Responses to nitrogen, phosphorus and irrigation by grain sorghum on cracking clay soils in central Queensland

Abstract

The yield and dry matter (DM) responses to nitrogen (N), phosphorus (P) and irrigation in sorghum hybrids E57 and Goldfinger were measured near Emerald, Queensland, on a cracking clay soil (Ug 5.12) deficient in N and P. A raingrown experiment received factorial combinations of 0, 25, 50 and 75 kg N/ha as urea, and 0, 10, 20 and 40 kg P/ha as superphosphate. An irrigated experiment received the same rates of superphosphate in conjunction with 0, 50, 100 and 150 kg N/ha and irrigation applied at 80 mm deficit. Plants were sampled on days 20 (floral initiation), 41 (early booting), 55-57 (anthesis) and 142 (grain harvest). Factors affecting yields on a soil type fairly representative of a large area of the cultivated soils of the Central Highlands were, in decreasing order of importance, N, water and P. Goldfinger initially produced higher DM yields than E57, but E57 produced higher grain yields. Mean grain yields for the 2 hybrids from control plots were 1150 kg/ha (raingrown) and 1190 kg/ha (irrigated). Under raingrown conditions, a grain yield asymptote (1600 kg/ha) was reached with 50 kg N/ha, but rates higher than 150 kg N/ha (2700 kg/ha) would be required to achieve maximum yields under irrigation. Responses to P were smaller, with rates of 10-20 kg P/ha (raingrown) producing maximum yields for both hybrids. Phosphorus rates greater than 40 kg/ha would be required to achieve maximum yields under irrigation. Apparent recovery of fertiliser P was significantly (P<0.05) lower at the highest P rate of 40 kg/ha at anthesis in the raingrown experiment. All other P recoveries either at anthesis or in the grain were not significantly (P>0.05) different. Phosphorus recoveries between the raingrown and irrigated experiments showed little difference. Nitrogen, however, showed much higher apparent recoveries in the irrigated experiment, with no significant (P>0.05) differences between N treatments. A month after harvest the only plots on the raingrown site with an increase in soil nitrate were the 75 kg N/ha treatments; residual values from all other treatments were low.