Effects of Phosphorus, Potassium, and Sulfur on Alfalfa Nitrogen‐Fixation under Field Conditions2

Abstract

AbstractThe influence of P, K, and S on symbiotic fixation of N2 in alfalfa nodules under field conditions is not well documented, even though through this mechanism shoot yield may be limited. The objective of this experiment was to study the effect of P, K, and S fertilization on alfalfa shoot yield, nodule numbers, and N2‐fixation as estimated by acetylene reduction by taking core samples (80 mm diam) at two sampling times at two locations (a silt loam and a loamy sand soil). Potassium fertilization increased shoot weight per plant, nodule number per core, and N2‐fixation rate at both locations more than the other nutrients studied. Shoot weight per plant increased 166% on the silt loam soil and 178% on the sandy soil with the addition of 448 kg K ha−1. Phosphorus fertilization also increased shoot weight at both sites but less than did K fertilization. Addition of P and S increased nodule numbers on the sandy soil but not on the silt loam soil. Nitrogen‐fixation rate was increased by 2.8 fold with the addition of 224 kg K ha−1 on the sandy soil and 1.7 fold on the silt loam soil. Increased N2‐fixation resulted from K fertilization only when P was added to the silt loam soil. Phosphorus addition increased nodule numbers and N2‐fixation only on the sandy soil. The presence of a K ✕ S interaction for N2‐fixation but not for shoot weight or nodule number per core indicates that some differences in fertility response exist between N2‐fixation and other variables, even though all tend to be positively correlated. Sulfur fertilization had no effect on shoot weight but increased nodule numbers in alfalfa grown on sandy soil.