate climate and having less aeration in the soil.
Here are some practises which cause increase or decrease in soil organic matter and are as,
Decrease in soil om is caused by Less biomass production, low OM supply and higher rate of decomposition due to intensive tillage practices and well drained soils
Agronomic practices that enhance soil organic matter include more biomass production due to more water availability, cover crops, high OM input through compost, mulching and crop residue management and low decomposition rates due to zero or no tillage.
SOIL ORGANIC MATTER AND CROP PRODUCTION:
Soil organic matter is of great significance as regard to crop production. It is primarily soil organic matter which provides essential nutrients and also makes soil conditions favourable for a good crop growth and ultimately a good crop. If a soil has a high amount of organic matter in it, then it is considered high in nutrients and well drained and well moistened soil. Soils rich in organic matter help the plants to grow and furnish in different ways. Some of them are as follows,
- It is a source of inorganic nutrients and also a source of food for microbes.
- It increases ion exchange capacity, it is a chelating agent and also act as buffer.
- Soil aggregation and hence promote root growth
- Water conservation and efficient use of water
- Soils rich in organic matter also help to control plant diseases.
Overall, soils rich with OM produce high yields from both qualitative and quantitative perspective but it is also important to manage the soil in such a way that the soil organic matter of the soil if not improved then at least remain in a beneficial range for crop productivity.
Jabir Riaz*,Imran Ramzan1, and Asrar Masood3.
Author * & Co- Author1 Department of Agronomy, University of Agriculture Faisalabad
3Department of Soil and Environmental Sciences, University of Agriculture Faisalabad