Oregon state fertilizer guides are based on legumes fixing the majority of their nitrogen from the air because this is the most economical source for the grower. However, where low cost sources of nitrogen are available such as with re-use water from a food processor or manure from cows, applied nitrogen can be substituted for fixed nitrogen.
Legumes are sensitive to nitrate level in the soil.
Where nitrates are present nodulation and N fixation are inhibited. In other words, legumes generally take up nitrogen from the soil first before they will fix their own N. Legumes are lazy before going to work to fix nitrogen they will consume what is available in the local soil environment. Nitrogen fixation is expensive to the plant consuming carbohydrate resources that might otherwise go to yield. The substitution of applied N for biologically fixed N by alfalfa has been documented in the literature cited in this letter. OSU fertilizer guides were not written for a situation where alternate nitrogen sources are available from manure or re-use applications.