5. INTERACTION BETWEEN TREES AND CROPS

5.3  Adverse effects of trees on soils

Trees can sometimes have a direct adverse effect on soil properties with consequences for crop performance. Such adverse effects that are relevant in the tropics are:

• Loss of organic matter and nutrients in tree harvest

• Nutrient competition between trees and crops

• Moisture competition between trees and crops

• Production of substances which inhibit germination or growth.

Loss of organic matter and nutrients in tree harvest

When trees are harvested and the biomass is removed, some loss of organic matter and nutrients occurs. However, there is a great difference between the situation where only the stem wood is removed and where the whole tree, including leaves and small twigs, is removed. In the latter case, a lot of nutrients are also removed, but use of the wood alone does not involve such a large loss of nutrients. If leaves and twigs are used for fodder and the animal manure is returned to the soil, the nutrient loss can be further significantly reduced.

Nutrient competition between trees and crops

In general, trees are less demanding on nutrients than crops. It is generally desirable that trees growing with crops should have deep roots and few roots near the soil surface where most crop roots are found.

Moisture competition between trees and crops

Moisture competition is discussed in Section 5.4.

Production of substances which inhibit germination and growth

This phenomenon is sometimes called the allelopathic effect. Some tree species, e.g. Eucalyptus spp. and Gmelina arborea, have been reported to produce substances that may inhibit the growth of crops. In practice it is difficult to distinguish such effects from the effects of competition between trees and crops for nutrients and moisture.

Footnote__________

Sections 5.2-5.3 are based largely on Young, 1989.