Phosphorus (P) use efficiency has been identified as a key issue for Australian grazing systems. This project examined the spatial variability in soil P concentration from two separate surveys of grazed pasture fields. A field on the central tablelands of NSW had a range in Bray P of 1.2 to 140 mg/kg and a COV of 107%. The other field on the northern tablelands of NSW reported a range in Colwell P from 13.0 to 121.1 mg/kg and a COV of 59%. Maps of the spatial variability of soil P demonstrated that there is a relationship with field elevation. Application of critical P values to both fields enabled an estimation of the value of site specific fertiliser management. For one field, fertiliser inputs could potentially be isolated to 37% and the other 56% if nutrient additions were targeted at responsive areas. The opportunity for increased fertiliser use efficiency through site specific management (SSM) warrants further investigation. Research is required into both the value of SSM and the techniques that might enable the development of this strategy.

Precision Agriculture (PA) is changing how producers manage their land. PA involves the use of sensors and management strategies that target the spatial and temporal variability that occurs across a landscape. The introduction of PA has increased profitability and resource use efficiency across many agricultural systems and is now widely applied in cropping and horticultural enterprises. However, development of PA strategies for grazing systems has largely been ignored, possibly due to the complex relationships that exist when considering soil, plant and animal interactions across variable pastoral landscapes. There is a growing interest in the potential of PA management strategies, for example Site Specific Nutrient Management (SSNM) to assist in increasing the fertiliser use efficiency in grazing systems (Simpson et al., 2011). Technologies such as soil EM38 mapping and plant vigour sensors (Crop Circle - Active Optical Sensor) have been extensively used in PA cropping operations and these tools offer some ability to monitor the soil and plant systems in a pasture. The more recent development of GPS livestock tracking has now unlocked the ability to monitor the spatial and temporal variability of the animal component of a grazing system. The integration of these technologies holds significant potential in providing an understanding of how grazing systems vary and how this variability can be managed, particularly through SSNM. This study aims to investigate how common PA tools such as soil EM38 and plant vigour sensors along with GPS tracking information from livestock can be used to understand the spatial distribution of soil nutrients in grazing systems. It is anticipated that this information will lead to an understanding of how producers can zone pasture paddocks to apply SSNM strategies in a similar way to what is currently applied in cropping systems.

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.