Inspection Services Blog

Research Update: Assessing Nitrogen Content to Improve Crop Management

Note: This is part of a Research Update series that highlights projects funded by the California Department of Food and Agriculture (CDFA) Fertilizer Research and Education Program (FREP).

Read the first update on this project.

Project Title: Assessment of Harvested and Sequestered Nitrogen Content to Improve Nitrogen Management in Crops

Project Leaders: Charlotte Gallock, Dr. John Dickey and Dr. Ken Cassman

Project Status: Year 3

Overview: This project is refining nitrogen (N) removal rates for Central Valley crops so growers can plan their N management more accurately and better comply with regulatory requirements. Researchers are determining N removal values through three approaches: field sampling of harvested crops, estimating N in perennial tissues with crop growth models, and integrating data from other research projects. The culmination of these three approaches should refine N removal values for 23 crops. To date, over 900 samples have been collected and analyzed. Additionally, the project leaders have disseminated information about this work at eight outreach events during the past two years.

Background: Through the Irrigated Lands Regulatory Program (ILRP), the Central Valley Regional Water Quality Control Board (Regional Board) requires growers to implement management practices that protect groundwater quality. To document the effectiveness of those practices, the Regional Board requires that coalitions provide information on growers’ field N balances. These balances consist of N applied (A) minus removed (R) at harvest to gauge grower progress in reducing leachable N. To comply with these reporting requirements, growers and the ILRP coalitions need reliable data about the amount of N removed from fields in harvested crop and sequestered in perennial (woody) tissues. Growers also use N crop removal rates to develop Irrigation and Nitrogen Management Plans (INMPs) to minimize the risk of nitrate leaching below the root zone.

The objectives of this project include assessing N concentration of harvested material for 23 major Central Valley crops. Additionally, work from this project will establish values for N sequestered annually in standing biomass for seven perennial crops. Based on the data collected, the researchers will refine crop-yield-to-N-removal (Y-to-R) conversion factors and estimate N-sequestration rates. This information will expand knowledge and awareness of appropriate uses of N-removal coefficients and N-sequestration rates by growers, advisers, researchers, and coalitions.

ApproachTo achieve the objectives listed above, the project is using crop growth models for seven crops to determine N sequestration in perennial tissues. The crops included in this analysis are as follows: almond, English walnut, pistachio, orange, tangerine, peach, and pomegranate (Table 1). These estimates are complemented with whole-tree sampling from other studies for all except pomegranate and peaches. Additionally, the researchers have developed protocols for, and sampled, 11 crops to determine N removal rates for pomegranate, peach, plum, carrot, raisin, sunflower, safflower, processing tomato, pistachio, corn and sorghum silage (Table 1). They coordinated sample collection with grower/packer/shipper partners, and the samples are currently undergoing lab analysis.

*Central Valley Irrigated Acreage Estimate

ActivityCropsNo. of CropsPercent CVIA*
Update and Remove Coefficients
Crops for which N removed data are currently available and will be incorporated into an updated N-removed report. Almond, English walnut222%
Crops for which N removed data will be sampled and analyzed as a part of this investigation.Pomegranate, peach, plum, carrot, raisin, sunflower, safflower, processing tomato, pistachio, corn silage, sorghum silage 1119%
Crops for which N removed sampling and analysis are currently being funded by other sources. Results will be incorporated into the updated N-removed reportPima cotton, prune, bok choy, ong choy, chives, lemongrass, orange, tangerine, daikon, table grape 10 18%
Estimate N Sequestration on Perennial Tissues
Crops for which data on N sequestration in perennial tissues are available or in development and results will be incorporated into the updated N-removed report. Almond, English walnut, pistachio, orange, tangerine, pomegranate629%
Crops for which N accumulation in perennial tissues will be estimated with growth models as part of this investigation.Peach and all the previously listed perennial crops11%
The sampler at a Processing Tomato Advisory Board (CDFA) grading station is collecting a subset of tomatoes from a truck transporting the crop from harvested fields to the processing facility.
Processing tomatoes being sorted and sampled at a tomato grading station in California.
Processing tomatoes are being sorted to determine grade, and subsampled to determine solids, pH, and color. A portion of the pureed subsample is taken to analyze N content.

The data for the remaining crops will come from existing sources and research currently underway through other research projects. As this project has evolved, the researchers have included more crops in the study than originally planned.


For this project, the researchers collected over 900 crop samples in the past two years and will continue to collect samples in the 2020 growing season. Once they process and analyze these samples, the project leaders will distribute the information through presentations, publications, online calculators and grower coalitions. This task will commence as results become available in 2020.

Examples of project outreach to date include:

As the work from this project helps to refine N-removed coefficients, the revised coefficients will be used to satisfy the following regulatory requirements:

  • Irrigation and Nitrogen Management Plan (INMP) Summary Reports: Coalitions process these reports with N-removed coefficients, and then report back to growers. This process facilitates proper accounting for N removal values by growers as they plan fertilizer scheduling. N applied to and removed from irrigated lands in each township are reported by coalitions to the Regional Board.
  • N Management Plan Summary Report data from throughout the Central Valley: The ILRP coalitions analyze the INMP Summary Reports from an agronomic perspective, and results are shared with commodity groups, growers and farm advisors to evaluate and verify future nutrient management studies. The N removal coefficients are employed directly in calculation of the N balance in this analysis.

Preliminary results based on samples analyzed to date are shown in Table 2. Note that the concentrations shown for the current study are the simple averages of analyzed results, which is not always the same as a final Y-to-R coefficient. For example, early peaches contain more N than late peaches and are equally represented in sampling with mid-season and late peaches. However, later crops yield more tonnage, so a finished coefficient might be weighted to strengthen representation of the later harvested peaches. Also, N contained in non-measured parts of the harvest (e.g., trash and culls) will be built into coefficients

Table 2. Preliminary sampling results for nine of the 11 sampled crops. Concentrations are entered as received material, wet-weight basis, except as noted for pistachio.

Table showing the sampling results across two years for carrot, corn silage, peach, pistachio, pistachio trash, plum, pomegranate, safflower, sunflower, and processing tomato.
Table 2.

Work Planned for 2020

Sampling continues for a few crops (sunflower, raisin, and plum). Estimates of perennial tissue N content are being developed. Also, the results are being documented in an update of Daniel Geisseler’s 2016 publication, “Nitrogen concentrations in harvested plant parts – A literature overview.”

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