Sweet potato is grown year-round and is planted on all islands of Hawaii. During the 20th century, sweet potato was a profitable export crop for Hawaii, but production and sales have been declining for decades. Environmental conditions, such as continued droughts or precipitations and poor soil nutrients, contribute to low sweet potato yields. Sweet potato yields can be determined by the number of storage roots (tubers) per plant, and the size of each storage root at harvest. To maximize the marketable yields of sweet potato, it is necessary to achieve the greatest possible number of storage roots. In a field trial at the Kula Agricultural Park in the island of Maui, Okinawan sweet potatoes were grown under different irrigation regimes, both with and without fertilizer application. Control plots were irrigated daily regardless of soil moisture, and irrigation treatments were designed to initiate irrigation events based on soil moisture thresholds as follow: high moisture (irrigated to a soil water tension threshold of -60 kPa), medium moisture (irrigated to a soil water tension threshold of -80kPa), low moisture (irrigated to a soil water tension of -100 kPa). Fertilizer (16-16-16 and potassium sulfate) was applied to half of each plot in a split plot design. Marketable yield was evaluated in the different treatments. The fertilizer treatments were only significantly different in the irrigated control and the lowest moisture treatment. Marketable yield was highest in the irrigated control that received no fertilizer, and lowest in the low moisture treatment that received fertilizer. As irrigation was reduced, yield was also reduced, and the number of off-grade small tubers increased. In the irrigated control, adding fertilizer reduced the number and weight of marketable tubers, especially in the AA grade, and increased the number and weight of large, off-grade tubers. In the lowest moisture treatment, high numbers of “pencil roots” were observed due to inadequate moisture during tuber development, and few marketable tubers in both the fertilized and unfertilized treatments.
