Effectiveness of phosphate rock on ferralsols in Tanzania and the influence of within-field variability

Crop yields in Tanzania are often limited by P deficiency. Direct application of the locally mined Minjingu phosphate rock (MPR) is considered a possible option in addressing the problem. Being poorly soluble, it is more effective in low pH soils with sizeable P and Ca sinks. In soils rather high in pH, mechanisms are required to promote its dissolution. This research was initiated with the overall objective of increasing our understanding of the multiple interactions affecting the availability of P from MPR in the soil-plant system. This knowledge is required to enable optimal exploitation of the various factors influencing MPR effectiveness for increased crop production. The research program consisted of one laboratory experiment, six greenhouse experiments and four field trials.

The laboratory and greenhouse experiments dealt with single factors influencing the effectiveness of PR under controlled conditions. From the laboratory experiment it was established that the high content of CaCO ₃ in Minjingu PR was the major factor that determined the amount of HCl extractable P. However, agronomically it did not differ from Khouribga and Mali PRs, which have relatively lower CaCO ₃ content. High application rates lowered the effectiveness of PR pointing to a need to balance application rates with proton supply. In a study relating yield and soil pH, increasing dry-matter yield of maize, cowpea and pigeonpea could not noticeably affect the soil pH mainly due to the pH buffer capacity of the soils. However, when intercropped with maize, cowpea or pigeonpea took up more P from MPR and even improved the uptake of MPR-P by maize. This point to a possible proton induced dissolution of MPR by the legumes. Hence, when intercropping involves legume crops capable of fixing N lower rates of PR can be used which is an attractive option for resource poor farmers. A study on the response of maize, cowpea and pigeonpea to MPR and TSP on soils of low and high pH showed that response to MPR was stronger in the low than in the high pH soil. Both legumes gave a higher DM production with MPR than with TSP on the high pH soil and yields were higher on the high than on the low pH soil. This indicates that the legumes prefer a high soil pH to a low soil pH and are able to make use of MPR at a relatively high pH. The later is possible when the legumes can modify rhizosphere soil conditions with respect to pH.

Maize dry-matter yield response to P on soils with different pH and available P was observed where soil available P was low. There was no yield response to P when P-Bray-I was > 7 mg kg ^-1 (P-Olsen > 10 mg kg ^-1 ). Utilization of absorbed P was better from MPR than from TSP in low pH soils indicating a liming effect by the MPR.

Field experiments dealt with multiple factors influencing the effectiveness of PR. The influence of spatial soil variability on the effectiveness of PR and crop performance was studied using the Post-mortem Residual Analysis and Nearest Neighbor Means techniques. These techniques were effective in isolating environmental from treatment effects and they were more useful in the large than in the small trials. A study on the method and rate of P application showed that method of application was more important for MPR in high pH soils and that response to MPR will be obtained at modest rates. The residual effects of TSP and MPR were almost the same. Combined application of MPR and the acidifying fertilizer sulfate of ammonia showed that there is little scope for improving the effectiveness of MPR through the acidifying effect of sulfate of ammonia. Combined application of MPR and TSP gave best results in low pH soils when both of them were applied using the same method. Meanwhile, incubating MPR with farmyard manure could not stimulate MPR dissolution given the high pH of the two materials even after 40 days of incubation (pH (H ₂ O) = 8.35). It is concluded that the use of MPR could be extended to less acidic P deficient soils when mechanisms are employed that could stimulate its dissolution. They include use of low amounts of MPR (low input strategy), incorporating legumes in the cropping system and using the same method of application of mixtures of MPR and soluble P fertilizers.

Keywords: Minjingu phosphate rock, triple superphosphate, Rhodic Ferralsols, farmyard-manure, protons, dissolution, legumes, maize, management options, spatial soil variability, Tanzania.