Monitoring Nutrient Uptake and Growth Patterns of Onion Crops to Improve Fertilizer and Water Management

Information of nutrient uptake and water use of onions in California is very limited. The crops shallow root system and sensitivity to mild water stress creates a conducive environment for off-site movement of nitrate. The objective of this study was to develop information of crop growth and nutrient uptake patterns to help growers target the right amount and time of water and fertilizer applications. Aboveground biomass nitrogen (N), phosphorus (P) and potassium (K), crop canopy coverage and root growth were estimated by 8 to 15 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 189.4, 52.8 and 276.0 lbs/A of N, P and K, respectively. Canopy growth patterns of varieties A and B were very similar, with steady growth (10% to 95% of ground coverage between 80 to 130 days after planting (DAP), while the canopy of variety C increased from 10% to 85% between 70 to 90 DAP. Root depth development (measurements not replicated) was also very similar between varieties A and B, and reached maximum depth of 21 and 22 inches (respectively) at 115 DAP, while variety C reached the maximum depth of 16 inches at 98 DAP. Overall, similar canopy coverage and root depth patterns between varieties A and B could be explained by similarities in variety type (intermediate-day) and seeding-harvesting dates. Greater aboveground biomass nutrient accumulation of variety A may be explained by a longer growth season (172 days) and consequently greater biomass accumulation. Nitrogen uptake and canopy growth data were analyzed by regression procedures for developing algorithms that estimate N uptake and coefficients for calculating crop water use from reference evapotranspiration data. The parameters of these models will be incorporated into the web-based software, CropManage (https://ucanr.edu/cropmanage), which assists growers in determining appropriate irrigation and N applications for their crops.