Nitrogen, Phosphorus and Potassium Uptake Patterns of Fresh-Market Onion Production in Southern California.

Information of nutrient uptake of onions (Allium cepa L.) in California is very limited and most fertilization programs are based on information developed in other regions, such as the Pacific Northwest. Discrepancies of nutrient uptake amounts and time are conducive for both over and under fertilization and consequent detrimental impacts on yield and on the environment. The objective of this study was to develop information of nutrient uptake amounts and patterns to help growers apply the right rate and time of fertilizers. Aboveground biomass nitrogen (N), phosphorus (P) and potassium (K) were estimated by 9 to 13 measurements throughout the 2013 growing season of three onion varieties in three fields located in Lancaster, California. Varieties A and B (intermediate-day) were seeded on 18 and 28 Feb. and harvested on 8 Aug., while variety C (long-day) was seeded on 12 Apr. and harvested on 16 Sept. Mean seasonal aboveground biomass nutrient accumulation for variety A was greater than varieties B and C and estimated at 212.1, 58.0 and 309.1 kg ha^-1 of N, P and K, respectively. Overall, greater nutrient accumulation of variety A may be explained by a longer growing season (172 days) and consequently greater biomass accumulation. Nutrient concentrations of aboveground biomass were estimated from bulbs and leaves together until approximately 100 days after planting (DAP), when they were separated for laboratory analysis. Nitrogen and K concentrations had a significant decrease until 100 DAP, while P concentrations oscillated and presented a modest decrease. From 100 DAP until harvest, N, P and K concentration trends in the leaves and bulbs varied according to each variety. Total N, P and K uptake at harvest were in average 207, 616 and 114% greater (respectively) on bulbs than leaves. Nitrogen uptake data was analyzed by regression procedures for developing algorithms that will be incorporated into a web-based software, CropManage (https://ucanr.edu/cropmanage), which assists growers in determining appropriate N fertilizer applications for their crops.