Soil affects inoculum activity
Legume inoculation is the process of introducing commercially prepared sources of rhizobia to promote nitrogen fixation. This usually is done by applying inoculum directly to the seed prior to planting, or by metering the inoculum into the seed furrow during planting.
When in doubt about the rhizobial population in a field, it is a good practice to apply inoculum, especially if the legume has never or not recently been grown in that field.
Maintain proper soil fertility to ensure nodulation and nitrogen fixation. Some legumes normally get most of their nitrogen from the atmosphere through symbiotic nitrogen fixation. Attempts to supplement the legume nitrogen supply by fertilization usually are counterproductive, because plants tend to stop nitrogen fixation when soil nitrogen is high. Phosphorous and potassium can affect nodulation and nitrogen fixation. Research shows that additions of phosphorous and/or potassium increase the number of nodules formed, fresh weight of nodules, and amount of nitrogen fixed per nodule.
An important micronutrient for nitrogen fixation is molybdenum. Soils with a pH below 6.0 usually have low molybdenum availability. Other soils that could be low in molybdenum include those that are strongly weathered or leached, sandy soils, or soils high in manganese and iron. If molybdenum is a limiting factor, apply it as a seed treatment with the inoculum. Some inoculants have molybdenum already incorporated.