S U M M A R YSeven 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.

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S U M M A R YUptake of P " from labelled KH a PO 4 solutions, injected in the top-soil at points distributed over 4000 cm." in the zone of high root activity, was measurable soon after application with accumulation of P 88 in leaves increasing thereafter with time. The recently-acquired P 8 t was distributed uniformly in the tree canopies. Uptake of P 3a reached a maximum when the number of injection points was increased from 15 to 30 per tree, with 45 and 60 injection points leading to little further change in P " content of leaves. In the zone of high root activity, strip application of P " was as efficient as application in a circle around each tree. The practical implications of these observations in relation to P-fertilization of coffee are discussed.Studies on the distribution of the functional roots of coffee trees, carried out in Kenya during 1968/69 with P 32 -labelled solutions of KH 2 PO 4 injected at 16 equidistant points around the tree trunk, gave information on the pattern of root activity throughout the soil profile down to a depth of 180 cm. and to a distance of 135 cm. from the trunk of the tree (Huxley et al., 1974). The P 32 contents of leafy shoot tips were used as an index of P 32 uptake from the sites of injection. Variability in the level of radioactivity in samples from replicate trees was very high, and the results were not thought to permit quantitative assessment of the uptake from each site, but the results were useful for making a rough prediction as to the best method and timing of P-fertilizer application in the field for maximum utilization by coffee trees. Consequently, it was decided to re-evaluate the results of root distribution studies carried out from March 1969 to January 1970 with a view to showing the rates of P 32 uptake from each individual site.For practical purposes of fertilizer application in a coffee plantation, root activity below the upper layer of the soil did not seem to warrant exploration, since fertilizer is not normally injected into the sub-soil. The present investigation was therefore limited to the top-soil. The rates of P 32 uptake from 15 cm. depth and from distances of 30, 82 and 135 cm. from the trunk of the tree (Fig. 3) showed that the activity of surface roots was highest during wet periods (March/April 1969) and at 82 cm. from the tree trunk. The results also showed that there was reasonable P 32 uptake from 82 cm. during July/August/September and November 1969 (lower than during March/April but higher than from the other two distances). These findings suggest that the 82 cm. point of injection is within the zone of highest root activity, and that the applied fertilizer is more efficiently utilized during wet periods (i.e. periods of highest root activity) from this zone. •Present address: 111 Bedford Hill, London SW12. 2 R. Z. PATEL AND A. M. KABAARAThe standard practice of application in Kenya is to broadcast fertilizer from the middle of the row of coffee trees (by mechanical means in Estates). Effectively, this is a form of band placement, as...

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