The Central Coast Regional Water Quality Control Board (CCRWQCB) approved Ag Order 4.0 in April of 2021. This regulatory action requires growers to calculate the quantity of nitrogen (N) that leaves their fields in harvested product. This value is needed to calculate the metric applied (A) minus removed (R) nitrogen (A-R) which indicates the amount of N that remains in the field over the season and is considered the load of N at risk for nitrate leaching to groundwater resources that municipalities rely upon for drinking water. In Table 1 crop removal coefficients are provided for 75 crops and crop products. They include the larger acreage vegetable crops such as lettuce and broccoli as well as smaller acreage crops such as summer squash, green beans and Asian vegetables. Given the large number of species, gerberas (no foliage on the stem) and snap dragons (foliage on the stem) were evaluated as representative flower crops. Growers producing different flower crops can substitute gerberas or snap dragons to supply the N removal estimate of their crops.

The crop removal coefficient is multiplied by the total weight of harvested product to provide an estimate of the pounds of N that are removed from the field. This information can then be used to report N removal from production fields to the CCRWQCB.

For instance, in the case of whole heads of bulk romaine lettuce which in this example has a net* yield of 36,000 lbs/acre:

36,000 lbs harvested product/acre x 0.00149 = 53.6 lbs of N/acre removed

*subtract box weight from gross weight.

N removal coefficients were determined by multiplying the percent moisture content of the harvested product by its total N content. We worked with harvest crews to get freshly harvested products and immediately measured the moisture content. The samples were then dried and sent to the UC Davis Analytical Lab for total N content. We collected 4 to 6 samples per field from at least 10 to 15 production fields with diverse soil types, production practices and growing conditions (see Table 1 for exact number of fields sampled for each commodity), and as a result, there is variability around the mean values which is shown in Table 1.

Coefficients tend to be higher for commodities with high N content and high percent solids. The CCRWQCB allows growers to use different coefficients if they can justify their use; and growers can also develop their own coefficients using CCRWQCB guidelines. However, N removal coefficients for a given crop will tend to fall in certain ranges. For instance, high N content leafy vegetables will have higher coefficients that will tend to vary from 0.005 to 0.006. Head vegetables tended to range from 0.002 to 0.003. So, although coefficients may vary to some degree, they will tend to fall within certain ranges.  In the above example if the maximum coefficient of 0.00166 was used for bulk romaine, the increase in the estimate of N removal would only be about 6 lbs N/acre (59.8 lbs N/acre removed).  

Table 1. Mean and range of crop coefficients (coeff) developed by this FREP funded project and additional crops evaluated. Factors used to develop the coefficients are also included: percent dry matter (%DM) and percent nitrogen (%N)

Table 1. continued. Mean and range of crop coefficients (coeff) developed by this FREP funded project and additional crops evaluated. Factors used to develop the coefficients are also included: percent dry matter (%DM) and percent nitrogen (%N)