Factors affecting potato petiole sap nitrate tests

Many factors can affect the nitrate-nitrogen concentration of potato petioles. They include but are not limited to the following: N-fertilizer applications, mineralizable soil N, time of sampling, position of the petiole on the plant, age of the plant, potato cultivar, time of the day when plants are sampled, and the environmental conditions prior to sampling. A better understanding of these factors should lead to an improved sap test. Five irrigated potato studies were conducted between 1991 and 1993 to evaluate this variability. Sap nitrate-N concentration was found to be closely associated with N-fertilizer rates and soil mineralizable N. Sap nitrate levels were usually highest early in the season and declined with the age of the plant. The decline with time can be interrupted or delayed by in-season N applications. Although the sap nitrate level among six different cultivars and diurnal sampling periods was found to be statistically significant, the differences were relatively small. By careful selection of the most recently matured petiole (4th petiole from the top) and sampling between the hours of 10 AM and 12 noon, the variability of the sap nitrate levels can be minimized. Under Michigan's irrigated conditions, the fresh sap nitrate test is a valuable tool for managing N-fertilizer and reducing potential N losses to groundwater.