YIELD RESPONSES AND NITROGEN-FERTILIZER REQUIREMENTS OF WHEAT IN RELATION TO SOIL NITRATE LEVELS AT VARIOUS DEPTHS

Wheat grain yield responses and estimates of fertilizer nitrogen requirements in 57 experiments over five years (1985-9) were related to soil nitrate values at various depths (to 90 cm) in the soil profile. These experiments were on representative soils of the north western slopes and plains of New South Wales. Profile distributions of nitrate differed in the first three years, when nitrate concentrations at 15 or 30 cm intervals declined regularly with depth, to the last two years, when it was much more uniformly distributed and much lower in the top 30 cm. This was apparently caused by very high mid-fallow (pre-sampling) rainfall that occurred in the last two years but not in the earlier years. There were large differences in relationships of N fertilizer responsiveness and requirements to soil nitrate values between the two time periods. For each period, responsiveness and requirements were closely related to soil nitrate values at 0-15 cm and deeper depths, but not at 0-10 cm, with >50% of the variance being accounted for. The optimum sampling depth was 0-15 or 0-30 cm. Fertilizer responsiveness and requirements in relation to soil nitrate values (0-90 cm) were much lower in the last two years than in the first three years. This was attributed to very low in-crop rainfall in the last year and excessive rainfall in the penultimate year causing significant nitrate leaching beyond 90 cm depth. The critical nitrate concentrations varied from 24 mg N/kg at 0-15 cm down to 12 mg N/kg at 0-90 cm depth for 1985 7 and from 6 to 7 mg N/kg for all sampling depths in 1988-9. It was concluded that relationships obtained for the first three years were representative of years when normal or above average (but not excessive) rainfall occurred in the fallow and crop growth periods, while those for the last two years represented years when excess pre-sampling rainfall leached soil nitrate beyond the sampled depth or when yield responses to fertilizer were limited by in-crop conditions such as low rainfall.