As citrus growers in California’s Central Valley grapple with significant water quality and quantity issues, the need for precise fertilization and irrigation practices is more critical than ever. Currently, citrus growers in the valley lack access to a specialized, free decision-support tool that could guide them in making informed decisions about fertilizer application. The crops’ physiological characteristics are unique amongst commodities grown in the valley, adding a layer of complexity to developing such a tool.

According to a recent study conducted at University of California-Davis funded by the California Department of Food and Agriculture (CDFA FREP), fertilizer rate decisions in citrus orchards should be based on nutrient export in expected yield while fertilizer application rates and timings should be based upon the pattern of nutrient accumulation in fruits. Yield potential determines fertilizer strategy and there is a large negative impact on overall efficiency that occurs in years or orchards of poor yield in which standardized fertilization strategies are used. Therefore, fertilizing according to predicted yields will dramatically enhance nutrient use efficiency. The methodology used in this research allowed the researchers to estimate the annual amounts of nutrients that field grown orange and mandarin trees absorbed and allocated to above ground organs improving nutrient use efficiency from around 50% up to 90%. Thus, knowing the dynamics of nutrient uptake during the season is a requirement to allow the management of the timing of nutrient supply with nutrient needs and to avoid nutrient losses, especially N-Nitrate. 

To make this information accessible to growers, a new project funded by CDFA FREP will aim to develop a free decision-support tool that will guide farmers in making informed decisions about irrigation and fertilizer application. This project builds off years of research, experimentation, and software development efforts and will culminate in the development of three new citrus-specific modules for UC’s CropManage online platform (oranges, mandarins, and lemons). This project stands to positively impact a diverse group of thousands of citrus growers in California's Central Valley, as California citrus growers supply over 90% of the nation’s fresh citrus, and export to over sixteen foreign countries, in harvests valued at over $2 billion per year according to USDA. 75% of that production is in two Central Valley basins, the San Joaquin Valley and Tulare Lake Basin (also referred to as the Southern San Joaquin Valley), where growers are often farming in nitrate-affected areas with intense competition for groundwater resources. 

The potential impact of this initiative is significant. By providing citrus growers with access to a refined decision-support tool, this project can lead to increased adoption rates, resulting in more efficient fertilizer and water use. This, in turn, enhances the likelihood of regulatory compliance with the ILRP and SGMA and contributes to the long-term sustainability of California’s citrus production. Ultimately, this project not only addresses the immediate needs of citrus growers but also supports the broader goal of preserving the quality and quantity of California’s water resources for future generations.