In this article we will discuss about annidation and allelopathy.
Annidation:
The complimentary interaction between intercrops in the intercropping system is known as Annidation. When plants are grown as components of intercropping system, interactions between the components species occur. Such interactions are essentially the response of one crop species to the environment as modified by the presence of another species. One component species helps the other. Annidation occurs both in space and time.
(a) Spatial Annidation (Annidation in Space):
Here the complimentary interactions between the intercrops occur with regard to spatial position (space/place). This principle is used in Multistorey cropping. The component species occupy different vertical layers by spreading out their canopies or roots either in air or in soil.
As far as aerial annidation is concerned the taller intercrops occupy higher vertical layers and shorter intercrops the lower layers. Such taller component species are comparatively tolerant to strong light and high evaporative demand than shorter one.
On the other hand, shorter component species are relatively shade- loving and acclimatised to high relative humidity. The principle of spatial annidation may also occur in soil, where rooting patterns and its systems of component species exploit nutrients from different soil layers. Therefore for efficient utilization of soil resources, intercrops of different root system viz. deep rooted species + shallow rooted species are grown.
(b) Temporal Annidation (Annidation in Time):
The complimentary interaction between the intercrops in the time aspect is called Temporal Annidation. Such intercrops have different natural habit and zero competition. Both the component species have widely varying duration and different peak demands for light and nutrients,
Thus competition between the intercrops is reduced. In the Maize + Urad/Moong intercropping, the peak nutrient demand for Maize is 50 days after sowing (DAS) whereas around 30-35 DAS for Urad or Moong. Other intercropping utilising this principle are Groundnut + Redgram, Shorghum + Redgram, etc.
Allelopathy:
The interactions between the crops may also occur through other means, i.e. competitive or non–competitive. The competitive interaction between the crop species in intercropping and sequential cropping through the release of chemical substances or toxins is called Allelopathy.
Such chemical substances or toxins generally exuded by the roots or produced by decomposition of the residues of one crop species have direct or indirect harmful effect on the other crop species. It affects germination, establishment and growth of the associated crops.
Such crops are unsuitable for intercropping or sequential cropping. Sunflower, Sorghum, Walnut, Cucumber,Peach and Eucalyptus are the examples of such crops which are known for allelopathic effect.
Sunflower affects the associated crops through the release of allelopathic chemicals in the soil by the roots. It inhibits the germination process of subsequent crops. Sunflower residues also produce allelo chemicals during decomposition. Therefore a sufficient time period of 15-20 days is allowed to lapse between the harvest of sunflower and sowing of the subsequent crop.
It results into benefiting the subsequent crop in two ways:
(i) Subsequent crop escapes from the allelopathy effect.
(ii) The rapid decomposition of sunflower residues causes mineralisation of soil nitrogen.
(a) True allelopathy.
(b) Functional allelopathy.
(a) True Allelopathy:
The direct or indirect harmful effect on the other crops through the release of toxic substance as such from the plant.
(b) Functional Allelopathy:
When precursor is released which is converted into active substances by some microorganisms, is categorized under functional allelopathy.
Allelopathy is observed in two ways:
(i) Allelo-Inhibition:
The chemical substances released by one species may inhibit species of plants other than one releasing it.
(ii) Auto-Inhibition:
The toxins may inhibit more strongly plants of the producer species itself.
Allelo-Chemical:
Such types of chemicals, released by plants which show allelopathy are known as allelo-chemicals. The genetic make-up of the plant and the surrounding environment may affect the type and quantity of allelo-chemicals. Some of the organic substances exudated by the roots may inhibit the growth of the neighbouring species. The known examples are walnut, cucumber and peach whose living roots exudate allelo-chemicals and inhibit the growth of the plants growing near them. Some of the allelo-chemicals may be produced by the aerial portions of the plant.
These allelo-chemicals may reach the ground through raindrops, falling leaves or insects or animal activities. The leaves of Eucalyptus globules exudate some allelo-chemicals which drastically reduce the germination of mustard seeds when sown underneath.
The crop residues of sunflower may produce allelo-chemicals after its decomposition which adversely affects the germination, establishment and growth of the subsequent crop. When sunflower stalks are applied as such, it inhibits the establishment and growth of sorghum in an untilled plot.
On the contrary, the non-competitive interaction between the crop species through the release of organic substances may also be possible. The release of hormone- like substances by one crop species may stimulate the growth of the other crop species. It may be called Negative Allelopathy. But the release of Nitrogen from the root nodules of legumes is not considered as negative allelopathy.
Legume Effect:
The beneficial effect of the legumes in any crop rotation and intensive cropping system is termed as Legume effect.
Inclusion of legumes in the cropping system is beneficial in many ways:
(i) Legumes fix atmospheric nitrogen in root nodules and thus improves the nitrogen status of the soil.
(ii) It saves upto 25% of recommended level of Nitrogen application to the associated cereals when grown as intercrop.
(iii) The crop residues and root nodules of legumes release nitrogen during decomposition for the use of the succeeding crop.
(iv) Legumes absorb soil phosphorus more efficiently and part of this mobilized phosphorus in organic form is available to the succeeding crop. It means legumes convert Inorganic Phosphorus into Organic form of phosphorus and thus is able to extract insoluble forms of soil phosphorus.
(v) Legumes have greater root cation exchange capacity (CEC) than cereals. The plants with greater root CEC are capable of absorbing more of divalent cations such as Ca2+ and Mg2+ But they can’t compete effectively with the cereals in the absorption of monovalent cation like K+.
The nutrient status of soil is exhausted by any fast growing cereal. The crop residue of such cereal having wide C: N ratio takes a long time to decompose. During the process of decomposition, part of soil nitrogen is temporarily immobilised affecting the succeeding crop.
Such effect is more pronounced in sorghum in low fertile soil causing to temporary deficiency of nitrogen in the soil for the succeeding crop. To reduce shorghum effect, 25% more nitrogen is applied at the time of first fertiliser dose of the succeeding crop.
It hastens the process of decomposition and overcomes the immobilised nitrogen. Ragi is the exception because its residues decompose rapidly resulting in mineralization of soil nitrogen.
Cotton feeds in the deeper layers of the soil, and removes comparatively smaller quantities of nutrients. The succeeding crop having the shallow root system is able to tap on the unused pool of nutrients in the surface layers of the soil. This effect may be termed as cotton effect.