Abstract
The Peel–Harvey estuarine system in Western Australia's south-west is affected by poor water quality, algal blooms, and fish kills. Phosphorus (P) discharge from agricultural activities is the main source of poor water quality. The catchment’s soils are naturally infertile, but P application has increased P fertility. This paper draws on and undertakes a meta-analysis of 20 200 surface (0–10 cm) and profile (to 100 cm depth) soil samples collected in the period 1983–2018. Soil P content was high, with 70% of samples with Colwell P content in excess of agronomic requirements; Production is more likely limited by low soil pH(CaCl2) and low K (92% and 67% of paddocks respectively). Strong P stratification in the soil is evident, particularly topsoil; sandy soils are saturated to depth; and clay soils show signs of P saturation in the topsoil. Management of P in sandy soil near the estuary is a high priority as is P stratification in highly P retentive soil. Soil P stocks increased since clearing compared with uncleared soils (1221 kg ha–1 m–1 and 285–694 kg ha–1 m–1, respectively). Thirteen percent of samples had P content in excess of agronomic requirements in 1983, rising slowly to 69% in 2018. Landholder practices need to be analysed in detail to confirm if this accumulation occurs everywhere or is only confined to actively farmed land.
Highlights
Soils in south-west Western Australia (SWWA) were deficient in phosphorus (P) when land clearing for agriculture commenced in the 1800s
The accumulation of P, measured as soil test P and often termed legacy P, is a well-documented threat to water quality and requires effort to draw down soil P reserves to manage the threat
This challenge posed by legacy P stores and soluble P (Summers et al 2014; Weaver and Summers 2014) exists in the Peel–Harvey coastal catchment and is a problem that is increasing with time (Figs 1g, h, 8)
Summary
Soils in south-west Western Australia (SWWA) were deficient in phosphorus (P) when land clearing for agriculture commenced in the 1800s. To overcome P deficiency, superphosphate application, in the period 1945–1975 became an integral part of farming practice (Birch 1982; Yeates 1993) in the Peel–Harvey coastal catchment. An unintended consequence of P use in agriculture has been increased P loss to waterways, removing natural limitations to algal growth in the Peel Inlet and Harvey estuarine system (Kinhill Engineers 1988). Of the coastal catchments in SWWA, the Peel–Harvey coastal catchment has the most P discharge (Ruprecht et al 2013), with ~90% of the P sourced from ~210 000 ha of predominantly farmland in the coastal section of the catchment (Kinhill Engineers 1988). Agricultural P loss has been widely reported (Sharpley and Withers 1994; Weaver and Reed 1998) and attributed to longterm P application to agricultural watersheds, leading to elevated soil P and widespread P losses from legacy P
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