SITE FACTORS ASSOCIATED WITH THE PERFORMANCE OF LEUCAENA-LEUCOCEPHALA(LAM.) DE-WIT AND SESBANIA SESBAN (L.) MERILL IN PURE AND MIXED STANDS IN THE NORTHERN HIGHLANDS OF RWANDA

Alley cropping with leguminous tree hedgerows planted on contours is a n emerging practice in the northern highlands of Rwanda where field sl opes range from 4 to 76% and loss of soil fertility due to erosion is the principal impediment to food production. Sesbania sesban and Leuca ena Leucocephala, the two woody species recommended for alley cropping in the region, do not perform equally well across sites: Sesbania is faster growing, but is more sensitive to pruning than Leucaena. We tes ted the hypothesis that the two species could be grown together to mak e the best use of their relative advantages. Species were planted in p ure and 1:I mixed stands at 26 sites to give a range of altitudes, slo pes, azimuths and soil fertility levels representative of the region. During two cropping seasons',growth spanning 19 months, in pure stands , Sesbania produced more biomass, grew taller and had thicker stems th an Leucaena: for the same species, stem diameter and height were not d ifferent in pure and mixed stands. Biomass and number of poles from 1: 1 mixed stands of Sesbania and Leucaena were equivalent to biomass and pole numbers from Sesbania in pure stands. However, mixed stands prod uced more biomass and pole numbers than Leucaena in pure stands. Growt h of S. sesban was positively correlated with soil pH, potassium and a ltitude; whereas, growth of L. leucocephala was positively correlated with soil phosphorus, but negatively correlated with field slope. All other soil and site variables tested were nonsignificant. The study su ggests that farmers can plant S. sesban and L. leucocephala in 1:1 mix tures in an alley cropping setting and retain levels of biomass produc tion and bean pole numbers equivalent to those from S. sesban alone wi thout compromising, future growth and production by Leucaena. As well, the impact of pH, K and P on growth and production of Sesbania and Le ucaena as a function of field slope and altitude should be determined before extending their use to other regions.