Impact of volumetric versus gravimetric assessment on Olsen P concentrations

Soil Olsen phosphorus is used for regional and national environmental reporting, and for farm soil fertility assessment. In New Zealand, the laboratory measurement using sieved and air-dried soil is undertaken gravimetrically (by weight), or volumetrically, so can be reported on either basis. Olsen P ‘stocks’ can also be calculated using field bulk density. These methods have led to inconsistencies in environmental reporting. This study compares and quantifies the relationships between the laboratory-determined gravimetric and volumetric Olsen P for a range of soil orders, by developing a statistical model. Our study showed there is a significant difference in Olsen P determined by volumetric and gravimetric methods, and the relationship varies with soil order. The gravimetric method gave elevated values for all soil orders compared with the volumetric method. Bulk density was generally significantly different from, and greater than, volume weight, for many soil orders. From the statistical modelling, look-up tables are provided for converting from volumetric-to-gravimetric, and gravimetric-to-volumetric, in the absence of volume weight, such as for legacy data. Several recommendations to improve national environmental reporting are provided, including that, if the volumetric method is used, volume weight should be requested from the laboratory to enable consistent conversion for national reporting.