Soil P forms and P uptake under intensive plant growth in the greenhouse

Abstract

The concentration of available soil (P) is a function of the equilibrium established among different soil P forms through numerous and different reactions in soil. The objective of this study was to examine the changes in P forms and P supply under exhaustive extraction conditions in soils from 3 different land use areas. In order to establish a greenhouse experiment, representative soil samples (0-20 cm) were taken from three fields located adjacent to one another, in a Typic Hapludands in Costa Rica. One field was a coffee plantation (Coffea arabica var Catuai), the second a sugar cane plantation (Saccharum spp. var 611721), and the third a secondary forest. Sorghum (Sorghum bicolor var Glazer 41) was planted in 1-liter pots and harvested 4 times consecutively. Treatments were no P and P application (100 mg kg-1) for each of the different land-use soil samples. Shoot and root dry matter and total P uptake were determined. Soil samples were taken before and after each of the 4 plant growth cycles and analyzed using a modified Hedley et al. (1982) soil P fractionation methodology. Labile-Pi, NaOH-Pi, HCl-Pi, extractable-Po, and residual-P were determined. Applied P increased labile-Pi, NaOH-Pi and HCl-Pi. Statistical changes were not observed in extractable organic P and residual-P due to P application. The NaOH-Pi and HCl-Pi seemed to act as a temporary pool of applied P. The possible participation of residual-P in replenishment of labile-P and NaOH-Pi was observed. The amount of plant P uptake was closely related to the initial amount of labile-Pi and was higher in coffee than in forest and sugar cane soils. The labile-P was depleted by plant uptake. Rapid changes in reversibly available soil P forms (NaOH-Pi and HCl-Pi) were observed during the experiment. Our results suggest the occurrence of very rapid and dynamic changes between available and unavailable soil P forms in response to fertilizer application and plant uptake, supporting the idea of a continuum among the P forms.