Evaluation of the efficacy of Malawi Tundulu phosphate rock for maize production

The objectives of this research were to evaluate the extent of phosphorus deficiencies in Malawi as related to maize (Zea mays L.) production, and to assess the value of Malawi Tundulu phosphate rock (PR) in supplying P for maize production. Soil samples from 303 unfertilized farmer sites in major maize producing regions in Malawi showed that 53% of fields tested high in P. An experiment was carried out to compare the effects of Malawi Tundulu PR, a low-reactivity P source, and triple superphosphate (TSP) on maize yields. The trial was carried out at 4 locations testing low to medium in soil P A partially confounded factorial design was employed using 3 P rates (8.8, 17.5, and 35 kg P ha(-1)), 2 placement methods (point placement and banding), and 3 N fertilizers of different acidulating abilities (calcium ammonium nitrate (CAN), urea, and ammonium sulfate (SA)). Residual effects were monitored the following season. Both TSP and PR were more effective when band applied, compared to the traditional point application. Response to banded PR was site-specific, and was related to P sorption capacity, with best responses occuring on low P-sorbing soils. Rates of P above 8.8 kg ha(-1)] did not generally improve yields the first season, but did result in increased residual response. While the most acidifying N source, SA, did not improve yields significantly over the least acidifying, CAN, yields using both SA and CAN were significantly better than yields under urea. This is probably due to higher ammonia volatilization losses using urea. A further trial implemented at 2 sites in 1992/93 indicated that broadcasting PR gave greater yields than did banding. The results indicate that Tundulu PR, applied as a band or broadcast, has potential for replacing conventional P fertilizers on some soils in Malawi. Lower P rates than the currently recommended 17.5 kg ha(-1) may be equally effective and more economical for either P source.