Three broad categories of competition can be recognized:
1. Actual yield of each species is less expected. This is called mutual inhibition. This is rare.
2. The yield of each species is grater than expected. This is called mutual co-operation. This cannot unusual.
3. One species yield is less and the other is less and other is more than expected. This can be termed compensation.
The species which yields more than expected is believed to have greater competition ability and called the dominant species. The other species is called the dominated species.
Legume effect: they are widely used for food, fodder, shade, fuel, timber, green manure and cover crops. Legumes either increase the soil N status through fixation, excretion or in absence an effective N fixing system, compete for N. thus they have potential for self-sufficiency for N, the nutrient most limiting to productivity. Nutrient self-sufficiently is desirable characteristics of agronomic ally sustainable cropping system. Inclusion of legumes in intensive cropping systems has many ramifications. They are less demanding on soil resources, many of them can tolerant some amount of shading, fix atmospheric N in root nodule contributing part of N to associated crop and improving soil fertility, capable of extracting less soluble forms soil P and K, thus, making it available to other crops and also complete better for higher valence cations like ca and mg due to more CEC of roots of legumes. But in low K soils they are likely to be deprived of their due share of K, especially in mixture with cereals. Hence with the overall view of maintaining soils fertility and economizing on fertilizer, it is beneficial to include legumes as components of intensive cropping systems.
The quantity of N fixed by the legume component in cereal legume intercropping depends on the species, morphology, and density of legumes in mixture, type of management and competitive abilities of the component crops. Legumes of indeterminate growth are more efficient in terms of N fixation than the determinate types (fore and stern, 1957) shading by the cereals reduces both the seed yield and N fixation potential of the companion legume (whoa and miller, 1978).
Interaction in Sequence Cropping:
Competition for light water and nutrients as in mixed crop communities does not occur when sole crops are grown in sequence. It occurs only in relay cropping where there is a short span of overlapping between two crops in a sequence and the relay crop experience the shortage of light especially at seedling stage. Such type of competition may be minimized by proper choice of crops and varieties and adjustment of time and method of planting.
In intensive multiple cropping involving two or more crops in sequence, the main objective is to harvest as much solar energy per units area per unit time as possible. The important purpose in sequential cropping is to increases the use of solar radiation. It is achieved by longer field duration and rapid ground coverage crops are raised one after another to keep the land occupied by the crop for longer period. In rice based cropping system, the solar energy use efficiency ranged from 1.58 to 2.03 % of PAR in U.P inclusion of a C4 plant in summer increased the efficiency. If the crop development is slow, much of the solar radiation reaches the ground, favoring weed growth and increasing evaporation losses from the soil surface.
In sequential the preceding crop has considerable influence on the succeeding crop mainly due to
i) Changes in soil condition,
ii) Complementary effect such as release of N from the residues of the previous crop, particularly legume,
iii) Presence of allelopathic chemicals,
iv) Shift in weeds,
v) Temporary immobilization of N due to wide C: N ratio of these residues and
vi) Carry over effect of fertilizer, pest and diseases.
Field preparation is difficult after rice crop since structure is destroyed due to pudding. Crops like sorghum and sunflower leave toxic chemical in the soil, which do not allow germination of subsequent crop. The phosphorous applied to the previous crop is available for the succeeding crop. Weed number and species differ in the succeeding crop due to the effect of the previous crop. Wheat crop after rice suffers from high density of weed phallaris minor. The pest and diseases in crop stubbles and other residues of the previous crop may infect subsequent crop.
Cotton rose after legumes performed well than when it is followed cereals, particularly sorghum.
Prasad and power ( 1991 ) after reviewing number of experiments concluded that legume N contribute substantially to the N needs of succeeding crops in rotation, but the amount contributed would depend on the time of legume growth, amount of bio-mass incorporated and species of succeeding crop grown.