UH Hilo researchers: Sweet potato production in Hāmākua can be improved with select fertilizers

Horticultural researchers Chantal Vos and Norman Arancon highly recommended soil testing to determine if current fertilization practices are sustainable and can maintain the production of sweet potato tubers.

Norman standing in field of sweet potatoes.
Norman Arancon collects leaf samples from sweet potato field. Photo by Chantal Vos.

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, UH Mānoa. Leaf samples were analyzed by Waters Agricultural Laboratories, Camilla, GA.

Norman Arancon
Norman Arancon

“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.

Local sweet potato production

Sweet potato is an important crop in Hawaiʻi both for local consumption and as an export crop and more than 90 percent is produced along the Hāmākua Coast on the island of Hawaiʻi.

“Most commercial sweet potato farmers on Hawaiʻi Island do not test their soil or crops on a regular basis for potential nutritional problems,” write the researchers. “Fertilizers are often applied indiscriminately based on prior experience or current practice from other growers, whether these areas have been cropped for many years or are newly cleared for cultivation.”

This results in soil fertility that is often not optimal, even on land that has never been cultivated with sweet potato (virgin land). Further, during crop production, available nutrients are lost through leaching, run-off, and crop harvest. Vos and Arancon write that nutrient balances are distorted, and fallow periods have demonstrated limited capacity to adequately restore and build soil fertility. This being said, fallows will generally reduce many disease and pest problems.

Large sweet potato field with ocean in the background.
A sweet potato field operated by Mitch Anderson. Farm owners Mitchell and Lili Anderson run a local eight-acre sweet potato farm north of Hilo. Photo by Chantal Vos.

Access to lands not previously cultivated with sweet potato or fallowed long enough to break disease and pest cycles (weevils, nematodes, etc.) is increasingly becoming a problem for growers. This challenge is magnified by new land owners who are unwilling to lease their lands for row crop production.

Vos and Arancon note that developing improved post-harvest field sanitation and short rotation management strategies will be key in addressing disease and pest issues.

“Large increases in sweet potato yields can be obtained from a modest increase in nutrient supply to address mineral deficiencies,” they note, adding that diagnosis and correction of nutritional problems are essential for the sustainable production of sweet potato production.

But too much of a good thing can cause other problems, they add: Unbalanced or excessive use of fertilizers can cause environmental pollution and is an unnecessary expense.

Soil testing is key

Average fertilizer costs to address nutrient deficiencies with conventional fertilizers at the 16 sampled fields are estimated at U.S. $3,000 per acre per cropping cycle. Lime (coral limestone or dolomitic limestone) and calcium fertilizer (gypsum) comprise 60 percent of these fertilizer costs.

“Adequate fertilization can increase overall yields and improve the shape of the sweet potato storage roots,” explain Vos and Arancon in their summary. “Annual soil testing is highly recommended to determine if current fertilization practices are sustainable and can maintain the production of sweet potato tubers.”

Study funding

Funding for this study was provided by the USDA-PBARC Integrated Cropping System Project. Agreement No. 5320-43000-016-17S/agreement 58-5320-016-17S under Bruce Mathews, dean of the College of Agriculture, Forestry and Natural Resource Management, UH Hilo.

Adapted from study summary in the May-June 2018 issue of the CAFNRM Newsletter.