Weeds are plants, normally of an indigenous type, which become established on land where they are not wanted. Many weed plants are well adapted for seed dispersal and the efficiency of this mechanism makes them effective in becoming rapidly established on land which is not fully occupied with a crop.
Many weed seeds also have a fairly long period during which they can remain dormant in the soil, so that they may remain ungerminated until conditions favour their activity, Weeds often have efficient root systems so that they can obtain water, even from relatively dry soils.
Where weeds have become well established, their roots may dry out the upper layers of soil to such an extent that a following crop may yield considerably less than it would have done if the ground had remained fallow.
Weeds are therefore not generally recommended as a soil cover crop, much better results can be obtained by establishing a leguminous cover crop. Mulching, which has already been discussed in Chapter 2, is also an effective means of checking weed germination and growth. Weeds also compete with plants for soil nutrients.
As with all cultivated crops which are harvested at maturity and removed from the land where they have been grown, the removal of established weeds from land which is required for cultivation deprives the soil of nutrients which the weeds have absorbed. In addition to competing with plants for water and nutrients, weed roots also require oxygen for their respiration. If they are growing close to crop seedlings, the more vigorous growth of the weed roots can result in depriving crop seedling roots of their required supply of oxygen. The result of this competition may not be immediately obvious but may become so later in the growth of the plant. Young cocoa seedlings, for example, are particularly sensitive to weed competitor rains begin and may compete seriously with annual crops sown at this time. Crops particularly sensitive to early weed competition are maize, groundnuts and cotton; later weeding, when the crops are established does not compensate for this early check to crop growth. Early soil preparation, therefore, is recommended so that the soil can be prepared and cultivated to suppress the early growth of weeds before a crop is sown. Excessive soil cultivation, however, should be avoided, since this could lead to a loss of structure, particularly as this loss is also promoted by heavy rainfall.
ion which may not become apparent until the young trees produce yields which are lower than those from trees which had no weed competition. An additional factor of weed competition is their ability to grow more rapidly than many crop plants, resulting in a shading of the crop seedlings which reduces yields due to a lack of sunlight.
Weed seedlings usually germinate rapidly as soon as their ins begin and may compete seriously with annual crops sown at this time. Crops particularly sensitive to early weed competition are maize, groundnuts and cotton; later weeding, when the crops are established does not compensate for this early check to crop growth. Early soil preparation, therefore, is recommended so that the soil can be prepared and cultivated to suppress the early growth of weeds before a crop is sown. Excessive soil cultivation, however, should be avoided, since this could lead to a loss of structure, particularly as this loss is also promoted by heavy rainfall.
