Natural Resources Research Articles

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

Document Type

Article

Publication Date

4-3-2022

Journal Title

Geoderma

ISSN

0016-7061

Keywords

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

Disciplines

Agricultural Science | Environmental Monitoring | Soil Science

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.

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Digital Object Identifier (DOI)

https://doi.org/10.1016/j.geoderma.2022.115862