A study about the competition between organic anions (oxalate and malate)—currently present in the rhizosphere and phosphate for adsorption sites in tropical soils—indicate that in the presence of organic anions, phosphate adsorption by soils is reduced. The extent of such reduction is dependent on the way in which either phosphate or the organic anion are added to the soil. The organic oxyanions studied are more rapidly adsorbed to the soil surface than the phosphate; consequently those anions, when existing in the rhizosphere, can improve the phosphate status of soil.

No abstract

This study compared three previously published methods of estimating gross P mineralization: (i) an isotope dilution method that relies on specific activity, (ii) a later isotope dilution method that uses isotopically exchangeable P as the response variable, and (iii) a differential extraction method. We adapted the isotope dilution method (KB) commonly used for gross N mineralization for gross P mineralization. We evaluated two methods used to correct for adsorption of 32P: sterilized soil incubations and a simulation model. Finally, we examined the necessity of including microbial biomass P as a component of labile P for the isotope dilution methods. The three previously published methods gave highly variable estimates of gross P mineralization, and our data suggest that critical assumptions of each method were violated. We recommend the KB method because its assumptions were generally met and it requires no sterilized samples. The KB method represents net mineralization because there is no correction for adsorption/desorption, which we have shown to be complex and difficult to interpret in nonsterilized samples. Modeled and estimated adsorption were often different, and relative differences varied among soil types. We also recommend combining the extractable inorganic P and microbial biomass P fractions into a single “labile” pool for isotopic‐dilution studies and that the incubations are run over no more than 3 to 5 d. Although the KB method represents a conservative estimate of P mineralization as net P mineralization, it corresponds to a useful indicator in comparing potentially any soil type.

No abstract

No abstract

No abstract

No abstract

Phosphate sorption capacity estimated by Piper's (1942) 'anion exchange capacity' and Bache and Williams's (1971) phosphate sorption index were correlated with soil pH, clay, organic matter, 'free iron oxides' and 'extractable aluminium ' (McLean et al., 1958) for topsoil and subsoil samples from twenty tropical and twenty British acidic soil profiles. These two groups of soils 'did not differ significantly in phosphate sorption. Extractable aluminium and free iron oxide were well correlated with phosphate sorption, free iron oxide being superior to aluminium in freely drained British soils but not in poorly drained ones. Organic matter content correlated well with phosphate sorption for the poorly drained British soils, and for the tropical soils when sorption capacitywas measured using a high phosphate concentration.