Although many growers in California's Salinas Valley monitor nitrates in plant tissue, soil, and water, fewer of those who monitor incorporate the data collected into their fertilizer scheduling.

That was one finding from a voluntary survey of growers by the Monterey County Water Resources Agency (MCWRA) during 2001 in an effort to profile nitrate management practices in the county, which for years has been concerned about nitrate content in wells.

The State Water Resources Control Board requested the study and funded it for $75,000 in two grants for a four-year project leading up to the final report. Other boards and industry groups collaborated broadly to represent all areas of the county.

Kathy Thomasberg, program manager for the MCWR in Salinas, said although only 107, or 35 percent, of the growers in the county responded, the 99,000 acres they represent is 49 percent of the county's 202,000 irrigated acres.

That, she added, was sufficient to establish a baseline for future focus on where best management practices (BMPs) for nitrogen use are most needed in the county.

She outlined the results recently in Tulare at a conference of the Fertilizer Research and Education Program, a partnership between the California Department of Food and Agriculture and several other governmental agencies and plant health industry groups.

Groundwater reliance

The study area, the Salinas Valley from Monterey Bay to San Ardo, has about 3,000 agricultural wells and 314 well operators. The area, Thomasberg said, “depends 100 percent on groundwater, and that sets us aside from any other area in California.”

The nitrate issue, and later the study, emerged because domestic wells have been shut down due to high nitrate content in the water. In 1986, 113 of the county's 1,207 small public water systems were in violation of safe drinking water standards.

In 1998, 31 percent of the agricultural production wells studied were found to contain nitrates exceeding the standards.

Thomasberg said the MCWR is searching for a double solution: what to do about existing nitrate and how to prevent more of it.

“For what's already there, we are promoting best management practices to incorporate those nitrate values in irrigation water, soil, and plant tissue into growers' fertilizer schedules.”

Nitrate prevention, she continued, is more complicated because of differing soils and climates and first needed a nitrate management survey to see what growers are presently doing. The survey generated a baseline to gauge future action for improvement in areas that most need promotion of best management practices.

Growers in the upper Salinas Valley reported the most nitrate monitoring and the most incorporating of data into fertilizer scheduling.

Offered guidance

“We were looking for a qualitative measurement, and the survey also served a purpose in leading us to ask why others are not using the data. It gave us some idea about where we need to focus our outreach in the future,” she said. University of California specialists assisted with interpretation of survey results.

The survey, among farms as small as five acres to as large as more than 8,000 acres, also revealed organic soil amendments were applied to over half the 99,000 acres farmed by responding growers. For nearly 90 percent of the acreage, respondents said they calculated the amount of nitrogen needed per acre of crop, coordinated fertilizer application and irrigation scheduling, and applied their crop nitrogen fertilizer budget in multiple applications.

Thomasberg said her agency promotes use of multiple applications as an important part of nitrogen management.

“Fertilizer was stored on the ranch for over 70 percent of the irrigated acres represented, and, of that number, just under 90 percent applied their own fertilizer.”

Among other findings related to BMPs, 65 percent of the respondents used outside consultants to guide their fertility programs, 65 percent used mobile irrigation lab services, 45 percent grew non-legume, fallow, cover crops, and 37 percent diverted and confined water for reuse.

Management gains

According to Thomasberg, 88 percent of the 107 respondents said they had improved their nitrate management practices over a 10-year period. Sixty-seven percent, she added, said they had implemented four or more best management practices.

However, she could not account for the difference between the number of growers who monitor for nitrates and those among them who use the data in fertilizer scheduling.

The confidential survey had a brief, four-page format and queried growers about the amount of fertilizer they applied, their fertilizer storage and handling, application methods, and other practices. It also invited comments from growers.

Since the survey was voluntary, she said the agency offered free nitrate quick tests with the questionnaire and a free flashlight when the completed questionnaire was submitted. Some 900 phone calls were made to follow up the questionnaires.

Thomasberg said her agency distributed the questionnaires in May 2001 but later learned they should have been sent out earlier, in February, when growers had more time to respond.

She credited the much of the success of the survey to “public and private partnerships” that are making progress toward finding local, voluntary solutions to water quality issues and to hopefully avoid regulation of nitrate management. Local farm groups helped with publicity on the purpose of the survey before it was taken.