Estimation of phosphorus needed for attaining sufficiency levels in volcanic soils from the south of Chile

María Adriana Carrasco R.1-2 y José Domingo Opazo A.1-2

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 PRSsoil = (needed P-initial P) PRUsoil, where PRUsoil is the quantity to increment the Olsen-P in one unit (mg kg-1) and was calculated as PRUsoil =(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

Keywords: P sufficiency level, phosphate soil incubation, P rates estimation, total extractable acidity, soils derived from volcanic ashes.
1 Departamento de Ingeniería y Suelos, Facultad de Ciencias Agrarias y Forestales, Universidad de Chile, Casilla 1004, Santiago, Chile.
2 Correo electrónico: acarrasc@abello.dic.uchile.cl