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Crop Rotation System: A Productive and Sustainable System for Food Crops in Kenya
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By: Hillary M. O. Otieno

Crop Rotation System: A Productive and Sustainable System for Food Crops in Kenya

Crop rotation is a system that involves production of different crop species on the same piece of land at alternate seasons. The impact and benefits of the system which is always hailed by researchers is greatly linked to its design in terms of crops included- with researchers concluding that nitrogen fixing legumes are the best to be rotated with cereals for the system as they offer several benefits in terms of efficient use of soil water and nutrients, improved soil physical and chemical characteristics and reduced soil erosion. It is one of the greatest principles under Conservation Agriculture.

Crop rotation has great short term and long term influences for better crop growth and yield production. The system helps in soil and water conservation through greater soil surface cover that minimizes evapotranspiration; and salinity problems.

Weeds, especially those that are specific to either legume or cereal crops, are effectively controlled­ through this practice. As explained by various scientists, the system effectively changes the pattern of disturbances thereby diversifying selection pressure.

Crop rotation has also been found to increase the microbial diversity of predatory organisms thereby offering constant check on the population pressure of some of the common pathogenic microbes and pests. This mechanism has been confirmed on several pathogens with narrow and specific host ranges like nematodes.

The system plays a major role in soil fertility management through inclusion of nitrogen fixing leguminous plants into the sequence. The mechanism is through modification of soil structure which has substantial effects on soil nutrient cycling.The important interactions between soil physics and biology ensure availability of plant nutrients through increased soil nitrogen fixed as by Rhizobia bacteria and soil organic matter residues. Scientist have confirmed and reported a range of up to about 280 kg N per hectare from annual legumes such as peanuts, cowpeas, soybeans, and fava beans and up to 560 kg N per hectare from perennial forage legumes such as alfalfa, sweet clover, true clovers, and vetches legumes in a season, depending on the presence of right bacteria for colonization, soil chemical, physical and biological characteristics. This is an important benefit since it brings the economic aspect of the system. Farmers are relieved of pressure to apply nitrogen from various commercial sources like urea, calcium ammonium nitrate etc. for these legumes and other subsequent cereal crops.

Under cereal-legume rotation system, cereal residues which are more fibrous stabilize soil and provide protection against erosion through the period. The soil also develops better structure and increase aeration and water infiltration for better root growth and development.

Cumulatively, these benefits result in high crop yields which is of great importance to farmers. For example; higher accumulation of crop biomass than when grown in monoculture; and double yields of the cereal crops that followed legume crops have been reported.

Despite all these clear and great evidences, why is crop rotation not common in Kenya? Well, the system is faced by a number of challenges: Farmer own small farm sizes, less than 1 hectare, which do not permit adequate room for rotation leading to intensive intercropping as families struggle to provide adequate food for their members; farmers’ practice of uprooting legumes during harvesting which are then carried away from the farms leaves no residues for nitrogen release and organic matter build up; inadequate and competing uses for these crop residue- farmer are using these same crop residues as source of fuel, building material and important animal feed. This leaves less or none at all for better soil cover and release of nutrients upon decomposition.