ChileanJAR

The volcanic soils from the south of Chile, Andisols and Ultisols, receive high rates of phosphorus applied in bands in which a high P concentration appears. This P reacts with the solid phase of the soil and hence it is important to evaluate the available or labil P. In this study, by means of a laboratory incubation experiment, the P availability was evaluated, the soil chemical properties that control it were identified, and equations to estimate the P rate needed for attaining a given P sufficiency level are proposed. The incubation experiment ínvolved eight soil samples taken in the VIII, IX and X Regions and was carried out in an incubator, for 15 days, at 25 ºC, and at a relative humidity equivalent to 60-70% of the field capacity. The rates of P applied varied from 150 to 900 mg kg^-1 P. The available P was measured by the Olsen method (Olsen-P) . The equations obtained for estimating the Olsen-P (y) as a function ofthe P applied (x) are ofthe type y = a + bx. They show that the method of incubation is a reliable technique for determining the relation between Olsen-P and the applied P. The "a" constant was related to the k₂, constant of the twosurface Langmuir equation (a = -27.247 + 20.286k₂ ) and "b" with the total extractable acidity (TEA) (b = 0.4268 - 0.005433TEA). The slope "b" was inversely related to the soil P retention capacity. The P rates needed for a given sufficiency level (PRS) were estimated using the equation PRS_soil = (needed P-initial P) PRU_soil, where PRU_soil is the quantity to increment the Olsen-P in one unit (mg kg^-1) and was calculated as PRU_soil =(0.4268 - 0.005433TEA)^-1. The estimated rates would be applicable to field conditions if it is considered that in the incubation experiment a situation similar to that in the fertilization bands occurs in sowings carried out with a combined