Observations on phosphate upkeep manuring

In this experiment, carried out in slightly acid alluvial soil well supplied with assimilable phosphorus, the yearly addition of soluble phosphates to compensate for the uptake by crops has enabled the phosphate fertility of the soil, characterized mainly by the phosphorus isotope dilution kinetics, to be maintained at its initial level. However owing to the difficulties inherent in studies based on long-term experiments and the conditions peculiar to the test (slightly positive P balance, negative humic balance) the first results given are not sufficient to disprove the hypothesis recently put forward for the same type of soil, of some irreversible fixation of phosphate ions. Nevertheless it is safe to assume, even if this hypothesis is verified, that little more than the strict upkeep would be enough to maintain fertility. In fact manuring recommendations based on the compensation of uptake, in the type of soil investigated and on formerly enriched land, are unlikely to lower the fertility by this effect in the short or the long run