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Nitrogen Sources for Bean Seed Production¹

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 N₂ 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 N₂ (AR) fixation. The symbiotic N₂ fixation was also estimated by the equation, N₂ = N_up – (Ni + N_m – N_h)–N_f, where N_up is the accumulated N uptake measured near physiological maturity, N₁ and N_h are the amounts of soil NO₃-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 ¹⁵N-depleted (NH₄)₂ SO₄.

The symbiotic N₂ 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 N₂ 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 N₂(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 N₁ 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.

Key Words: Acetylene reduction • Fertilizer N utilization • N fixation • N uptake • Phaseolus vulgaris

² Soil scientist, Snake River Cons. Res. Ctr., Kimberly, ID 83341; crop physiologist, Univ. of Idaho Res. and Ext. Ctr., Kimberly; and soil scientists, U.S. Dep. of Agric., Fort Collins, Colo., and Kimberly, Idaho, respectively.

Received for publication June 5, 1980.