Quantifying the benefits of accounting for yield potential in spatially and seasonally responsive nutrient management in a Mediterranean climate

Crop yield potential is a chief determinant of nutrient requirements, but there is little objective information available on the gains in profitability that can be made by accounting for the influences of soil type and season on yield potential when making fertiliser decisions. We conducted such an assessment using crop growth simulation coupled to nutrient response curves for wheat-growing at 4 locations in the low-medium rainfall zone of Western Australia. At each location, the yield potential was simulated on 10 soil types with plant-available water capacity (PAWC) ranging from 34 to 134 mm, which represent the major soils types in Western Australia. Soil survey maps were available to quantify soil type variability and the historical climate record (1974–2005) for seasonal variability.The benefits possible for fertiliser (NPK) management that takes account of variation in crop yield potential due to season and soil type by having ‘perfect knowledge’ ranged from $2 to 40/ha. Seasonal variation was more important than soil type for the better soils (high PAWC), providing two-thirds of the benefit of perfect knowledge. On low PAWC soils, knowledge of soils and seasonal influences on yield potential were similar contributors to profit gains. An assessment of one yield forecasting system showed that about 50% of the maximum gains could be captured if seasons could be categorised as below, at, or above average at the time the fertiliser decision is made. In each catchment, 30–40% of fields showed scope for benefits in accounting for within-field variation in soil type due to large variation in PAWC, and therefore yield. Maximum profit gains and reductions in nutrient excess were greater in the low rainfall locations and also on the low PAWC soil types.