The draft Agricultural Order issued by the Central Coast Regional Water Quality Control Board (CCRWQCB) on Nov. 19 increased the regulation of discharges of nitrate-nitrogen to surface and ground water from agriculture.

As written, all vegetable operations that produce over 1000 acres of lettuce, cole and several other ‘high risk’ crops and that use chlorpyrifos or diazinon are placed into Tier 3 compliance category which has specified regulations concerning the movement of nitrates to surface and ground waters.

The new regulations may require growers to implement a certified Irrigation and Nutrient Management Plan (INMP) to document information on nitrogen applied to crops vs nitrogen removed by crops. This information would be used to calculate a nitrogen balance ratio and growers are given three years to demonstrate nitrogen balance ratios of 1.0 for annual rotations that are double cropped. In other words, if double cropped lettuce annually removes 240 lbs of N/A (120 pounds N/A/crop), the annual amount allowed to grow two crops of lettuce in order to comply with the nitrogen balance ratio would be 240 pounds N/A/year. Given current production practices and traditional fertilization programs, complying with these new restrictions will require many growers and their fertility consultants to make a shift in their approach to fertilization of leafy green vegetables.

The ultimate goal of the regulations issued by the CCRWQCB is to reduce the load of nitrate that is added to agricultural operations in the hope to improve the quality of ground and surface waters in the valley. It is therefore important for us to explore ways that we can be more efficient with applied nitrogen fertilizer. It is important to keep in mind that there are tools that can help growers to deal with this new regulatory era and which safeguard yield.

There are a variety of approaches that can be taken to improve nitrogen use efficiency, but I will just discuss two simple first steps that can be taken to achieve better nitrogen use efficiency. The first is the much discussed nitrate quick test. It is particularly useful in double cropped leafy green vegetable production for the following reasons. Nitrate levels in Salinas Valley soils typically follow a predictable pattern over the course of the growing season (Figure 1).

Soil nitrate levels at the beginning of the growing season are typically low in the soil due to loss of nitrate from the prior season from leaching by rain that occurs during the winter. Once the first crop has been planted and the growing season progresses, soils begin to warm and nitrogen from applied fertilizer and from mineralization of nitrate from soil organic matter begin to increase the pool of nitrate in the soil.

Typically, higher amounts of nitrogen need to be applied to the first crop because of lower initial soil nitrate levels. As we get to the second crop however, there are higher levels of soil nitrate which can be accounted for with the nitrate quick test. For example, a value of 20 ppm nitrate-N is equivalent to 80 pounds of nitrogen in the soil. This amount of residual soil nitrate can be used to for growth of the second crop and fertilizer rates can be reduced accordingly. This is the reason that we often grow the second crop of lettuce with substantially less nitrogen than the first crop. The nitrate quick test gives you the information that you need to make an informed decision on the nitrogen needs of the crop without jeopardizing yield.

The second practice to reduce the load of nitrate added to vegetable operations is whether to use fall applied preplant nitrogen. We had an opportunity to follow the fate of an application last winter and observed a dramatic loss of nitrogen during a series of storms in 2010 (Figure 2).

The data indicates that the money spent on this application of nitrogen fertilizer was rapidly and nearly completely lost in one series of storms. This is clearly “low hanging fruit” in terms of nitrogen savings that can be achieved for lettuce production and a cost savings to the production budget. I realize that fall nitrogen applications are often mixed with decision regarding phosphorus and potassium applications; for more information on phosphorus applications to cool season vegetables check out the following publication: les/Monterey_County_Crop_Notes8723.pdf

The draft rules issues by the CCRWQCB are yet to be finalized by the full board in March 2011.

Whatever shape the final rules take, it appears that we have to begin the process of rethinking our approach to fertilizing lettuce and other leafy greens. The good news is that there are solid tools that can help the industry cope with this new regulatory era.