Elsevier

Geoderma

Volume 418, 15 July 2022, 115862
Geoderma

Valuable phosphorus retained by ironstone gravels can be measured as bicarbonate extractable P

https://doi.org/10.1016/j.geoderma.2022.115862Get rights and content
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Highlights

  • Phosphorus retained by the > 2 mm fraction of ironstone gravel soil is measurable as plant available.

  • Plant available P in ground and intact soil fractions < 2 mm and > 2 mm was correlated 1:1.

  • Sodium flouride pH has potential as a surrogate for phosphorus retention measures of unsieved soil.

  • Phosphorus retention decreased with increasing particle size.

  • The > 2 mm fraction had higher specific surface area than implied by physical size.

Abstract

The > 2 mm fraction of soils is often excluded from laboratory analysis and glasshouse experiments, but is known to influence whole soil physical, chemical and biological properties. The historical focus on the < 2 mm fraction has led to a knowledge gap in analytical procedures and flow-on effect for agronomic response and advice when the > 2 mm fraction is present, especially when it is porous and chemically reactive.

An ironstone gravel soil (Endopetric Pisoplinthic Plinthosol (Arenic)) from south west Western Australia was separated into < 2, 2–4, 4–6, 6–8 and 8–10 mm fractions. Physical analysis included specific surface area (SSA), optical mineralogy, XRD and SEM. Chemical analysis included phosphorus (P) sorption, P retention index (PRI), P buffering index (PBI) and sodium bicarbonate extractable P (Cowell P) on intact and ground samples, and intact mixtures of the < 2 mm and > 2 mm fractions. Δ NaF pH was used as a surrogate P retention measure on intact mixtures of the < 2 mm and > 2 mm fractions.

P adsorbed onto the > 2 mm fraction can be measured as Colwell P, suggesting this fraction can provide P to plants. Colwell P determined on ground samples was correlated 1:1 with Colwell P on intact counterparts. Grinding of samples resulted in large increases in PRI and PBI, and is not a supported sample preparation, neither is the use of end over end shakers due to surface abrasion of the > 2 mm fraction increasing P sorption, however the use of Δ NaF pH as a surrogate measure of P sorption offers some potential.

Phosphorus adsorption decreased with increasing particle size but adsorption by the > 2 mm fraction is likely significant in the context of an entire unsieved soil and was influenced by SSA. The > 2 mm fraction had higher SSA than their physical size would imply, possibly influenced by the thickness and mineralogy of the exterior coating (rind). Our results show that the > 2 mm fraction contains reactive surfaces that can contribute to the nutrient holding capacity and plant available P of soils.

Keywords

Phosphorus
Ironstone gravel
Adsorption
Bicarbonate extractable P
P retention index
Endopetric Pisoplinthic Plinthosol (Arenic)

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