Intensive pH sampling and variable rate surface application of lime: does it pay?

The principles of production economics were used to generate profit-maximising lime 'prescriptions’ for each homogenous zone (HZ) within 10 case-study cropping paddocks in the Victorian HRZ, and to quantify the net benefits of the precision liming strategy. The initial pHca distribution within each paddock was obtained using intensive point sampling at the rate of two soil cores per hectare followed by spatial interpolation to a resolution of 10 square metres. The method used to determine the lime rates for each HZ zone involved optimisation, simulation and accommodating the dynamic nature of the acidity of the soil. The expected payoff from the precision strategy was positive for all 10 paddocks. It was shown to depend mostly on the physical attributes of the soil (i.e. in-paddock pH variation and buffering capacity). Net benefits increased substantially as pHca fell from about 5.0 to 4.2. Productivity gains due to increased yield were most important in determining the size of the benefits and more than offset the additional costs. If farmers plan to grow acid tolerant crops and have a relatively homogeneous paddock (CV less than 5%), then they need not worry unduly about the most appropriate method (precision or traditional) for applying lime. But if they want the option of planting high value, acid sensitive crops such as pulses, and if the in-paddock variation in pH exceeds 5%, then it pays to pursue a profit-maximising precision strategy involving intensive pH sampling and variable rate surface application.