ROOT DISTRIBUTION OF DACTYLADENIA (ACIOA) BARTERI AND SENNA (CASSIA) SIAMEA IN ALLEY CROPPING ON ULTISOL .1. IMPLICATION FOR FIELD EXPERIMENTATION

Root observations were carried out on a typic Paleudult in the humid forest zone of south eastern Nigeria in an alley cropping trial using Dactyladenia (Acioa) barteri and Senna (Cassia) siamea as hedgerow trees and the interrow space planted to maize/cassava intercrop. Rooting depth of D. barteri and S. siamea exceeded 1.6 m. Lateral root propagation of S. siamea was 15 m, and of D. barteri was 5 m from the hedgerows. The whole no-tree control plot was within the range of roots of the adjacent hedgerows. Rooting density and depth of S. siamea in the no-tree control plot was generally higher than of cassava. S. siamea and cassava root density were inversely correlated. Assuming radial symmetry of root propagation, water and nutrients were available from an area 6.1 and 2.3 times larger than the allocated plot size of S. siamea and D. barteri, respectively. Data obtained in alley cropping trials, not considering lateral root propagation, can be invalidated through exploitation of the no-tree control treatment and nutrient acquisition by hedgerow species from a larger area than allocated, thus underestimating and overestimating the performances of the respective treatments. Possible alternative measures for avoiding root interference are discussed but no good solution can be given.