Potato Nitrogen Fertility and the Potential for Groundwater Contamination.

Areas of the Southern San Joaquin Valley have high measureable nitrate levels in the groundwater.  A State Water Resources Control Board commissioned report has indicated that crop land agriculture is the main source of nitrates in the groundwater. The area has also had a significant dairy presence for many years.  Annual rainfall is less than 20 cm, thus irrigation is necessary for a high productivity vegetable crop endeavor. A project was undertaken to evaluate current cropping practices in potato production and its contribution, or lack thereof, to nitrate movement and potential nitrate contamination of groundwater.  Nine potato fields were monitored for potential nitrate leaching. Pre-plant and post-harvest soil samples were collected to a depth of 2 meters.  Plant, root and tuber samples were collected and analyzed for nitrogen content. Soil moisture and irrigation amounts were monitored. Substantial soil nitrate-N was available prior planting and generally remained in the rooting zone following harvest. Total nitrogen required for the yields obtained in the Southern San Joaquin Valley were consistent with previously published data and nitrogen use efficiency exceeded eighty percent. Appropriate irrigation scheduling did not have water from moving through the root zone. Nitrate not taken up by potato plants remained in the root zone for subsequent crops.