Agronomic evaluation of phosphorus fertilizers developed from Sokoto rock phosphate in Nigeria

Sokoto rock phosphate (SRP) obtained from Sokoto in the Northern Nigeria was evaluated with some other phosphorus (P) sources viz: Partially acidulated rock phosphate (PARP) and single superphosphate (SSP) in the greenhouse and field studies. These fertilizers were also compared with nitrogen-phosphorus-potassium (NPK) 11-12-11-9.7S-1.4Zn and NPK 10-20-5-6.5S which have 80% of their P as rock phosphate and were formulated through compaction. The fertilizers were applied in the greenhouse at 0-400 mg kg soil on the Oyo Arenic Haplustalf and Alagba Kandiudult soil. Field trials were carried out at four locations-at Ikenne in the humid, Samaru in the subhumid, Gumi and Gusau in the semi-arid zones of Nigeria. The fertilizers were applied at 0-150 kg P2O5 ha(-1) in the humid zone and 0-100 kg P2O5 ha(-1) in the subhumid and semi-arid zones. Maize was used as test crop in most sites except at Samaru where sorghum was planted. The results of the greenhouse study showed that on the Haplustalf, PARP, and NPK 10-20-5-6.5S gave almost a similar relative agronomic effectiveness (RAE about 70%) as SSP, which was followed by SRP and NPK 11-12-11-9.7S-1.4Zn (RAE between 50 and 60%). On the Kandiudult, the RAE of the fertilizers increased significantly. The PARP and NPK 10-20-5-6.55S gave high effectiveness (RAE about 90%) relative to SSP. The field trials results indicated that ground SRP was suitable for direct application on slightly acid soil in the humid zone (annual rainfall >1,200 mm). Its efficiency was fairly moderate in the subhumid and quite low in the semi-arid zones (annual rainfall <900 mm). The PARP gave higher RAE than SRP and had almost similar efficiency as for NPK 10-20-5-6.5S. The PARP was well comparable to SSP in the humid and subhumid zones and was fairly comparable to the later in the semi-arid. This suggests that PARP may be suitable for humid and subhumid zones and the physical quality was also superior to SRP and SSP. Application of SRP on soils in the semi-arid zones of low rainfall gave relatively low yields which could be due to inadequate moisture availability required to enhance P solubilization.