Inspection Services Blog

Research Update: Phase 2 Assessment of 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) annual grant program.  

Read the first and second update on this project.

Project Title: Assessment of Harvested and Sequestered Nitrogen Content to Improve Nitrogen Management in Crops, Phase 2

Project Leaders: Charlotte Gallock, Kings River Watershed Coalition Authority (KRWCA), John Dickey, Formation Environmental, and Kenneth Miller, Formation Environmental

Project Status: Year 3

Overview: This project is refining nitrogen (N) removal rates for Central Valley (CV) crops so growers can plan their N management more accurately and better comply with regulatory requirements. The researchers are determining N concentration values and N removal coefficients that can be used to calculate the N removed from the field by harvested crops. This is accomplished by collecting field samples and estimating N in perennial tissues with crop growth models. Field samples include both the marketable component and any other plant material such as hulls, culls, trash, etc. The results will be incorporated into an online database and calculator that growers can use to calculate pounds of N removed per acre based on yield.

Background: Through the Irrigated Lands Regulatory Program, the CV Regional Water Quality Control Board now requires producers in the CV to implement management practices that are protective of groundwater quality. Producers are required to document the effectiveness of those practices by providing information on field N balances in an Irrigation and Nitrogen Management Plan (INMP). To accurately complete their INMPs, producers and their coalitions need accurate N removal coefficients that convert yield information into the amount of N removed from the field in the harvested crop material. This information can also help inform nutrient management planning, allowing N application rates to be scaled to meet crop requirements while minimizing excess N at risk of leaching below the root zone.

In 2016, Nutrient Management Cooperative Extension Specialist Daniel Geisseler published Nitrogen Concentrations in Harvested Plant Parts – A Literature Overview. This presented literature-based N removal coefficients for 72 crops representing more than 98% of CV irrigated lands. However, the report noted that most of these coefficients were based on datasets that included a small number of field studies that were conducted more than 20 years ago, or were collected outside the CV with cultivars, yields, cropping systems, and soil types that didn’t reflect contemporary CV conditions. The report showed that well-established coefficients were available for only 10 of the 72 crops, accounting for approximately 19% of CV irrigated lands. There were even fewer data on the amount of N sequestered in perennial crop biomass, which growers need when planning N fertilizer programs.

The Phase 1 FREP grant assessed the N concentrations of harvested material for 11 major CV crops and incorporated these revised coefficients into the Crop Yield to N Removed calculator for public use on the SSJV MPEP website. Geisseler’s N Concentrations in Harvested Plant Parts was also updated. The results from the Phase 1 research refined N removal coefficients for about 74% of CV irrigated lands.

However, due to difficulties in obtaining information and samples during the Phase 1 project, many important CV crops still required analysis to establish accurate coefficients. The project organizers applied for a Phase 2 FREP grant to complete the dataset of N removal coefficients and N sequestration rates to cover 98% of CV agricultural acreage. A summary of the Phase 1 project can be found on the SSJV MPEP website.

Aerial view of Central Valley farmland showing the mosaic of crops grown in the region.

Approach: This Phase 2 research aims to continue assessing the N concentration of harvested crop materials for approximately 33 crops. N sequestration rates will also be calculated for the eight perennial crops. Field samples collected over several seasons are currently being analyzed for N concentration to refine the nitrogen removal coefficients and N sequestration rate estimates. The basis for N removal rates (e.g., nuts in shells or shelled, grapes with stems) is determined in consultation with each industry to ensure that values relate to tonnages readily known by growers and capture total biomass removed from fields as closely as practicable. Existing sampling data and crop growth models are used to estimate N sequestration rates into perennial tissues.

Results: During 2022, the following crops were sampled and analyzed: apples, apricots, bell peppers, lemons, mandarins, oranges, cherries, cucumbers, figs, kiwis, melons (cantaloupe, honeydew, watermelon), nectarines, Irish potatoes, sweet potatoes, squash (butternut, spaghetti), sweet corn, fresh market tomatoes, table grapes, wine grapes, and table olives.

Results presented at the 2022 FREP conference indicated variability in N concentration values derived from field data. The main sources of variation were year-to-year variability, N availability (including differences in fertilizer application rates, residual soil nitrate, nitrate available in irrigation water, soil mineralization potential and loss via excess irrigation), variety, site differences, dry matter content, growth stage and season when harvested, and availability of other nutrients. These factors contributed to differences between the crop N removal coefficients derived from the initial literature review and those calculated during Phase 1 of the research, underscoring the importance of collecting samples that are representative of current CV cropping systems.

For example, the data from oranges (Table 1) shows how collecting field data through the Phase 2 research results in a more accurate accounting of the potential N remaining in the field. The current value derived from the literature review, 2.96 lb. N/ton fresh weight underestimates the amount of N removed at harvest compared to data collected from the Phase 2 samples (Table 1). By correcting the N concentration to account for the greater N content in the harvested fruit, growers can provide a more accurate account of the N balance in their fields.

Table 1. N removed (pounds per ton fresh weight) for Valencia and Navel oranges from phase 2 of the project.

Reaching Growers: In 2023, the project organizers will continue to develop and refine sampling protocols as they carry out the third year of the Phase 2 research. Once all the samples and data have been analyzed, final N removal coefficients and N sequestration values will be incorporated into the SSJV MPEP Crop Yield to N Removed calculator and Geisseler’s N Concentrations in Harvested Plant Parts online reference. Outreach through meetings with growers, Certified Crop Advisors, and commodity groups is ongoing. The updated N removal coefficients will also be employed directly in the calculation of Irrigation and Nitrogen Management Plan (INMP) Summary data that the southern CV water coalitions use to assess regulatory compliance. This N balance data will be shared with commodity groups, growers, and technical assistance providers who use this information when evaluating future nutrient management study and outreach needs.

For additional information about CDFA’s FREP-funded projects on research and education regarding the agronomically safe and environmentally sound use of fertilizer in California, please click here to visit the FREP Grant Program.

For details about current and completed FREP-funded projects, as well as a searchable database that aims to make the research available, understandable, and convenient for growers to implement, please click here to visit the FREP Research & Project Database.

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