The application of site-specific nutrient management (SSNM) in grazing systems may offer considerable benefits in terms of optimised nutrient use efficiency and pasture productivity. There is significant variability in soil nutrients and plant growth potential across pasture paddocks, suggesting that SSNM may prove viable (Virgona and Hackney, 2008). Research has led to increasing uptake of SSNM in cropping systems. Despite this, there has been little research into the value of SSNM strategies within more complex grazing systems and even less is know about how producers might implement these techniques. This experiment examines the value of SSNM of nitrogen (N) in a pasture using soil EM38 mapping to guide paddock zoning for variable rate fertiliser application. Soil EM38 is strongly linked to soil moisture characteristics and this study explores how variability in soil water may influence nutrient use efficiency at the sub paddock scale.

Nutrient-use efficiency is a key issue for grazing systems in Australia. Spatial variability in soil pH and nutrients at the sub-paddock scale may affect the efficiency of utilisation of, and provide an opportunity for, site-specific management (SSM) of fertiliser and soil ameliorants. However, there has been little research exploring the potential for SSM in grazing systems. This study examines the spatial variability of soil test pH, phosphorus (P), potassium (K) and sulfur(S) in two typical pasture fields (a native and an improved) on the Northern Tablelands of New South Wales and evaluates the potential for SSM based on a comparison with critical values. In both fields, the overall paddock mean from a grid survey containing >80 samples for pH, P, K and S (0-10 cm) exceeded the critical values, suggesting that the addition of fertiliser or lime was not required. However, considerable sub-paddock-scale variability was observed, with CV ranging from 35% to 66% for the key nutrients (P, K and S). The Sprengel-Liebig Law of the Minimum was applied to evaluate the proportion of each field constrained by one or more soil characteristics. Up to 55% of the improved paddock and 78% of the native pasture was potentially responsive to amendments. The results of this study suggest that SSM of fertilisers and ameliorants could provide substantial improvements in productivity and possibly reductions in fertiliser use. The development and application of appropriate systems and tools to effectively quantify this spatial variability remain a challenge, coupled with management strategies that optimise the placement of amendments and account for the variability in other production limiting factors.