Growth response of second-rotation Pinus radiata on an Orthic Allophanic soil to P fertilizer and weed control.

Information on the interactive effects of soluble and less-soluble P fertilizers and weed control on the growth and P nutrition of second-rotation radiata pine (Pinus radiata) trees is required to determine appropriate management practices of P fertilizer and understory vegetation in radiate pine forest plantations. A field trial was conducted to investigate the growth of second-rotation P. radiata and determine the relationships between needle P concentrations and soil P forms in an Orthic Allophanic soil 2 yr after application of 4 doses (0, 50, 100, and 200 kg P ha^(-1)) of P applied in 2 forms of P fertilizer (triple superphosphate (TSP) and Ben-Guerir phosphate rock (BGPR)) in combination with 2 weed control practices (weeds present and weed-free). The application of TSP and BGPR increased the tree needle P concentration although the needle P concentrations before fertilizer application were marginally higher than the critical P concentrations, despite the soils being P deficient according to traditional soil P tests (Bray and Olsen tests). The application of P fertilizers had no effect on tree growth during the 2-yr period of the trial, although it increased radiate pine needle P concentrations. However, weed removal increased the diameter at breast height (DBH) and basal area (BA). Trees in this forest site had needle P concentrations higher than the critical P concentration. This suggests that the growth increase due to weed removal treatment was probably due to an increase in the availability of soil water and nutrients other than P. The needle P concentrations of P. radiata can be predicted by soil tests, and Bray-2 P, Olsen P, resin-P(subscript i), and NaOH-P(subscript i) tests. Of these soil tests, Bray-2 P seemed to be the best test for predicting soil P availability for radiata pine. P concentrations in the needles had a strong relationship with the NaOH-P(subscript i) fraction in the soil but it had only a weak relationship with the H2SO4-P(subscript i) fraction. These results suggest that radiata pine was probably taking up P more from the pool of P-adsorbed onto allophane and Fe+Al oxides (NaOH-Pi) than from the Ca-P pool in this high P-fixing acidic soil.