Note: This is part of a Research Update series that highlights projects funded by the California Department of Food and Agriculture’s (CDFA) Fertilizer Research and Education Program (FREP).
Project Title: Development of Site-specific Nitrogen Fertilization Recommendations for Annual Crops
Project Lead: Daniel Geisseler, Cooperative Extension Specialist, Department of Land, Air and Water Resources, University of California, Davis
Project Status: Complete
Project location: Field trials in commercial fields in Colusa, Fresno, Sacramento, Siskiyou, Solano, Tulare, and Yolo counties
Overview: This FREP-funded project develops site-specific estimates of the contribution of nitrogen (N) mineralization to the plant-available N pool for different regions in California and incorporates these estimates into user-friendly online N fertilization calculators.
Background
Growers need reliable tools to determine optimal rates and times of N applications to minimize nitrate leaching to groundwater while maintaining high yields. These tools should be based on field-specific information, including availability of N from non-fertilizer sources, such as residual soil nitrate, nitrate in the irrigation water, and N mineralization from soil organic matter (SOM). The existing online calculator, which was developed by Geisseler’s lab through a previous FREP-funded project, estimates the N fertilizer requirements based on expected yield and non-fertilizer sources; however, the calculator currently estimates in-season N mineralization based on an N budget for Central Valley cropping systems and does not consider site-specific soil properties and cropping history. The objectives of this project include: i) validating N mineralization estimates in field trials in the Central Valley, including the Delta, and Tulelake basin, ii) characterizing the chemical composition of SOM using the Fourier Transform Infrared (FTIR) spectroscopy method and correlating it to soil quality, and iii) developing user-friendly and site-specific online N fertilization calculators for different crops.
Approach
Researchers established 36 field trials over three growing seasons from 2021 to 2023 across the seven counties covered by this project. Each trial consisted of two treatments, (i) no N fertilizer applications in plots within the field and (ii) grower’s N management. Soil samples were collected from the top 120 cm of the profile pre-plant and post-harvest and analyzed for residual mineral N content (ammonium-N and nitrate-N). The aboveground biomass of fertilized crops was harvested in three-week intervals to determine biomass dry matter accumulation and N concentration. Prior to harvest, the aboveground biomass in the fertilized and unfertilized plots were also collected. Also, samples from each 30-cm layer were incubated to determine the distribution of N mineralization in the soil profile.
Findings
Nitrogen accumulation in the aboveground biomass followed an S-shaped curve (Figure 1). Generally, less than 40 kg N per hectare (ha-1) accumulated in the biomass during the first four weeks after seeding. The following weeks were characterized by rapid N uptake, before slowing down during generative growth stages. Across all sites monitored from 2021 through 2023, the average amount of N in the aboveground biomass at harvest was 185 kg ha-1 (165 pounds per acre (lb/ac)) for sunflower, 265 kg ha-1 (237 lb/ac) for corn, 203 kg ha-1 (181 lb/ac) for cotton, and 223 kg ha-1 (199 lb/ac) for spring wheat grown in the Tulelake basin.
Figure 1: Seasonal nitrogen accumulation of field crops grown in California.
Daily net N mineralization over the growing season averaged 0.94 kg N per hectare per day (ha-1 d-1) (0.84 lb N/ac/d) in soils with a low SOM content and 2.41 kg N ha-1 d-1 (2.15 lb N/ac/d) in soils with a high SOM content. Net N mineralization decreased with depth, closely following total soil N content.
The proportion of total soil N mineralized decreased with increasing clay content in soils with a low SOM content and decreased with increasing total soil N in soils with a high SOM content (Figure 2).
Figure 2: Correlation between daily N mineralization at 25 °C in the top 60 cm of the profile and soil clay content (left, panels A and C), as well as total soil N (right, panels B and D) measured in the top 30 cm.
The Fourier Transform Infrared (FTIR) spectroscopy method predicted net N mineralization with anR2 (coefficient of determination) of 30.9% across all sites included in this study. The prediction of net N mineralization was not enough of an improvement over current estimates to warrant recommendation of FTIR as a tool for site-specific N mineralization estimates at this time.
The results of this project have been incorporated into a Crop Nitrogen Calculator for California on the UC Davis Geisseler Lab Nutrient Management website that allows for site-specific estimates of crop N demand, N mineralization from SOM and other non-fertilizer sources to determine N fertilizer needs.
Figure 3: This project’s Crop Nitrogen Calculator for California is available at http://geisseler.ucdavis.edu/Crop_N_Calculator.html.
Conclusions
Nitrogen budgets that take all major inputs and outputs into account are challenging and time-consuming to calculate. The data generated in this project, combined with results from previous studies, has enabled the researchers to create an online N calculator that assesses site-specific N budgets and N fertilizer application rates to meet crop demands. This free decision-support tool is designed specifically for growers and crop consultants to accurately estimate N fertilizer demand using typical soil and water measurements that are easy to enter. The online N calculator which was developed in this project performs these calculations based on readily available information entered by the user.
This blog post presents a summary of this project and some of its findings and does not include all the data collected in this study. The complete report will be published where other completed FREP projects are available on the FREP Research & Project Database. If interested in receiving further details about this project in the meantime, please email FREP@cdfa.ca.gov.