Quantification of the effects of citric acid on the availability of phosphorus in soils with different oxide content

M. Drouillon, E. Smolders and R. Merckx

Laboratory of Soil and Environment, Department of Land Management, Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, 3001 Heverlee, BELGIUM

Tel.: +32 16 32 96 75 Fax: + 32 16 32 19 97

E-mail: margriet.drouillon@agr.kuleuven.ac.be

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The Al³⁺ and Fe³⁺ complexating capacities of citric acid play a key role in enhancing phosphorus availability to plants. Soil humic substances associated with metal cations tend to be less soluble, larger in size and thus less accessible to soil phosphatases. Complexation of Al and Fe bonded to humic substances, containing organically bound phosphorus, eases their hydrolyzation by phosphatases. However, the importance of this mechanism as a factor in increased phosphorus availability is not clear-cut. Citric acid can also block phosphorus adsorption sites on oxide surfaces or cause oxide dissolution. This study attempts to determine the main mechanism in soils rich in organic matter and with a low phosphorus background concentration.

A range of soils, rich in organic matter (4 – 38 % C), with varying oxide content was selected. Soils were shaken for 2 h with citric acid (10, 25 and 50 mM) and CaCl₂ (at ionic strength of soil solution). Soil:solution ratio's were 1:4, 1:10 and 1:20. Before analysis, extractants pH’s were adjusted to soil pH, to eliminate effects due to differences in extractant pH. Total dissolved phosphorus (TDP) and molybdate reactive phosphorus (MRP) in the extracts were measured colorimetrically with malachite green [1]. Al, Fe (ICP) and pH were also determined. To part of the CaCl₂ extracts, citric acid at soil pH was added to make final citric acid concentrations of 10, 25 or 50 mM and analyzed as described above.

pH-differences between extracts will indicate to what extent citric acid complexates Al and Fe bonded to organic matter (i.e. no pH change). Differences in TDP, Al and Fe content will be combined to quantify the phosphorus availability increasing effect of citric acid. MRP before and after addition of citric acid may yield valuable information, but suffers from methodological difficulties.

1. Van Veldhoven, P.P., and Mannaerts G.P. (1987) Anal. Biochem. 161: 45-48.