Crop rotation has been practiced from the time immemorial, and every farmer is quite familiar with it. Crop rotation may be defined as a system of raising crops in a regular order one after the other on the same piece of land keeping in view that fertility of land may not be adversely affected and farmers profit out of land may not be reduced. We classify the crops according to the residual effect on the soil, i.e., Exhaustive rotation: It includes more number of exhaustive crops which take up the plant food nutrients and leave the soil poor in fertility, e.g., wheat, cotton, field mustard, and maize. Restorative rotation: It includes those crops which improve the soil fertility. These include leguminous crops and exhaustive crops. Crop rotation limits the build-up of weed populations and prevents major weed species shifts.

In a crop rotation, the timing of cultivation, fertilization, herbicide application, and harvesting changes from year to year. Rotation thus changes the growing conditions year to year, a situation to which few weed species easily adapt.

Principles of Setting Different Crop Rotations

•Crops belonging to the same natural order (family) should not follow one another.

•The deep-rooted crop should be followed by shallow- rooted crop and vice versa.

•Exhaustive crops (e.g., cereals which take more nutrients from soil and do not add anything to it) should be followed by restorative crops (e.g., legume crops which not only take nutrients from the soil but at the same time also add nutrients to soil).

•Green manure preferably legume crops should be included in the rotation.

•The fodder crops should also be included in the rotation.

•Diseases susceptible crops should be followed by disease-resistant crops.

•Alternating crops with different peak requirements of labour, water, and fertilizer, etc. should be included in crop rotation.

•Long duration crops should be followed by short duration crops.

•Crops with problematic weeds should be followed by clean crops/multicut crops (fodders) and other dissimilar crops.

•Broadleaved crops should be rotated by narrow-leaved crops.

•The crops with taproot should be followed by the crops with fibrous root system. This helps in proper and uniform use of soil nutrients from different depths.

•Both wide spaced crops and thickly planted crops should be included in rotation for control of weeds. Wide spaced crops control weeds due to frequent interculturing and dense forage or legume crops controls weeds by suppressing weeds.

•Effect of previous crop on succeeding crop should be considered for obtaining maximum yield and good quality of produce.

•The selection of crops should be problem based, e.g., on sloppy lands, an alternate cropping of erosion- promoting (erect growing crops like millet, etc.) and erosion-resisting crops (spreading types like legumes) should be adopted.

 •Selection of crops should suit the farmer’s financial conditions.

•In case of rainfed farming, some minor winter crops requiring less moisture like pulses may be grown on moisture retentive soils after harvest of summer crops.

•Crops with minimum water requirements should be grown in periods of water deficiency in canal irrigated areas.

•The rotation should be flexible enough to allow the farmers to make changes in the selection of crops with fluctuation in the economic condition of farmer and market demand and price.

Importance of Crop Rotation

Crop rotation helps return nutrients to the soil without synthetic inputs. It is an important strategy for managing some diseases, weeds, and insect pests of vegetable crops. Crop Rotation also helps to battle against the forces of erosion. Rotating crops helps to improve soil stability by alternating between crops with deep roots and those with shallow roots. By planting crops like legumes, for example, one can increase nitrogen in the soil as they contain nitrogen-fixing bacteria that fix nitrogen naturally into the soil. Each crop type adds up or absorbs different soil nutrients to the soil; therefore, it needs a mix up of a variety of plants to make them more balanced. The constant application of fertilizers to soils causes soil leaching, which is the excessive build-up of nutrients in the soil to a toxic and harmful level that does not allow plants to grow well. Crop rotation n increases the nutrients in the soil, and it prevents the accumulation of toxic chemicals or substances secreted by some crop plants. It helps in maintaining the amount of organic matter in the soil. Crop rotation such as cereal-cereal or non-legume-non legume results in reduced population of soil organisms. Population and activity of soil organisms can be increased by following legume cereal crop rotation. Biological activity can also be stimulated by simply allowing a grass cover to develop, through forestation or by adding organic material through rotation. Effect of Crop Rotation on Soil Properties

Soil organic matter is considered a key attribute for a sustainable agricultural production and influenced by the quantity and quality of the crop residue deposited on the soil surface. Therefore, different crop rotations could change the soil organic matter pools. Soil N levels often increase when N-fixing legumes are included as rotation crops. Deep-rooted legume crops, such as alfalfa, clover, beans, peas, chickpeas, lentils increases residual soil N and thus increase N availability to subsequent shallow-rooted crops Although there are obvious effects of rotation on soil mineral status, particularly N, researchers have concluded that there is a rotation effect beyond that which can be explained by soil mineral status alone. Crop rotation improves soil structure, increase soil organic matter levels, increase water use efficiency enhance mycorrhizal associations and reduce grain yield variability. Crop rotations also provide better weed control, interrupt insect and disease cycles, and improve crop nutrient use efficiency.

 Table-1: Crop rotation impacts on soil biological quality indicators at different depths (Averaged across tillage)

*See the table 2 foot note

Conclusion

Crop rotations provide us with the opportunity to profoundly modify the soil environment. The rate and direction of a modification can have a major impact on system level responses, such as nutrient exchange and storage and soil quality. The sequence of crops in rotation not only influences the removal of nutrients from a soil, but also the return of crop residues, the development and distribution of bio-pores and the dynamics of microbial communities. Farmers require rotations that are flexible and economic in order to respond to market demands. In order to manage the rotation, the farmer will need to measure progress, evaluate crop performance, and modify management as required. Thus, measurement of the benefits of a particular rotation on soil structure and the evaluation of these benefits in terms of the growth and productivity of the component crops are important. Future research directed towards a better understanding of the impacts of crop rotation on soil and plant responses will enable us to develop systems in the future with improved environmental, agronomic, and economic performance.

                        Table-2: Crop rotation and soil depth interaction Source- (Aziz et al., 2009)