Here is an exert from a publication that Dr. Scott Tubbs, UGA Cooperative Extension Cropping Systems Agronomist, has shared with us about reduced nitrogen fixation in peanuts after significant rainfall.
It is recommended to check fields that have received abundant amounts of rain recently for nodule activity and active N-fixation. To do so, select several plants from low spots in the field that may have experienced prolonged saturated soil conditions. Slice several of the nodules open on each plant. If the nodules are pink, red, or dark purple in color and appear moist on the interior, then those nodules are healthy and there are no concerns of reduced N-fixation. However, if the interior of the nodules are gray, white, green, or brown in color and they appear dry on the interior, then those nodules are most likely no longer active, so the chances of N deficiency will be greater as we enter into pegging and pod-fill – a very N demanding period of crop development.
When peanut roots and nodules are saturated and cannot access oxygen, N-fixation can cease either temporarily or permanently. Conditions can vary by soil type and micro-climate, and we do not have data to determine exactly how long a peanut plant can withstand adverse conditions, but we theorize that if a field was in saturated soil conditions for 48 hours or less, then the crop may experience brief yellowing from temporary shut-down of nodule activity, but the crop likely will not experience any serious injury or yield loss. Longer periods of water-logged conditions start to carry concern of damage which may need to be addressed.
If you feel that your peanuts may not be fixing nitrogen, contact your local county agent.