Low pasture productivity of a sedimentary soil in relation to phosphate and water supply

Reasons for the low pasture production of a soil derived from Ordovician sediments in the Yass Valley 50 km north of Canberra were investigated in field and glasshouse experiments. In the field, at a moderate phosphate level, application of water to the soil surface in the autumn increased yields although rainfall was above average and soil water was adequate for growth. At a high phosphate level there was no response to this surface wetting. In this soil, available phosphate (Truog) was confined to the 0-5 cm layer. In the glasshouse surface wetting increased yield and phosphorous uptake even though soil water was maintained at adequate levels below the immediate surface. Placement of phosphate at 8 cm depth where the soil remained wet for a longer time led to substantial yield increases over surface applied phosphate. In the field, a productive soil nearby derived from porphyry rock did not respond to surface wetting and yields were higher than those on the sedimentary soil. In the following moist spring, yields on the sedimentary soil were slightly better (P < 0.01) than yields on the porphyry soil. Available evidence suggests that surface drying limits the uptake of surface applied phosphate on the sedimentary soil. Thus phosphate levels that would be adequate for growth are inadequate in the early autumn when evaporation from the soil surface is greatest. On the adjacent porphyry soil, surface applications were fully effective.