NITRATE AND PHOSPHATE LEVELS IN SOIL AND WATER AND HOW THEY REFLECT PREVIOUS CROPS ON A FARM

The soil and water on four ranches in the Salinas Valley of California were analyzed for nitrate and phosphate to assess the impact of agricultural practices. The ranches sampled had different crops growing on them and included spinach, lettuce, broccoli and fallow, such as mustard greens. Nitrate and phosphate concentrations were measured in the soil of each ranch and in the suspended sediments and water in runoff streams.

Soil nitrate concentrations were lowest on ranches with fallow (4.8 ppm). Ranches with commercial crops had an average nitrate concentration of 10.1 ppm, with nitrate concentrations of 10.6, 6.9, and 4.7 ppm determined for those growing spinach, broccoli and lettuce, respectively. Ranches in which fertilizer and/or compost was applied had an average nitrate concentration of 13.1 ppm, while unfertilized ranches averaged 7.2 ppm. Phosphate concentrations of 0.7, 3.2 and 3.3 ppm were determined for fields of spinach, broccoli and lettuce, respectively. Results indicate that phosphate concentration is not as strong a function of crop type as nitrate. Fertilized fields had lower phosphate concentrations than unfertilized (2.7 vs. 3.3 ppm). Extremely high values of nitrate, (50-500 ppm), were observed in water samples. This indicates significant leaching from the surrounding fields. Phosphate values were much less concentrated and less variable in runoff, ranging from 1-4 ppm.

These data indicate that the concentration of nitrate and phosphate concentrations in soils can be utilized to determine the amounts and type of fertilizer necessary to maximize growth for various crop types and can be used in the development of beneficial crop rotation practices. They can also determine whether fertilizer or composting is necessary, using nitrate and phosphate concentrations < 50 ppm as a guide. Higher nitrate values in runoff may be attributed to large amounts of rainfall during the sampling interval. Low nitrate values for the fields with fallow can be attributed to nitrate banking by these plants as they take up nitrates as part of their metabolic processes.