Are Reactive Rock Phosphate and Superphosphate Mixtures Suitable for No-Till Soybean?

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Several studies showed that rock phosphate efficiency may be increased when this fertilizer is mixed with soluble phosphates such as triple superphosphate (TSP), but most of these findings were obtained after 1 yr under conventional tillage. This study examined soybean [Glycine max (L.) Merr.] response to mixtures of Arad reactive rock phosphate (RRP) and TSP as affected by previous P fertilization. Plots received no fertilization and TSP or RRP broadcast on soil surface at a rate of 35 kg ha(-1) of P in the first year. Mixtures containing 0, 20, 40, 60, 80, and 100% of RRP were then applied to soybean seed furrows at 35 kg ha(-1) of P for 3 yr. Soybean responded to broadcast TSP from the first year on, while broadcast RRP increased available soil P, soybean leaf P content and yields from the second year on. Soybean responses to an increase of TSP in the mixture was linear, except for the third year, when the mixture of 40% RRP resulted in similar yields as TSP. The mixture of RRP with TSP applied to seed furrows did not produce similar soybean yields as those obtained with TSP applied alone in the first 2 yr. Combining rock phosphate and soluble phosphate to substitute for soluble phosphate is not a viable option for no-tilled soybean, because the efficiency of this fertilizer is relatively low and it does not increase when mixed with TSP.