Rooting depth in cropping systems in the humid tropics in relation to nutrient use efficiency

A simple model is presented for calculating the rooting depth of a crop or crop combination required to intercept leaching nutrients for different climatic and soil conditions. Important parameters in this model are the amount of water moving through the soil, which depends on excess of rainfall over evapotranspiration, and the apparent adsorption constant, which depends on the nutrient and soil type involved. Calculations for three time patterns of nutrient supply in relation to nutrient demand show moderate effects of the degree of synchronization on rooting depth required if a high interception fraction is desired. In shifting cultivation systems a deep-rooted fallow vegetation can recover nutrients leached to the subsoil during the cropping phase. The simple leaching model can indicate the combinations of climate zone and apparent adsorption constant for which such interception is possible. It appears that recovery of leached nitrate is only possible in the subhumid zone. In the humid tropics the continuous presence of a deep root system as part of the crop combination on the field is necessary to use nitrogen efficiently, except when acid soil conditions keep all nitrogen in the ammonium form or when an almost ideal synchronization exists of nitrogen supply and demand during the growing season. Some data are discussed on the root distribution of food crops and on the possibilities to establish a "safety-net" under the crops grown in alleys between deep-rooted hedgerow trees.

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