Interannual variation in soybean yield:: interaction among rainfall, soil depth and crop management

Using data from large, grower-managed fields we investigated the variation in yield of dryland soybean in an area with low and variable summer rainfall, and soils that are variable in depth and poor in phosphorus (P). First, using data from unfertilised, wide-row (0.7 m) crops grown under standard management between 1989 and 1992 (Series 1), we quantified the relationship between yield and W a rainfall-based estimate of water availability during the period of pod and grain set. Separate functions were established for deep (depth greater than or equal to 1 m) and shallow soils (0.75 m greater than or equal to depth greater than or equal to 0.5 m). Second, we partially tested these functions using two independent data sets (Series 2 and 3). Third, we evaluated the effects on yield of large (18 kg P ha(-1), Series 4) or moderate doses of P fertiliser (8-12 kg P ha(-1)) in narrow-row crops (0.35 m, Series 5). To investigate water x management interaction we (1) calculated Delta Y the difference between actual yield in Series 4 and 5 and yield calculated with the functions derived from Series 1, and (ii) tested the association between Delta Y and actual W. In a set of 24 crops (Series 1), yield varied between 2.1 and 3.1 t ha(-1) in deep soils and between 1.3 and 2.6 t ha(-1) in shallow soils; nonlinear functions described fairly well, the response of yield to W. Fertilisation with 18 kg P ha(-1) increased yield by 0.6 t ha(-1) irrespective of water availability. The combination of narrow rows and a moderate dose of fertiliser increased yield in 73% of crops in deep soil but only in 53% of crops in shallow soil. There was a positive association between Delta Y and Win deep soil but no relationship between these variables in shallow soil. Yield responses to management were thus differentially affected by rainfall in deep and shallow soils. (C) 1999 Elsevier Science B.V. All rights reserved.