Characterisation of soil phosphorus forms in the soil-plant system using radioisotopic tracer method

Soil incubation and pot experiments were conducted to follow the sorption processes of added phosphorus (P) fertiliser using the radioisotope tracer technique. Increasing doses of P fertiliser (40, 80, 160, 320 mg P/kg soil) were added to Chernozem and Arenosol and incubated for 1, 3, and 13 weeks. After incubation, perennial ryegrass (Lolium perenne L.) was sown in one group of pots, and the experiment had been continuing for another 9 weeks. The yield, grass P uptake, isotopically exchangeable (P_IE), water-soluble (P_W), and ammonium lactate soluble phosphorus (P_AL) fractions of soils were measured. On Chernozem, plant P uptake, P_IE, P_W and P_AL were significantly less in the case of the longest incubation period compared to shorter incubations. This suggests a transformation of P into tightly sorbed form. On Arenosol, there were only small changes in the parameters as the incubation period increased, suggesting less intense P transformation to tightly sorbed form. The P_W/P_IE ratio enhanced with increasing P-doses, and the ratios were higher on Arenosol. On Arenosol, the higher P doses caused a greater increase of P_W than on Chernozem. The P_IE + P_W showed a good correlation with plant P uptake proving this value can be a good indicator of plant-available phosphorus.

Interferences in the determination of isotopically exchangeable P in soils and a method to minimise them

In soils with a high phosphate buffering capacity or in soils with large amounts of colloidal P in soil solution, E-values often overestimate P availability. These problems have been ascribed to a variety of causes including analytical difficulties in terms of measuring low concentrations of P, colorimetric interferences, and inadvertent measurement of non-isotopically exchangeable colloidal P. We investigated measurement of E-values in 8 soil types, of which 6 had properties likely to give overestimates of size of the available pool of P based on results of previous authors. The potential for overestimation of the E-value due to interferences described above was identified in several soil types. A simple anion exchange resin purification step was introduced to the E-value methodology. The resin adsorbs both 31P and 32P isotopes in proportion to their concentration in the solution and minimises transfer of colloids and Si into the analysed eluant, thereby providing a simple way to increase the eluant P concentration and to avoid analytical interferences.

Long-term effects of lime on soil-phosphorus solubility and sorption in eight acidic soils

Measurements of phosphorus (P) sorption, isotopically exchangeable, KCl soluble and extractable P (Bray(1)) were carried out on limed and unlimed soils from eight pasture experiments on the Northern Tablelands of New South Wales at intervals of 1, 2 and 3 years after lime application. Lime increased soil pH by a minimum of 0.5 to a maximum of 1.55 units, and there were corresponding decreases in soluble aluminium and manganese. Lime decreased P sorptivity in every soil and at every sampling, but decreases were usually largest at the first sampling. They were attributed to the pH-induced increase in surface negative charge and the smaller increases in calcium concentrations of these freely drained soils, compared with undrained potted soils, of a previous glasshouse experiment. Isotopically exchangeable P was increased by the highest lime rate (5 t/ha) in all but one soil at the first sampling, while soluble P was increased by both lime rates in all soils. Increases in exchangeable P tended to decline at successive samplings, but increases in soluble P sometimes increased and sometimes decreased with time. In general, lime-induced increases in soluble P were consistent with decreases in P sorptivity, although the primary cause of the increases was probably the dissolution of iron and aluminium phosphates. All these changes were conducive to the increased plant availability and uptake of soil and fertilizer P.