New Zealand Journal of Agricultural Research abstracts
Potential phosphorus losses in overland flow from pastoral soils receiving
long-term applications of either superphosphate or reactive phosphate rock
R. W. McDowell
R. M. Monaghan
AgResearch Limited
Invermay Agricultural Centre
Private Bag 50 034
Mosgiel, New Zealand
email: richard.mcdowell@agresearch.co.nz
P. L. Carey
AgResearch Limited
c/o Soil, Plant and Ecological Sciences Division
P.O. Box 84
Lincoln University
Canterbury, New Zealand
Abstract The forms and concentrations of P in overland
flow were measured from intact pastoral soils obtained from the Winchmore
long-term P fertiliser trial. Treatments under evaluation were soils that
received either 0, 188, 250 or 376 kg superphosphate ha–1 yr–1,
or 175 kg reactive phosphate rock (RPR) ha–1 yr–1. The
objective was to determine the magnitude of potential P transfers from soil
to water following P fertilisation, and to determine if losses were different
following RPR fertilisation compared with superphosphate. Overland flow was
induced by the application of artificial rainfall at 15 mm h–1,
maintained for 1 h after flow commenced. Concentrations of dissolved reactive
P (DRP) and total P (TP) mirrored the long-term application rates, although
prior to a fresh application of P, soils with P applied in RPR form lost more
P during an event than soils with the same rate of P applied as superphosphate.
After a fresh application of RPR and superphosphate treatments, up to 5.4
mg TP litre–1 was lost in flow from the 376 kg superphosphate ha–1
yr–1 treatment, while P in flow from soils fertilised with RPR
were commonly c. 0.11 mg litre–1, but still greater than from
the unfertilised control soils (0.02 mg litre–1). Regression analysis
indicated that DRP concentrations in flow from the fertilised soils were
elevated above that lost before fertiliser application for a period of approximately
60 days. These results support earlier studies that demonstrate the greater
risks of incidental P losses from soluble P fertilisers such as superphosphate
(up to 60 days), and conversely the potential environmental benefits from
RPR fertilisation of soils ‘at-risk’ of P loss (e.g., where much overland
flow occurs such as in very wet soils and near stream channels). However,
if good management practice is followed then the difference in P loss between
superphosphate and RPR treated soils should be minimal over a period of a
year.
Keywords phosphorus; superphosphate; RPR; pasture; overland
flow
A03004; Received 21 January 2003; accepted 13 June 2003; online publication
date 5 November 2003
New Zealand Journal of Agricultural Research, 2003, Vol. 46: 329–337
0028–8233/03/4604–0329 $7.00/0 © The Royal Society of New Zealand
2003
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