Distribution and forms of phosphorus and aluminum in acidic topsoils under pastures in south-eastern Australia

Abstract

The vertical distributions of pH, and forms of phosphorus (P) and aluminium (Al) in acidic topsoils (0-100 mm depth) under pastures were examined at 15 sites in New South Wales and Victoria. The soils were characterized by shallow surface strata of moderate pH, high P and low Al concentrations overlying more acidic, P-deficient and Al-rich subsurface strata. Significant amounts (35-65%) of the total P in acidic pasture topsoils were present in organic forms (Po). Some 20-40% of this Po was extractable using 0.5 M NaHCO3, and is regarded as easily mineralizable and a potential source of P for plant uptake. Substantial amounts of Al were present in crystalline and non-crystalline oxides (2000- 6000 mg kg-1) and complexed to organic matter (up to 400 mg kg-1). The amount of aluminium extracted using reagents commonly accepted to indicate Al toxicity in soils (1 M KCl and 0.01 M CaCl2) was not related to the amounts of those Al forms (extracted by ammonium oxalate, sodium citrate-dithionite) typically identified in pedological and mineralogical studies. Sampling and analysis of soils for determining P fertilizer requirements and diagnosing acidity and related problems (e.g. Al toxicity, Ca deficiency) should take into account the vertical heterogeneity in soil properties in the top 100 mm of the profile.