Because BNF provides society with a number of important economic and environmental benefits, there has been considerable effort to quantify the amount of N obtained via BNF in different crops. Scientists have used a number of methods to determine how much N is obtained by different crops. The values of BNF obtained by any of these methods should be viewed as estimates. The reason for this is twofold. First, all of the methods used to measure BNF have inherent limitations that prevent them from producing an absolutely accurate figure. Secondly, the magnitude of BNF in a given crop may vary greatly due to environmental conditions.

Estimated BNF values for a number of crops are given in the table below. A range of values is given for each crop because the magnitude of BNF varies according to environmental conditions as mentioned above. Further discussion of environmental variation in BNF is given in section 4. 


Species Range reported for total BNF 
(kg N/ha/growing season or year)
Alfalfa (Medicago sativa L.) 80-250
Alfalfa-Orchardgrass (Dactylis glomerata L.) Mixture 15-136
Birdsfoot Trefoil (Lotus corniculatus L.) 49-112
Hairy Vetch (Vicia villosa Roth) 50-150
Red Clover (Trifolium pratense L.) 22-150
Red clover - Reed Canarygrass (Phalaris arundinacea L.) Mixture 5-152
Lupine (Lupinus spp. L.) 50-100
Bean (Phaseolus vulgaris L.) 30-50
Cowpea (Vigna unguiculata L.) 50-100
Subterranean Clover (Trifolium subterraneum L.) 58-183
White Clover (Trifolium repens) 128
Table adapted from Brady and Weil (1996), p. 423 and Miller and Heichel (1995), p. 50



Estimated values for the magnitude of BNF (see table below) for different crops are sometimes used by scientists to help producers determine how much they can reduce application of N fertilizer in crop production. The term 'fertilizer N replacement value' or 'Nitrogen credits' are sometimes used to refer to the amount of N fertilizer reduction that can be obtained by using a particular N fixing crop in a rotation. As with measurements of BNF, the N credits given for a particular crop will vary, depending on factors such as density of the stand. For example, a 'good' stand of alfalfa (high density of plants) will typically be given a higher N credit value than a 'poor' stand (lower density of plants). Typical ranges for N credits vary from 40 lbs/acre (44.8 kg/hectare) for soybeans incorporated into the soil before corn to 150 lbs/acre (168 kg/hectare) for a good stand of alfalfa incorporated into the soil before corn. Producers should contact a local soil testing lab, agricultural extension agent, or crop consultant to determine N credits for legumes that are appropriate for their local conditions and production practices.