Assessing soil phosphorus status under different agronomic land use
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The study of the availability of soil phosphorus (P) to crops has been an important issue for years in different agroecosystems around the world. Because of the complex P cycling in soils this has been studied from different points of view. The study of soil P forms has been seen as a possible way to explain many processes and changes occurring in plant-soil interactions. The purpose of this research was to characterize soil P forms under different land uses and evaluate the relationship between different land use areas and the spatial distribution of soil P forms. The study was carried out in Costa Rica on a Typic Hapludand under coffee plantation (Coffea arabica ), sugar cane plantation (Saccharum spp.), and secondary forest. A modified Hedley soil P fractionation methodology was used for determining the soil P forms. Means of the relative content of P forms were 0.43% labile-Pi, 6.44% NaOH-Pi, 9.20% HCl-Pi, 32.55% extractable organic P and 51.37% residual-P. Inorganic fertilization was correlated with labile-P, NaOH-Pi, and HCl-Pi forms. In the first experiment the sugar cane yields were correlated with labile-Pi and NaOH-Pi. Organic fertilizer increased the extractable organic P suggesting an accumulation in this form. In the second descriptive study a spatial relationship was found with soil management areas. Sugar cane soil accumulated more P in HCl-Pi, extractable organic P, and residual-Pi forms. Coffee soil had the highest values in labile-Pi and NaOH-Pi that were correlated with the higher rate of fertilizer application. Secondary forest had intermediate values between these two cropped areas. The third experiment showed that under greenhouse conditions P uptake was closely related to labile-Pi and NaOH-Pi suggesting also that a sparingly available P form could be related to P uptake by plants in time. Extractable organic P and residual-P were suggested to act as a sink of the available P forms. It is concluded that under a sustainable crop production framework the adequate input of P is necessary in order to maintain the adequate nutrient supply through time.