2017 ASHS Annual Conference

In 2015, sweetpotato (Ipomea batatas) production in California had estimated production of 650 million pounds grown on approximately 18,500 acres. California produces is the second largest producer in the United States, with about 25% of total production. Soils are typically sandy to loamy sand in texture, and drip irrigation is the preferred method to irrigate this crop. Despite it’s wide adoption by the industry, there have been no documented irrigation studies to determine optimal water use in California to maximize yield. Therefore, irrigation trials were conducted from 2010 – 2013 to determine the amount of water needed to produce sweetpotatoes with drip irrigation by imposing deficit irrigation treatments of 40%, 60%, 80%, 100%, and 120% of crop evaporation. Crop evaporation was determined from the equation ETc = ETo x Kc where ETc is the evapotranspiration of the crop, ETo is reference evaporation using the Merced CIMIS weather monitoring station (CA Dept. of Water Resources), and Kc is the crop coefficient. Irrigation amounts were controlled through the use of battery operated timers connected to surface drip, and were initiated two weeks after transplanting to allow for root establishment. In-line water meters were used to determine applied water for the individual treatments and adjust the time accordingly based on daily crop ETc. Decagon 5TM (Decagon Devices, Pullman, WA) and Watermark (Irrometer Company, Riverside, CA) soil moisture sensors were installed at 5, 15, 30, and 60 cm depth. Plot design was a randomized block with 4 replications. Applied water root size, and root yield data were analyzed using Fisher’s Protected LSD at the 95% confidence level. Soil moisture ranged from 0.25 – 0.10 m³/m³ (25 – 10%), and < 25 centibars, during most of this trial except for the 5 cm depth. Applied water ranged from 36 – 101 cm depending or year and irrigation treatment. Yields were significantly reduced as applied water was reduced, however, there was no yield benefit to exceeding 100% ETc. Across 4 years of this study, 100% relative yield occurred around 76 cm of applied water (2.5 acre-feet), and declined about 20% with 50 cm of water, however, sweetpotatoes responded by sizing differently. Jumbo sized roots declined 40%, whereas medium and #1 root yields declined 10%. Since the value of #1 roots is typically twice that of jumbos, the economic impact of deficit irrigation is disproportionate to the impact on yield: a 20% yield loss resulted in 16% lower return per acre.