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Increasing contamination of ground water by nitrates has been a long-standing problem in the United States, and in California it has been listed as one of six major water quality problems by the State Water Resources Control Board. In 1985, the U.S. Geological Survey found that more than 10% of the 2,732 water wells sampled in the state contained nitrates at concentrations exceeding the federal drinking water standard of 10 milligrams per liter (mg/L) nitrate-nitrogen (nitrate-N). Sources of nitrogen in soil include onsite domestic waste disposal systems (e.g., septic tanks and cesspools), dairies, animal feedlots and sewage effluent used for irrigation. In farming, applying fertilizers is a major contributor to the total load of nitrogen in the soil. Since the 1960s, one study found, increased use of nitrogen fertilizers has been accompanied by increased levels of nitrates in ground water. Ideally, only the amount of fertilizer that can be used by the plant is applied, leaving no residual to move below the root zone. However, in most cases, the plant does not assimilate all of the applied nitrogen; some does move below the root zone. Nitrogen in the soil that is not returned to the atmosphere in the form of nitrogen gas or ammonia is generally converted by bacteria into the nitrate form. Nitrate is very mobile; with sufficient water in the soil it can move readily through the soil profile. Careful management of nitrogen and water applications should minimize the amount of nitrogen moving below the root zone, thereby also minimizing potential nitrate contamination of ground water. The purpose of our project was to study the influence of nitrogen and irrigation management on the movement of nitrate in the soil below the root zone of mature avocado trees. This is part of a larger study investigating the effects of low-volume trickle and drip irrigation and fertilizer management on avocado yield and quality. This report offers the results of studying the influence of different levels of nitrogen applications and irrigation levels on nitrate-N concentrations in soil "water. Methods Trials were conducted near Corona, California in an orchard of mature Hass avocado trees. Three different irrigation levels were maintained in the orchard: 80, 100 and 120% of the crop evapotranspiration (ETC) level. The ETC is calculated using the formula:

In a study of avocado trees, using less fertilizer more often can reduce nitrate leaching