Tracer studies with 32P on the distribution of functional roots of Arabica coffee in Kenya

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Abstract
SUMMARY: Seven experiments were done over a period of 21 months at Ruiru, Kenya, using 4 mCi/tree of 32P tracer to study the distribution of functional roots of mature Arabica coffee trees. Tracer was placed at sixteen equally spaced sites around individual trees at one of five depths (to 180 cm) and three distances (to 135 cm = mid‐row). Thereafter, samples of three‐leaf shoot tips were collected at 14‐day intervals for up to seven occasions and the radioactivity assessed after drying and ashing.There was negligible activity at any time at 180 cm depth but at other depths, and at all three distances, the relative level of activity changed markedly with season. After prolonged drought relatively high root activity was found at mid‐depth, near to the trunk; after the soil was re‐wetted by rain most root activity occurred in the topsoil at the quarter‐row distance; after the soil profile had been wet for some time there was a more general distribution of functional roots. Some departures from this general scheme are discussed as are practical implications and the need for further investigations.