Phosphorus status and saturation in soils that drain into the Peel Inlet and Harvey Estuary of Western Australia

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.

Sediment Budgets for Small Salinized Agricultural Catchments in Southwest Australia and Implications for Phosphorus Transport

“…A-sheet and rill erosion dominate sediment movement, and therefore fertilizer P, which is retained mostly in surface soil layers [39], can also be a significant source of stream particulate P. In this type, hillslope erosion may lag changes to erosive land uses, and valley-floor sedimentation absorbs most sediment and particulate P. The sediment (and particulate P) delivery ratio is low but can increase as overbank stores fill, allowing more transport in-stream.…”

“…The Takalarup catchment P yield would be about twice (260 kg TP) the channel erosion P loss estimates. However, increased TP concentrations in surface soils because of fertilization since clearing [39,40] indicates that this is probably an overestimate of the contribution of TP from channels and gullies. On average in the Kalgan River catchment, surface soils have twice the TP content as subsurface soils.…”

Lowering the water solubility of phosphorus fertilisers impacts leaching, plant growth and residual soil phosphorus

“…Phosphorus application has been required to increase soil P fertility on the naturally infertile soils to support pastures grazed by cattle and sheep on these poorly P retentive sandy soils. Many of the soils are now P saturated (Weaver and Summers 2021) and represent a high risk of P loss from legacy P stocks (McDowell et al 2020;von Arb et al 2021) with little opportunity for P attenuation as they are situated close to affected water bodies (Weaver and Reed 1998;Weaver and Summers 2021). The loss of P, particularly from farmland, contributes to water quality problems because P ingress removes limitations to algal growth.…”

“…While most of the soils in the region have sufficient or excess P to maintain pasture dry matter yields (Weaver and Summers 2021), some have P deficiency and there is some requirement for early season P (Grant et al 2001). LWSP fertilisers may not be soluble enough to provide adequate early season P in the rare occurrence of severe P limitation in SWWA.…”

Critical P, K and S Concentrations in Soil and Shoot Samples for Optimal Tedera Productivity and Nodulation

“…Most natural Australian soils are highly weathered and deficient in P when first cleared for agriculture, and phosphorus application has been required to overcome P limitation in major agricultural plant species. Appropriate P application requires evidence-based management through soil testing to provide suitable P levels while avoiding losses via leaching from sandy soils, overaccumulation in clay soils and limiting losses via erosion [15]. Plant biomass removal is another significant factor of P dynamics in soils [16].…”