Potatoes in Rotation When Nitrogen Is Under Permit.

Growing potatoes with wastewater and the requirements a wastewater permit placed on the system causes interesting management problems.  Simple calculations were derived to help understand the nitrogen flux of the system; apparent nitrogen uses efficiency (ANE) and cumulative nitrogen use efficiency (CNE). Apparent nitrogen efficiency is crop N uptake divided by N applied.  Over a two year period ANE varied between 75% and 125% averaging 96% in year one and 101% in year two.  CNE, which is crop uptake plus residual post-harvest soil N divided by N applied plus pre-season soil N, varied between 62% and 183% averaging 81% in year one and 117% in year two.   Potatoes leave a significant amount of N in the soil after harvest between 153 lb./a 370 lb./a.  Post-harvest N is the greatest concern for regulators.  Post-harvest N is from decomposed leaves and stems not excess fertilizer applied during the growing season.  A deficit irrigated wheat crop will utilize nitrogen in the profile to a depth of five feet.  Without the deficit irrigated wheat potatoes are much more difficult to manage in permitted situations.