Sulfur and phosphorus dynamics in an acid soil/crop system

The objective of this study was to evaluate different methods, forms, and times of sulfur fertilizer application on a highly weathered soil under different rates of added lime and phosphorus. A pot experiment was conducted using a medium phosphorus (P) and sulfur (S) sorbing red earth (Haplohumult) of pH (1:1 H2O) 4.9 soil. The experiment was carried out using an incomplete factorial combination of two rates of lime (0 and the equivalent of 1.5 x exchangeable Al), three rates of phosphorus (0, 10 and 30 �g P/g soil), two sources of sulfur fertilizer [elemental S (ES) and gypsum (GS)], two methods of placement of sulfur fertilizer (mixed with or separated from the P fertilizer), and two different times of sulfur application (ail treatments applied as a basal dressing and a split application, half applied as a basal and half applied 14 days after planting). Three treatments with P only (0, 10 and 30 �g P/g soil) were added as a check for S responses Liming had no effect on crop yield or S and P dynamics. The experiment demonstrated that movement and leaching losses of applied S fertilizers, and thus their efficiency, were not only determined by soil properties but also influenced by form, rate and time of application of the S fertilizers, and their placement relative to the placement of P fertilizer. In addition, the mobility and effectiveness of S fertilizers was affected by the S requirement of the plants and the fate of P fertilizer application. Gypsum provided available S immediately and thus was advantageous for rapidly growing corn. However, the immediate availability of sulfate-S from gypsum resulted in considerably higher amounts of S being lost by leaching, ranging from 2.2% to 15.7% of the applied S. On the other hand, the application of elemental S resulted in lower S losses by leaching (<1%), but the amounts of S taken up by the plant from the elemental S fertilizer were also lower, especially if the fertilizer was applied in a split application. The mixing of S and P fertilizers increased the effectiveness of gypsum and, more particularly, elemental S fertilizer. Mixing 30 �g P/g soil with a single application of elemental S increased fertilizer S uptake by the whole plant from 2.7% to 12.4%. The advantage of mixing S and P fertilizers has important agronomic implications, and suggests that combined S/P fertilizers should be investigated further on weathered soils.