Effect of rock phosphate and superphosphate on crop yield and soil phosphorus test in long-term fertility plots

A long-term (1968-1987) field study using corn-soybean in rotation was conducted to compare the effect of rock phosphate (RP) and superphosphate (SP) at two lime levels on crop yield, soil available phosphorus (P) as Bray P-1 (0.025M HCl + 0.03M NH4F) and Bray P-2 (0.1M HCl + 0.03M NH4F) tests, and on the relationship between crop yield and available P tests. Treatments included a control, application of RP and SP ranging from 12 to 96 kg P2O5 ha(-1) yr(-1), and combinations of RP with SP or sulphur at various rates. The RP was applied once in 1968 at 8 times the annual rate while SP was applied annually until 1985. Corn and soybean yields increased with P application, more with SP than with RP. Bray P-1 and Bray P-2 increased linearly with the amount of P applied as SP or TCP. A significant correlation (r > 0.64) was found between corn yield and Bray P-2 at low lime level with both P sources. In contrast, a poor correlation (r < 0.50) was found between soybean yield and soil P tests. Both RP and SP were effective sources of P fertilizers for corn on soils treated with a small amount of lime compared with a large amount of lime. Under low lime the Bray P-2. accounted for 41% and 66% variability in corn yield with applied XP and SP, respectively, On the other hand, Bray P-1 was only of value when SP was the source of P.