Agronomic and economic evaluation of organic and inorganic sources of phosphorus in western Kenya

Many soils in the highlands of East and Central Africa are depleted of soil nutrients, particularly P. Our objective was to compare cattle manure, Calliandra calothyrsus Meissner leaf biomass, and triple superphosphate (TSP) as sources of P for maize (Zea mays L.), both individually and as mixtures of organic (manure or calliandra) and inorganic (TSP + urea) sources. Field experiments were conducted on a Kandiudalf at two sites in western Kenya. Net benefits were computed as the difference between the value of additional maize yield accruing from nutrient inputs and the associated additional costs. Maize grain yield was 0.6 Mg ha(-1) for application of urea without P. Application of 10 kg P ha(-1) as organic, inorganic, and mixtures of organic and inorganic sources significantly increased maize yield. Grain yield for manure at least equaled and sometimes exceeded (P less than or equal to 0.05) yield for calliandra and TSP + urea. Net benefits in U.S. dollars (USD) for two seasons were highest for manure spot placed in the planting hole (293 USD ha(-1)), broadcast manure (255 USD ha(-1)), and broadcast TSP + 44 kg urea-N ha(-1) (313 USD ha(-1) at P = 30 kg ha(-1) and 98 USD ha(-1) at P = 10 kg ha(-1)). Net benefits for calliandra leaf biomass were highest (136 USD ha(-1)) when biomass was valued at cost of production and integrated with TSP, such that it provided all the N for maize and TSP provided the additional P not supplied by calliandra. Calliandra valued at its opportunity costs a protein supplement for dairy cattle was not an economic source of P. Sensitivity analyses suggest that organic materials most suitable for use as P sources have high P content and low cost of production.