Ag Order 4.0 Finalized: Implications for Nitrogen Management of Vegetables on the Central Coast

On April 15^th, the Central Coast Regional Water Quality Control Board (CCRWQCB) finalized and approved Ag Order 4.0. The new rulings affect several aspects of agricultural production such as buffer areas and discharge of pesticides to water ways. In this article we will focus on the impacts of Ag Order 4.0 on the use of nitrogen (N) fertilizers.

New targets and limits on the use of N fertilizer are calculated using the A minus R metric. In this scenario, “A” is N applied to the crop in the form of fertilizer (A_fertilizer), N in irrigation water (A_irrigation), N supplied by compost (A_compost) and N mineralized from organic fertilizer (A_{organic fertilizer}). “R” is N removed from the field by the crop (R_harvest), scavenged during the winter fallow by cover crops or immobilized by high-carbon compost (R_scavenge), N removed by denitrification bioreactors (R_treated), N sequestered in woody plant biomass (R_sequestered), or other unspecified forms of N removal from fields (R_other). Although AgOrder 4.0 outlines 3 pathways to compliance, pathway 1 is most likely the one that most ranches would use unless the wells have very high nitrate-N concentrations (> 40 ppm N).  A - R is not to exceed the targets or limits shown in Table 1 for pathway 1 compliance. The A-R is calculated over the growing season on a land acre basis. If two or more crops are grown on the same physical acre, each crop contributes to the value for the year. Below is a discussion of each component of the A-R metric.

Table 1. A-R regulatory schedule

 The “A” side of the equation:

A_fertilizer is the amount of N fertilizer added to grow the crop. The actual units of N in lbs/A are used in this calculation.

A_irrigation is the amount of N contained in the irrigation water that is taken up by the crop. For most vegetable and berry crops grown on the central coast the volume of water that must be accounted for in this calculation is equivalent to the volume used by the crop for evapotranspiration (ET).  For crops where less water is applied than ET, the volume of applied water can be used in the calculation.  To calculate A_irrigation use the equation:   

A_irrigation = water volume (inches) x nitrate-N concentration of water (ppm N) x 0.227

The factor 0.227 converts the units inches x ppm N to lbs of N/acre.

For example, if a lettuce crop uses 7.3 inches for ET, and is irrigated with water that has a 37 ppm N concentration, the A_irrigation would be:

 7.3 inches x 37 ppm N x 0.227 = 61 lbs N/acre

A_compost is the amount of N provided by compost. Given that the amount of N that is mineralized by the compost depends on the carbon to nitrogen (C:N) ratio not all the N in the compost becomes available. This fact is recognized in the Ag Order as follows:

For compost with a C:N ratio of <11, the amount of N in the compost is multiplied by 0.10, and for composts with a C:N ratio of >11 the amount of N in the compost is multiplied by 0.05. Only this amount of N is added to the A side of the equation. These discount factors were an important change made by the Regional Board staff to reflect the actual quantity of N provided by compost and to avoid a disincentive to the use of composts, a key soil health practice.

A_{organic fertilizer} is the amount of N that is mineralized during the cropping system. The amount of N mineralized depends upon the C:N ratio of the material and the CCRWQCB is using the regression curve in a recent paper published by Lazicki et al (2020) to determine the amount of N mineralized. For instance, a material like 4-4-2 has a C:N ratio of 7.3 (29% C/4% N) and has a discount factor of 0.39 (Table MRP-3 in Attachment B). This means that if 100 units of N are applied as 4-4-2, the amount mineralized from this material and that is attributed to the A side of the equation is 39 lbs/A (100 lbs x 0.39). This discount factor acknowledges the fact that not all N in organic fertilizer mineralizes during the cropping season and was an important correction made to Ag Order 4.0.

 The “R” side of the equation:

R_harvest is the amount of N that is removed from the field in the harvested product. The amount of N removed in the harvested product is calculated by a removal coefficient composed of the percent moisture multiplied by the percent N of the crop. This coefficient is then multiplied by the net pounds of product harvested from a field to determine lbs N/A removed. We have been working on a project developing N removal coefficients for a number of vegetable commodities. Table 2 shows data for full term romaine lettuce. Note that percent solids and nitrogen values observed in our evaluations vary significantly and that the mean value has a notable degree of variability which affects the estimate of N removed by the crop. Regardless of the variability, the important point to recognize is that the removed N is modest in relation to the amount of N applied. Figure 1 shows total fertilizer N to lettuce for a large number of vegetable operations in our area. It becomes evident that growers face some major challenges in complying with the application limits as the limits ratchet down over the next several years.

Table 2. Example of calculation of removal coefficient for a full-term romaine crop.

* 0.0569 x 0.0315 = 0.00178

Figure 1. Nitrogen fertilizer applied to lettuce in 2017. Data reported by growers to the CCRWQCB

R_scavenge is the amount of N that is captured by by cover crops or immobilized by high-carbon compost during the winter fallow period. The CCRWQCB agreed to credit non-legume winter cover crops with 97% of their N content that meet the following criteria: 1) are grown for ≥ 90 days during the winter fallow period, 2) accumulate more than 4,500 lbs/acre of dry biomass and 3) have a C:N ratio of ≥ 20 when incorporated into the ground. N scavenging credits granted for cover crops are helpful, but do not remove any of the current logistical barriers to using of cover crops in intensive vegetable systems. However, given that non-legume cover crops routinely contain 100 to 150+ lbs N/acre, as N application limits ratchet down, cover crop use may be incentivized to some degree.

High-carbon composts were also included in the R_scavenge category. This practice is still being researched to fully understand how  much N can be immobilized.  Growers already use compost (typical C:N ratio of 10-12) but could substitute high-carbon compost (C:N ratio of >30) which can quickly facilitate its use. Currently, high-carbon compost has been granted a credit of 30 lbs N/acre in Ag Order 4.0. However, once the research on this practice is completed, this practice may be granted greater credits as warranted on the R side of the equation.

R_treat is the quantity of N removed from tile drainage  and irrigation runoff by denitrification bioreactors or constructed wetlands. This practice can be implemented in the northern part of Monterey County where high nitrate tile drain water impacts the surrounding sloughs and creeks.  The bioreactors vary in size and sophistication, from sunken beds filled with wood chips to highly engineered portable treatment systems.

R_sequestered is the quantity of N that is captured in the woody plant tissue of perennial crops. This form of N removal is relevant to vineyards and orchards in our area and does not impact the vegetable industry.

R_other is the quantity of N removed from the field in other, unspecified ways. One form of N removal not addressed in Ag Order 4.0 is the gaseous loss of N by denitrification from soil. This is a topic that needs further research. Two studies done on the Central Coast showed that in sandy soils with drip irrigation, there is little nitrous oxide or dinitrogen loss (2-4 lbs N/acre/crop). However, an earlier study of celery and lettuce production fields in the 1980s showed that on heavier soil with furrow irrigation gaseous N losses ranged from 18 to 37 lbs N/acre. Further research is needed to understand denitrification rates more fully in coastal vegetable production.

What options does the industry have moving forward?

Basically, a timer has been started by Ag Order 4.0. The first dates are targets of 500 lbs N/acre/year 2 years from now and 400 lbs N/acre/year in 2025 (four years). Starting in 2027 the targets become limits ratcheting down to 300 lbs N/acre/year. In scenarios that we and others have run, the 300 lbs N/acre/year limit will become very challenging for growers to comply with in typical double cropped production. There are basically three key practices that will provide the most improvements in N use efficiency: 1) measuring residual soil nitrate and adjusting fertilizer applications accordingly, 2) accounting for the nitrate in irrigation water as part of the N budget, and 3) improving irrigation efficiency to help maintain residual soil nitrate in the active rootzone of crops. A concerted focus on these three practices will require a commitment from the decision makers at each farming operation. Farming operations have differed in their approach to the pending water quality regulations. Some have taken a proactive approach and are farther along on the learning curve. It is important to make attempts to begin implementing these practices and see what is possible for your operation given the crop mix, soil types, and nitrate levels in the irrigation water. The good news is that there is still time. To begin implementing these practices, it is important to start small to gain the needed knowledge base in efficient N and water management practices. Working with knowledgeable people will be essential.

 Other options that can help fine tune fertilizer applications and reduce the risk of cutting fertilizer rates are various nitrogen technologies such as nitrification inhibitors and controlled release fertilizers. In studies that we have done, there is clearly a benefit to the use of some of these materials, but again, there is a learning curve to obtaining the benefits that they can provide. Nitrapyrin (a nitrification inhibitor commonly used in the corn belt) was registered on lettuce and brassicas in 2019 and has not been widely used yet by the industry, but it along with other materials, deserves greater evaluation.

In addition to improving in-season N use efficiency, implementation of practices to minimize nitrate losses during the winter fallow period is needed.  Cover crops are a key practice for reducing leaching losses during the winter and finding opportunities for their use will become more important in the next several years. The credits given cover crops on the R side of the equation can help growers achieve compliance with Ag Order 4.0 as the A – R limits are reduced to less than 300 lbs N/acre/year.

In summary, the finalization of Ag Order 4.0 will have a significant impact on how vegetables are grown on the Central Coast in the coming years. There is a window of opportunity to experiment on how to address limits that will be applied to the use of N fertilizers. Now is the time to make the decisions needed to address this new reality.  Please do not hesitate to reach out to us for help or advice.

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