Effect of phosphite phosphorus on alfalfa growth

Abstract

Phosphite (PO^‐3‐P) was compared to phosphate (PO₄ ^‐3‐P) at 2.5, 5, 10, 20, and 40 mg ? kg^‐1) in a P‐deficient soil in a greenhouse pot study with alfalfa (Medicago sativa L.). Alfalfa growth (g dry matter pot^‐1) and ? accumulation (g ? pot^‐1) were measured by clipping each pot 3 times on 30‐day intervals. Although there was a significant (P<.05) growth response to ? during the first 30‐day growth period, it was significantly less from PO₃ ^‐3 than from PO₄ ^‐3‐P at the 10, 20, and 40 mg kg^‐1 rate. The growth response to ? continued through the next 60 days with only the 40 mg kg^‐1 PO₃ ^‐3‐P being significantly lower than PO₄ ^‐3‐P. During the first and second growth periods, there were no significant effects of PO₃ ^‐3‐P on total ? uptake, as compared to PO₄ ^‐3‐P, except at 40 mg kg^‐1. Tissue concentrations of PO₃ ^‐3‐P at 40 mg kg^‐1 dropped successively from 150, to 50, to 10 ppm during the first, second, and third growth periods. This suggests that 40 mg kg^‐1 of soil‐applied PO₃ ^‐3‐P had largely oxidized to PO₄ ^‐3‐P by 90 days after application.