A GLASSHOUSE EVALUATION OF SULFUR FERTILIZER SOURCES FOR CROPS AND PASTURES .3. SOLUBLE AND NON-SOLUBLE SULFUR AND PHOSPHORUS SOURCES FOR PASTURES

Decreasing terms of trade of animal producers and increasing awareness of environmental problems associated with fertilizer use has resulted in increased emphasis on improving fertilizer use efficiency. Elemental S and reactive phosphate rock offer two sources of fertilizers with slower nutrient release rates than gypsum or monocalcium phosphate. A randomized block experiment was conducted under glasshouse conditions. Undisturbed soil cores of an Aquic Haplustalf soil were oversewn with perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens, L.), and a factorial combination of two S sources (elemental S and gypsum), two P sources (triple superphosphate, (TSP), or North Carolina reactive phosphate rock (RPR), two methods of S and P application (S mixed with granulated TSP or RPR and S coated onto TSP or granulated RPR). An unfertilized control (C) was included in order to calculate fertilizer S recovery using the S-35 reverse dilution technique. Pots were either watered to field capacity (NL) or watered to 25% excess to promote leaching (L). Harvests of tops were made at 4 or 8 weekly intervals up to 96 weeks after application when the pots were destructively harvested. Yields were lower in the leached than the unleached pots between weeks 8 and 44 where TSP or G was applied. Clover yields were initially highest with G, but elemental S gave more sustained production. Yields and P content of tops were lower with RPR than TSP up to week 36. Recovery of fertilizer S was higher in the TSP than RPR treatments up to week 24. Leaching losses of S were higher from G than elemental S sources up to week 17 and were higher where TSP was the P source. Manipulation of P and S source has been shown to improve the efficiency of utilization of the applied fertilizers.