Landform and phosphorus effects on nitrogen fixed by annual clovers and its contribution to succeeding cereals in the Ethiopian highlands

Poor drainage of Vertisols and nitrogen (N) and phosphorus (P) deficiencies are some of the major constraints to food and feed production in the Ethio pian highlands. The effects of soil drainage and P supply on N fixation by annual Trifolium species and their residual value to succeeding crops was i nvestigated in field experiments on Vertisols at Addis Ababa and Ginchi. In one experiment, the Trifolium spp. fixed 55-122 kg N/ha when P fertiliser was applied, and the increase in grain yields of oats following different c lovers ranged from 20% for T. steudneri to 55% for T. rueppellianum compare d with yields of continuous oats. In another experiment, landform treatment s designed to effect drainage had no significant effects on N fixation and clover dry matter, or on yields of wheat following the clovers. Phosphorus applied at 50 kg P/ha increased N fixation from 11 to 72 kg N/ha on the bro adbed-and-furrow landform and from 10 to 64 kg N/ha on the flat landform, a nd herbage yield responded similarly. Only the wheat following P-applied cl over, particularly when supplemented with N fertiliser at 50 kg N/ha, had s ignificantly higher wheat grain yields compared with continuous wheat grown without N fertiliser. These results confirm the critical importance of P i n increasing the productivity of clovers in the Ethiopian highlands.