Constraints to the Maintenance of Organic Matter in Soils
Below are some of the constraints to the maintenance of organic matter in soils;
1. Competing Alternative Uses
In addition to soil incorporation and as mulch, plant and animal remains are also used by farmers for fuel, housing, fencing, animal feed, and industrial purposes.
These alternative uses invariably reduce the number of wastes and residues being returned to the soil; thus reducing the organic matter input into the soil.
2. Bulk
The amount of organic matter needed to achieve most of the benefits discussed above are enormous; sometimes to the tune of 20 tons/ha or more. This bulk involves a high labor force, high haulage cost, high processing (cutting and chopping), and application costs.
The lack of tractive power to replace simple hard tools used by farmers in soil tillage is a great handicap for effective residue incorporation.
Apart from the problem of returning the crop residue into the soil, farmers often do not produce the quantity sufficient to maintain soil fertility and conserve the soil.
3. Time
The practices of mixed and inter-cropping pose another problem in serving as a hindrance to the end-of-season plowing operation to incorporate crop residues. Most of the crops in the mixture have different maturity dates which make proper timing of plowing operations difficult.
The farmer must allow appropriate time between residue incorporation and planting to enable the crops to benefit from nutrients released from the organic matter. Failure to synchronize the time for mineralization with crop needs will lead to a waste of nutrients.
There may also be an injury to the crops following the heat of decompositions of freshly incorporated organic matter when enough time for equilibration is not allowed.
4. The Fallow Period
In very many areas of the country nowadays, the fallow period usually allowed for the farmland to rest and rejuvenates the soil fertility through the return of litter to the soil, which is shortened.
This is due to acute land hunger and increasing demands for farmland to produce enough food for the increasing human population and to meet the increasing demand for industrial raw materials.
As a way of overcoming the problem of soil deterioration brought about by short fallow periods, research now concentrates on bringing the fallow and the cropping periods together in what is referred to as Alley cropping.
Alley cropping is a system in which arable crops are grown in spaces (alleys) between rows (hedgerows) of planted woody shrubs or tree legumes. The fallow species are subsequently pruned periodically to mulch the soil surface during the cropping season.
The prunings serve as shade, provide nitrogen-rich green manure for the companion crops and maintain soil fertility which occurs during the traditional fallow period. The system is especially suited to areas where a minimum or zero tillage system of land preparation is practiced.
Tree legumes already found effective for alley cropping are Leucaena leucocephala and Gliricidia sepium with cereals such as maize, sorghum, or millet as companion crops.
Read Also: General Importance of Soil Organic Matter
5. Quality of Organic Matter
For arable crops to derive nutritional benefits from incorporated organic matter, they must be of high quality. That is, the Carbon/Nitrogen ratio of the organic matter being incorporated into the soil must be below 20:1 – 25:1.
Above this ratio, the native nitrogen will be used up by the decomposing microorganisms (immobilization) instead of releasing nitrogen into the soil (mineralization). The liginin/N or C/N ratio defines residue or litter quality.
The food legumes and leguminous shrubs such as groundnut, soybean, mucuna, gliricidia, and Casia trees are suitable as a high-quality organic matter which should supply almost all the nutrients needed by crops in addition to nitrogen.
Cereal straws provide an example of low-quality organic matter that, when incorporated into the soil requires some initial dose of nitrogen-fertilizer application for decomposition to proceed at the rate that would benefit the arable crop.
To derive maximum benefit of crop residue incorporation it may be necessary to accompany it with the application of inorganic chemical fertilizer.
6. Pests, Diseases, and Toxic Materials
Some of the products of decomposition or organic matter are organic acids which lower the soil pH and enhance the toxicity of micronutrients that are not favorable for plant growth. Some products are chemicals that hinder the growth of other plants.
In conclusion, plant and animal remains such as crop residues, animal and green manures, compost, fallen leaves, and forest litter makeup soil organic matter. Its decomposition is carried out by micro-organisms leading to the release of plant nutrients.
Several environmental conditions could either enhance or slow down the rate of decomposition. We must adopt farming practices that could help to maintain a reasonably high level of organic matter content in our agricultural soils.
Organic matter comes from crop residues, animal and green manures, compost, and other organic materials.
Plant nutrient elements, such as nitrogen, phosphorus, and sulphate, are released into the soil when microorganisms decompose soil organic matter.
Owing to the tremendous benefits of organic matter, efforts should be made to ensure that some reasonable level of organic matter in the soil is maintained. The soil is as fertile and agriculturally productive as the amount of organic matter in it.
Read Also: Factors Affecting the Decomposition of Organic Matter in Soil
