California Agriculture

Soil nitrate (NO₃ ⁻) tests are an integral part of nutrient management in annual crops. They help growers make field-specific nitrogen (N) fertilization decisions, use N more efficiently and, if necessary, comply with California's Irrigated Lands Regulatory Program, which requires an N management plan and an estimate of soil NO₃ ⁻ from most growers. As NO₃ ⁻ is easily leached into deeper soil layers and groundwater by rain and excess irrigation water, precipitation and irrigation schedules need to be taken into account when sampling soil and interpreting test results. We reviewed current knowledge on best practices for taking and using soil NO₃ ⁻ tests in California irrigated annual crops, including how sampling for soil NO₃ ⁻ differs from sampling for other nutrients, how tests performed at different times of the year are interpreted and some of the special challenges associated with NO₃ ⁻ testing in organic systems.