Phosphorus fertiliser source and weed control effects on growth of three-year-old second-rotation Pinus radiata on Orthic Pumice soil in New Zealand

Abstract

Little information is available on the interactive effects of soluble and less-soluble phosphorus (P) fertilisers and weed control on growth and P nutrition of second-rotation Pinus radiata plantations. A study was initiated on three-year-old second-rotation P. radiata and to determine the relationship between needle P concentrations and soil P forms in an Orthic Pumice soil two years after application. Four rates of P (0, 50, 100, and 200 kg P ha−1) were applied as two forms of P fertiliser sources (triple superphosphate [TSP] and Ben-Guerir phosphate rock [BGPR]) in combination with weed control (weedy and weed-free). The results showed that the applied TSP and BGPR can increase tree needle P concentrations even when the needle P concentrations before fertiliser application were marginally higher than the critical P concentrations. The application of P fertilisers had no effect on tree growth during the two-year period of the trial, though it increased P. radiata needle P concentrations. However, the weed removal increased tree height, diameter at breast height, and basal area. In this plantation site the higher needle P concentrations than the critical P concentration 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, Bray-2 P, Olsen P, resin-Pi, and NaOH-Pi tests. Of these soil tests, Bray-2 P seems to be the best test in predicting soil P availability to P. radiata. The P concentrations in the needles had a relationship with the NaOH-Pi fraction but had no relationship with the H2SO4-Pi fraction. These results suggest that P. radiata was probably taking up P mainly from the pool of P-adsorbed to allophane and Fe+Al oxides (NaOH-Pi) in this high P-fixing acidic soil.