The practice of growing different kinds of crops, one at a time, in a definite sequence on the same piece of land is referred to as crop rotation. In designing a good crop rotation, the farmers must decide what crops to have in the rotation, in what sequence the crops should occur, and for how many years or season each cycle of the rotation must run.

A good rotation that provides for maintenance or improvement of soil productivity usually includes a legume crop to promote fixation of nitrogen, a grass or legume sod crop for maintenance of humus, a cultivated or inter-tilled crop for weed control and fertilizer. Perennial legumes and grasses may leave 2 to 3 tons of dry weight per acre of roots residues in the soil when plowed down.

Factors that Affect Crop Rotation

The choice of a rotation for a particular farm depends upon the following:

Adaptation of the crops to a particular soil, climate, and economic conditions.
Prevalence of weeds, plant diseases, and insect pests may also limit the kinds of crops that can be grown in a locality.
Crops may be selected for rotation so as to spread labor throughout the year.

Factors to Consider in Deciding the Sequence of Crops (Principles of Crop Rotation)

The target crop (the main crop) should be planted immediately after the legumes or fallow period. At a time the fertility of the soil is at its peak and the optimum realization yield of the target crop is possible. Crops which are known to have a high demand for nutrients are also timed for the first season after the fallow.

Crops which are deep feeders should alternate with shallow feeders. In this way, nutrient removal occurs uniformly from the various soil layers rather than occurring in only one layer.

Crops that are botanically similar or are likely to be attacked by the same diseases and pests should not normally follow each other in the rotation. Yams, for example, should not follow cowpeas in rotation if the root-knot nematode is prevalent, as the nematodes left over from the cowpea crop will severely reduce yam yields. However, if the nematodes problem does not exist in the area, yam could conveniently follow cowpeas.

The number of years for which each cycle of the rotation should run is determined by the number of crops in the rotation, the length of their growing seasons and how frequent the farmer can grow the target crop without running into problems of disease and soil fertility. For example, the time interval between the harvesting of the target crop and its being planted again on the same piece of land should be long enough to prevent the carryover of pathogens in crop residues from one cycle to the next.

Crop Rotation: Types, Advantages and Disadvantages

Types of Crop Rotation

In planning crop rotation, the farmer may decide to consider his entire field as one plot. He then rotates the crops in sequence on the field. At any given time, there is only a crop on a field, and that crop would not return again until the next cycle some years later. This is commonly practiced in Northern Nigeria.

Example of a 3-year crop rotation as practiced in Savanna zone of Nigeria.

Table 2.1

	Year 1	Year 2	Year 3
Winter (with irrigation)	Rice	Vegetables	Wheat
Summer (Rainy season)	Cotton	Groundnut/maize	Beans/millet

This system, however, has certain disadvantages:

The growing of one crop means that the demand for labour occurs in peaks. Labour demand is more evenly spread if many crops are grown simultaneously.
The risk of crop failure is ever present, and the risk is greater where only one crop is grown.
Since each crop occurs on the farm only once every several years, specialized facilities for the target crop, can only be utilized once in several years, a situation which is definitely inefficient.

Most farmers who practice crop rotation find it more convenient to divide their field into as many plots as there are years in the rotation.

The farmer then starts with a different crop on each plot and progress through the rotation. In the crops are present on the farm at any given time. Example of such type of rotation is given below.

Example of a 3-years crop rotation found in guinea savanna region of West Africa

Table 2.2

	Year 1	Year 2	Year 3
Plot A	Cotton	Guinea corn	Groundnuts
Plot B	Guinea corn	Groundnuts	Cotton
Plot C	Groundnuts	Cotton	Guinea corn

Advantages

It is an effective means of controlling diseases and pest. The pathogens and pests of a particular crop are more likely to die off when their host crop is followed by a completely different non-host crop. Many insects are destructive to only one kind of crop. The life cycle is broken when crops grown are unfavorable to the development of the insect pest. Cotton root-knot can be reduced by the growth of immune crops in the rotation.
Crop rotation is the most effective practical method for controlling many farm weeds. Some weeds are particularly adapted to cultivated crops, the absence of such host crop in the field for many years due to rotation, effectively control the weeds. Rotation may include smother crop as a means of controlling certain weeds.
The type of crop rotation where the field is divided into several plots, offers the farmer some insurance against crop failure, and enables him to spread out his labour needs.
Crop rotation is an effective means of reducing erosion in comparison with continuous cropping. Grass legumes mixtures in a rotation have been very effective in the reduction of erosion.