Research was conducted in Malawi and Tanzania to determine cowpea (Vigna unguiculata L. Walp) grain yield responses to applied P and K, the agronomic and economic efficiency of nutrient application, and the importance of other nutrient deficiencies. Nine site-years of research were conducted. Cowpea did not respond to fertilizer P and K in Malawi. In Tanzania, the yield response to applied P was linear with 21 kg of grain yield increase per kg of P applied. Overall, the effect of P applied at the economically optimal rate (EOR) were mean cowpea grain yield increases and profit to cost ratios (PCR), respectively, of 0.47 Mg ha-1 and 3.2 in Tanzania. Similar effects for K application in Tanzania with an EOR of 17 kg ha-1 were 0.264 Mg ha-1 yield gain and a PCR of 2.3. There were no responses to application of Mg, S, Zn and B. Financially constrained farmers are often not able to apply fertilizer at EOR for all of their cropland. The mean effect of applying K in Tanzania at 50% compared with 100% of EOR to twice as much land was 35% more production increase and 52% more PCR. The results indicate the importance of adequate availability to farmers of straight P and K fertilizers for farmer profitability. Use of multi-nutrient fertilizers implies paying for nutrients that will not give a yield response, thereby reducing the profit potential.
Key words: Economically optimal rate, net return to fertilizer, optimization, phosphorus, potassium, response functions.