Impact of volumetric versus gravimetric assessment on Olsen P concentrations

ABSTRACT

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.

KEYWORDS:

• Soil quality
• soil health
• soil fertility
• phosphorus
• bulk density
• environmental reporting

Acknowledgements

Thanks to several farmers, consultants, and members of the Land Monitoring Forum for their questions and discussions on regional soil quality reporting of Olsen P, that led to this study. Thanks to the farmers for access to monitoring sites, Auckland Council, Waikato Regional Council, Horizons, Greater Wellington Regional Council, and Tasman District Council for providing soil quality samples and data, the Manaaki Whenua – Landcare Research Environmental Chemistry laboratory team for chemical analyses, Danny Thornburrow, Pip Rhodes, Izak Iksteen, and John Dando of the Manaaki Whenua – Landcare Research soil physical laboratories. Thanks to Lucy Burkitt for comments on a draft, Anne Austin for editing, and several anonymous reviewers for helpful comments.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This study was funded by the Manaaki Whenua – Landcare Research led research programme, Soil health and resilience: oneone ora, tangata ora, funded by the New Zealand Ministry of Business, Innovation and Employment (contract C09X1613). Auckland Council, Waikato Regional Council, Horizons, Greater Wellington Regional Council, and Tasman District Council funded the soil quality monitoring programmes.