Agricultural researchers at the University of Hawaiʻi at Hilo analyzed soil and tissue samples from sweet potato fields along the Hāmākua Coast to see if current fertilizer practices can be improved at the sites. Horticulturalists Chantal Vos and Norman Arancon, from the College of Agriculture, Forestry and Natural Resource Management, randomly collected soil samples from at least 30 different locations throughout the fields using a hand shovel at a depth of 0–6 inches.

Tissue samples were collected from farms growing the Okinawan purple sweet potato variety, which is the principal variety grown for export to the continental U.S. Recently fully-developed leaves without petioles were harvested randomly from 20 to 30 sweet potato plants throughout the planted fields.

Between April and October 2017, a total of 16 commercial sweet potato fields were surveyed. All soil samples were analyzed by the Agricultural Diagnostic Service Center of the College of Tropical Agriculture and Human Resources at UH Mānoa. Leaf samples were analyzed by Waters Agricultural Laboratories, Camilla, Georgia.

“Sweet potato (Ipomoea batatas) production along the Hāmākua Coast can be increased by addressing the nutrient imbalances in the soil,” conclude Vos and Arancon. They write that results show adequate concentrations of nitrogen and sulfur, but low phosphorus and potassium, and very low calcium and magnesium levels.

Vos and Arancon recommend applications of dolomitic limestone to increase soil pH and plant available calcium and magnesium.

“Increasing soil exchangeable potassium to at least 200 ppm may increase the quantity and quality of sweet potato yields,” they write. Muriate of potash or alternatives such as sulfate of potash and sulfate of potash magnesia can be used to increase available potassium.

The fertilizer recommendations were shared with sweet potato farmers based on soil reports per field.