Effect of soil and plant nutrients on starch yield of potato for processing under growers’ management in Hokkaido, Japan

The demand for potatoes for processing in Japan has been increasing due to a shift in consumption preference from table potatoes consumption to processing. However, the domestic production remains low due to a reduction in planted areas and stagnant yields. Soil nutrients and fertilization are crucial for achieving high yield and quality of potatoes for processing. Therefore, there is a need to develop proper potato nutrition management to meet the quality standards, especially starch yield, desired by processing industries. This study was carried out in Tokachi district, Hokkaido to examine how soil and plant nutrients affect the starch yield of potato for processing. It was conducted in two phases: (1) a field survey of 16 growers, where soil, shoot, and tuber samples were collected and analyzed, and (2) a field experiment testing two levels of N (60 and 120 kg ha⁻¹) and Mg (0 and 40 kg ha⁻¹). The growers’ fields varied in tuber and starch yields. Our findings indicate that higher soil available N decreases starch yield. In general, the soil available N in the growers’ fields was high to excessive, and shoot N concentrations were positively related to soil available N. Growth traits such as stem length, shoot biomass, NDVI, and SPAD values did not affect starch yield in the growers’ fields. The results also showed that among the nutrients and their molar ratios in the shoot, only the N/Mg ratio was significantly related to starch yield. Although no significant effects on starch yields were shown by the main factors (N and Mg), nor their interaction in the field experiment, tuber and starch yields trend increased with added magnesium, regardless of nitrogen levels, while higher nitrogen application rates promoted longer stems and greater shoot biomass. This study revealed that high N/Mg ratio were related with lower starch yields in farmers’ fields, which was supported by field experiment results where Mg was added tended to improve starch yield across N levels. Therefore, to increase starch yield in potato for processing should consider fertilization based on crop need by monitoring soil available nutrients to ensure a balanced nutrition.