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Phosphorus Retention in Coastal Plain Forest Soils: I. Relationship to Soil Properties¹

ABSTRACT

Phosphorus-retention capacities in the surface horizons of 42 forest soils of the US Southeastern Coastal Plain were determined from the adsorption maximum of the Langmuir isotherm and by saturation with 2,500 µg P/g soil. Both indices were closely correlated (r = 0.986) although on the average the saturation method gave retention capacities two to three times greater than those derived from the Langmuir equation. Range in retention capacities were from 0 to 1,288 µg P/g soil from the concentrated solution. Spodosols generally had the lowest retention capacities although there was considerable overlap between soil orders.

Several soil properties (% clay, % silt, pH, loss on ignition) were significantly correlated with P retention but partial correlation analysis showed that, except for pH and to a lesser extent clay, the relationship was indirect through association with extractable Al. Extractable Al and Fe provided the best indices of P retention. The most successful of the extractants of Al for predicting P retention (r = 0.934) was 1N NH₄OAc (pH 4.8), while 0.3M (NH₄)₂C₂O₄ (pH 3.2) was the most successful of the extractants of Fe (r = 0.903) for predicting P retention.

Multiple regression analysis of P retention on exchangeable, amorphous, and crystalline forms of Al and Fe showed that on a per unit weight basis, the order of activity in P retention was exchangeable > amorphous > crystalline with apparently little distinction between Al and Fe within these categories. The greater contribution of Al than Fe to P retention in these soils was primarily a function of the greater amount of Al than Fe in the active forms.

¹ Contribution from the Soil Science Dep. Florida Agr. Exp. Sta., Gainesville, as Journal Series Paper no. 4987.

² Graduate Assistant and Professor of Soils, respectively, Univ. of Florida. Senior author is Scientist, Forest Research Institute, Rotorua, New Zealand.

Received for publication July 27, 1973. Accepted for publication October 29, 1973.