Nitrogen Sources for Bean Seed Production¹

Abstract

Abstract Beans ( Phaseolus vulgaris L.) often respond to N fertilization; however, N fertilization is not practiced for maximum seed production in southern Idaho. This suggests that the symbiotic relationship and/or soil N sources can provide most of the N needed by this legume. Our objective was to evaluate the relative contribution of the symbiotic‐nonsymbiotic N sources by studying the effects of N fertilization on the symbiotic N 2 fixation and seed yields under field conditions. Experiments were conducted on silt loam soils belonging to the Portneuf series (Xerollic Calciortnids). An acetylene reduction (AR) method was used to determine the effect of N fertilization treatments on the relative seasonal N 2 (AR) fixation. The symbiotic N 2 fixation was also estimated by the equation, N 2 = N up − (Ni + N m − N h )−αN t , where N up is the accumulated N uptake measured near physiological maturity, N 1 and N h are the amounts of soil NO 3 ‐N in the root zone before planting and near physiological maturity, N m is the N mineralized from soil organic N sources, and a is the recovery of the N fertilizer (N f ) applied. Estimates of the N fertilizer recoveries were obtained from two experiments using 15 N‐depleted (NH 4 ) 2 SO 4 . The symbiotic N 2 relationship contributed up to 90 kg N/ha, which was 40 to 50% of the total N found in bean plants near physiological maturity. The amount of symbiotic N 2 fixed decreased as the available soil N or fertilizer N increased, and increased as the N required by the individual cultivars increased. The response to N fertilization depended upon the cultivar, as well as on the N available from soil sources. Measured fertilizer N recoveries ranged from 7 to 33%. An average of 52% of the total N uptake near physiological maturity was taken up after the maximum symbiotic N 2 (AR) rate occurred; while the seed contained an average of 60% of the total N uptake. A low N fertilization rate (< 50 kg N/ha) when the soil N 1 was low (<50 kg N/ha) ensured an early vigorous plant growth but did not always increase seed yields. Higher N fertilization rates may be required on soils with lower amounts of mineralizable N.