New Zealand Journal of Agricultural Research abstracts

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A capacity factor as an alternative to soil test phosphorus in phosphorus risk assessment

V. D. Nair
W. G. Harris

Soil and Water Science Department
University of Florida
P.O. Box 110510
Gainesville, FL 32611-0510, USA

Abstract  Soil test phosphorus (P) concentrations (STP) are often used as measures of environmental P risk. However, a low STP is not valid justification for further P application because P sorption capacity may be low and P added could be lost to surface waters. The degree of P saturation (DPS) normalises extractable P using extractable Al and Fe as a surrogate for P sorption capacity, but like STP, fails to convey a magnitude of capacity. We propose the use of a DPS-based prediction of the remaining soil P storage capacity (SPSC) that would capture risks arising from previous loading as well as inherently low P sorption capacity. The SPSC is a direct estimate of the amount of P a soil can sorb before exceeding a threshold soil equilibrium concentration. In this paper, we demonstrate the applicability of the SPSC for a variety of sandy soils impacted by dairy and poultry manure additions. The SPSC provides a means to assess the capacity of a soil to retain additional P and hence is a more useful indicator of P-related environmental risk than STP or DPS measures alone.

Keywords  degree of P saturation; phosphorus saturation ratio; poultry and dairy manure; sandy soil profile

A04074; Received 8 July 2004; accepted 23 October 2004 ; Online publication date 15 December 2004
New Zealand Journal of Agricultural Research, 2004, Vol. 47: 491–497
0028–8233/04/4704–0491 © The Royal Society of New Zealand 2004

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