Abstract

Soil responds dynamically to the addition of phosphates by establishing exchanges among the different reservoirs of P existing in the soil. The aim of this work was to find out the effects of surface applied fertilization with triple superphosphate at two concentrations (50 and 100 kg P ha -1 ) in soils of the Sierra de Gata mountains (Spanish Central System) on forms of soil P, the availability of P for plants, and the impact on above-ground production of the tree cover and grasses. Soil P was deter mined according to Hedley, and P availability was quantified using ion exchange membranes. The effects of P fertilization on P forms were observed mainly in the first 10 cm of the study soils; there were no significant differences at greater depths because of the high P sorption capacity of these soils. With a moderate fertilization of 50 kg P ha -1 only two of the four studied soils showed a significant increase in labile P contents and of P adsorbed by sesquioxides. An intense fertilization (100 kg P ha -1 ) had significant effects on the distribution of soil P, independently of the initial available P of soils. However, fertilization with triple superphosphate did not significantly affect the aerial production of the tree cover (except in the-rst year), whereas it did affect herbaceous production at two sites, possibly because they have a larger root mass near the soil surface.

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