Sweet potato production on the Hamakua Coast

It’s time to test your soil!

By Chantal Vos, Researcher, and Norman Arancon, Associate Professor of Horticulture.

Norman stands in large sweet potato field.
Norman Arancon collects leaf samples from sweet potato field. Photo by Chantal Vos.

Sweet potato (Ipomoea batatas, L.) production along the Hamakua Coast can be increased by addressing nutrient imbalances in the soil. Sweet potato is an important crop in Hawai‘i both for local consumption and as an export crop and more than 90 percent is produced along the Hamakua Coast on the island of Hawai‘i (Miyasaka and Arakaki, 2010). Most commercial sweet potato farmers on Hawai‘i Island do not test their soil or crops on a regular basis for potential nutritional problems. 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. Soil fertility is often not optimal, even on land that has never been cultivated with sweet potato (virgin land). During crop production, available nutrients are lost through leaching, run-off, and crop harvest. 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 (Bennett et al., 2012).

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. Developing improved post-harvest field sanitation and short rotation management strategies will be key in addressing disease and pest issues (Bennett et al., 2012). Large increases in sweet potato yields can be obtained from a modest increase in nutrient supply to address mineral deficiencies (O’Sullivan et al., 1997). The potential yield for sweet potato under optimum conditions can reach 80-100 t/ha (O’Sullivan et al., 1997). In Hawai‘i, the average yield was 17 t/ha in 2011 (NASS, 2013), however, good yields on commercial farms range from 34 to 39 t/ha (Valenzuela et al., 1994). Diagnosis and correction of nutritional problems are essential for the sustainable production of sweet potato production. Both deficiencies and toxicities of mineral nutrients can affect crop production and unbalanced or excessive use of fertilizers can cause environmental pollution and is an unnecessary expense (O’Sullivan et al., 1997).

Large sweet potato field with ocean backdrop.
A sweet potato field operated by Mitch Anderson. Photo by Chantal Vos.

Between April and October 2017, a total of 16 commercial sweet potato fields were surveyed by College of Agriculture, Forestry, and Natural Resource Management, University of Hawaii at Hilo, researchers Chantal Vos and Norman Arancon to obtain insight into the current soil and tissue nutrient status of these fields along the Hamakua Coast and to elucidate if the current fertilization practices can be augmented using nutrient data from the soils and plant tissue collected from these sites. Composite soil samples were randomly collected from at least 30 different locations throughout each field 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 US Mainland (Miyasaka and Arakaki, 2010). The most recent fully-developed leaves without petioles were harvested randomly from 20 to 30 sweet potato plants throughout the planted fields.

All soil samples were analyzed by the Agricultural Diagnostic Service Center (ADSC) of the College of Tropical Agriculture and Human Resources, UH Mānoa, for pH (saturated paste method), organic carbon (modified Walkley-Black method), total nitrogen (micro-Kjeldahl method), extractable phosphorus (modified-Truog method), and exchangeable calcium, magnesium, potassium, and sodium (by extraction with ammonium acetate (1M, pH 7.0)) as described by Hue et al. (2000). Leaf samples were analyzed by Waters Agricultural Laboratories, Camilla, GA. Plant minerals phosphorus, potassium, magnesium, calcium, sulfur, boron, zinc, manganese, and iron were analyzed by open vessel wet digestion using an inductively coupled argon plasma spectrometer (ICAP, DigiBlock 3000). Total nitrogen was determined using a nitrogen gas analyzer (LECO).

Results show adequate concentrations of nitrogen (N) and sulfur (S), low phosphorus (P) and potassium (K), and very low calcium (Ca) and magnesium (Mg) levels. Applications of dolomitic limestone are recommended to increase soil pH and plant available Ca and Mg. Increasing soil exchangeable potassium to at least 200 ppm may increase the quantity and quality of sweet potato yields. Muriate of potash or alternatives such as sulfate of potash and sulfate of potash magnesia can be used to increase available K. Fertilizer recommendations were shared with sweet potato farmers based on soil reports per field.

Average fertilizer costs to address nutrient deficiencies with conventional fertilizers at the 16 sampled fields are estimated at US $3,000 per acre per cropping cycle. Lime (coral limestone or dolomitic limestone) and calcium fertilizer (gypsum) comprise 60% of these fertilizer costs. Adequate fertilization can increase overall yields and improve the shape of the sweet potato storage roots. Annual soil testing is highly recommended to determine if current fertilization practices are sustainable and can maintain the production of sweet potato tubers.

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, CAFNRM, UH Hilo.

–May-June issue of the CAFNRM Newsletter.