Plant Availability of Isotopically Exchangeable and Isotopically Nonexchangeable Phosphate in Soils

Isotopically exchangeable P (IEP) is usually considered to be completely plant-available and the major source of P for plant uptake. The aim of the present study is to test whether plants can, besides IEP, also use non-IEP and if part of the IEP has an equilibrium concentration in soil solution which is below the minimum concentration, C-Lmin, and can therefore not be taken up by plants. A pot experiment was carried out with maize for two years on two soils, an acid sandy and a neutral loamy soil, either without P fertilizer or fertilized with ten P sources of different solubility. Throughout both years of the study, pots were kept moist either without plants or planted twice with maize (Zea mays L., cv. Athletico). At the end of the experiment, plant P uptake, P concentration in the soil solution (C-L), and P accessible to isotopic exchange within 5 d (E-5d) were measured. Plant growth decreased the E-5d which was about equal to P uptake by maize for most treatments in the acid soil. But for some treatments, i.e., five in the acid and eight in the neutral soil, P uptake was up to 50% larger than the decrease of E-5d, indicating that plants had, besides IEP, also used P from non-IEP sources. At adequate P supply, both soils had an E-5d of about 100 mg P (kg soil)(-1), but about 30 to 40 mgkg(-1) of this IEP had an equilibrium P concentration in the soil solution below C-Lmin of 0.1 mol L-1 at which P would actually not be plant-available. This study shows that plants take up P mainly from IEP, but not the whole IEP is plant-available. Furthermore, plants may also use P from non-IEP sources.