Phosphate retention by New Zealand soils and its relationship to free sesquioxides, organic matter, and other soil properties

Abstract

Phosphate retention from KH 2 PO 4 solution at pH 4.6 was measured in some New Zealand soil profiles, arranged in sequences of increasing weathering and leaching, from sedimentary and volcanic parent materials. P retention from KH 2 PO 4 solution correlated closely with Pretention measured by adding solid Ca(H 2 PO 4 ) 2 H 2 O to moist soil and subsequently measuring water‐soluble P. P retention by topsoils correlated closely with organic carbon, total nitrogen, loss on ignition, organic phosphorus, Tamm Al and Fe, and neutral citrate‐dithionite Fe. Subsoil P retention correlated closely with Tamm Al and Fe and citrate‐dithionite Fe. Soil organic matter was not considered to be directly concerned in Pretention; the close correlation with P retention followed from a close correlation between soil organic matter and Tamm Al and Fe. On each parent material, Tamm Al, Fe, and P retention increased with degree of weathering to a maximum in moderately weathered soils of silt texture and then decreased in strongly weathered soils of clay texture. With increasing degree of leaching within the profile, P retention and Tamm Al and Fe decreased in topsoils and increased in subsoils. P retention per unit of Tamm Alar Fe was similar in topsoils and subsoils. Allophane in soils lowered the P retention per unit of Tamm Alar Fe. Soils with allophane contained appreciable Tamm Si (> 5 mM Si). Possibly the Tamm Si combined with Tamm Al and Fe, thereby preventing P retention by the Al and Fe.