Evaluation of Rock Phosphate, Sulfur and Thiobacillus Application Affecting the Yield and Quality of Soybean and their Residual Effects on Corn Growth

This experiment was carried out at Karaj Soil and Water Research Station during 2002 and 2003 for evaluation of rock phosphate, sulfur, and Thiobacillus on the yield and quality of soybean and their residual effects on corn growth. The experiment was based on a randomized complete block design with six treatments and four replications. The treatments were: T₁=control; T₂=Triple superphosphate; T₃=rock phosphate; T₄=rock phosphate + sulfur; T₅=rock phosphate + sulfur+ Thiobacillus inoculum; and T₆=rock phosphate + sulfur + manure. No phosphorus fertilizer was applied in the control treatment (T₁) during the first year, while in the second treatment (T₂) 150 kg ha^-1 of triple superphosphate was applied in a band below the seeds. Each one of rock phosphate and sulfur was applied in the form of powder at a rate of 300 kg ha^-1.  Thiobacillus inoculant was used at a rate of one kg ha^-1 (10⁸cells g^-1 inoculum). Composted cow manure was also used at a rate of 10 tons ha^-1. Soybean seeds of Williams variety were inoculated with a Bradyrhizobium japonicum before sowing. A  SC 704 variety of corn planted in the second year in the same plots used for soybean. No phosphorus fertilizers were applied during the second year, while in T₅, Thiobacillus inoculant was used. Soil samples were collected each year from 0-30 cm depth following harvest and leaf sampling. Results of first year indicated that the application of triple superphosphate improved the yield as compared with the control though not significantly (even at 5 % level). Rock phosphate application also increased the yield though less than triple superphosphate. T₅ increased the yield significantly as compared with the control but no significant differences (at 5 % level) were observed among yields of T₅, T₂ and T_3.  There was no significant differences among treatments respect with seed oil concentration. In the second year, the highest forage yield of 67190 kg ha^-1 was obtained in T₅ which was significantly (at 5 % level) differ from T₆. No significant yield differences were obtained with the other treatments at 5 % level. T₂ (triple superphosphate) improved the yield as compared with the control but this increase was not significant.