Assessing soil phosphorus status under different agronomic land use

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.