Site-Specific Nutrient Management for Onion Production In Southern California: Soil Fertility and Fertilizer Usage Comparison

There have been limited to no studies regarding nutrient site-specific management (SSM) of onion crops in California, and variable rate (VR) fertilizer application is uncommon in this area and crop. The objectives of this project were to assess phosphorus (P) and potassium (K) spatial variability in six High Desert onion fields, and to compare fertilizer usage and cost differences between uniform rate (UR) and VR application methods. Seventy nine soil samples were collected based on a grid of 1 sample/2.5 ha in six fields located near Lancaster, CA totaling 159.6 ha.  Soil samples were analyzed for Olsen P and exchangeable K. The VR fertilizer recommendation maps were created based on the fertility maps and on the grower’s current fertility program. Differences in nutrient variability and fertilizer usage between UR and VR varied according to each specific field and their location (north or south regions of Lancaster). Overall, P and K spatial variability and fertilizer savings due to VR application were greater in the North Fields (85ha), where savings with potassium due to VR application were $5,296. In contrast to North Fields where the use of VR application provided a fertilizer savings, the South Fields actually had greater P and K fertilizer usage since the intensive sampling with the VR method identified areas of the fields that were deficient. The balance of fertilizer savings due to VR application for all six fields was $2,992 (the economical analysis does not include costs associated with extra soil analysis, labor and equipment acquisition). Whether or not VR application results in an actual fertilizer savings is secondary, and depends on whether conventional sampling (field average) generally over or underestimates the fertility level. The important point is that with grid sampling the VR fertilizer application better matches the actual fertility needs of the field.