Phosphorus availability in samples of Aquatibia (Typic xerofluvent) and Hoda (Ultic haploxeralf) soils incubated with increasing amounts of organic materials (barley straw, cowpea plant tops, dairy manure and sewage sludge) was estimated by different chemical methods and by measuring the activity of the enzymes acid phosphatase and phosphodiesterase, at the Department of Soils and Environmental Sciences of the University of California/Riverside, from January to November of 1985. In a greenhouse study, yield of dry matter and P uptake by corn plants cultivated in the soil samples with the same organic materials were measured. Water extraction method correlated significantly with P uptake only when dairy manure and sewage sludge were added to Aquatibia and Hoda soils, respectively. The anion-exchange resin and the Mehlich 1 methods were suitable for measuring the P availability to plants when soils received increasing amounts of organic residues, as in both cases this variable was highly correlated with the yield of dry matter and the amount of P uptaken. The high amounts of P extracted by the anion-exchange resin procedure from the soil with high P sorption capacity indicated that this method not only took into account the intensity factor but also gave a better estimate of the capacity factor. Soil phosphodiesterase activity revealed to be a good index for estimating the P availability to the plant when both soils were amended with cowpea plant tops and dairy manure.
phosphorus availability; chemical extraction methods; enzime activity; sewage sludge; organic fertilizers; maize