The supply of phosphorus from native, inorganic phosphorus pools in continuously cultivated Mexican agroecosystems

Correlative and experimental approaches were used to determine the sources of phosphorus (P) that supply the labile P pools in nine soils from traditional Mexican agroecosystems. A correlation analysis was undertaken on phosphorus fractions, particle size fractions, soil pH and organic matter contents. Labile inorganic P (labile Pi) fractions correlated significantly with soil HCl-extractable Pi concentrations and soil pH. No significant correlations were found between labile phosphorus and any organic phosphorus pools or particle size fractions. Alfalfa (Medicago sativa L.) was grown in small quantities of soil and then P fractionations were carried out on the alfalfa-depleted and original soils. Alfalfa consistently depleted labile inorganic phosphorus fractions in all soils, and in five of the nine soils, inorganic phosphorus associated with calcium phosphate minerals and/or Al or Fe hydrous oxides were significantly depleted under cultivation. The bicarbonate extracted organic phosphorus fraction was significantly depleted in one soil, but no other organic phosphorus fractions were significantly depleted by the alfalfa treatment. These results suggest that inorganic P fractions are critical sources of phosphorus in the traditional agroecosystems studied, and that soil calcium phosphate contents may in part determine where continually cultivated agriculture can be sustained without the use of synthetic fertilizers.