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There is growing evidence of excessive use of phosphate fertilizer and high residual soil phosphate levels in arable cropping systems in Japan. However, there is a lack of regional-scale information on the current status of soil-available phosphate as well as its impacts on a potato (Solanum tuberosum L.) cropping system. Although phosphorus is an essential nutrient, there have been confounding reports about the effects of excessive soil-available phosphate on potato tuber yields. This study was conducted in Hokkaido, Japan, to determine the current status of phosphate fertilizer application rate and soil-available phosphate levels, and their effects on tuber yield. In two contrasting soil types varying in phosphate absorption coefficient (PAC), soil and potato tuber samples were collected from 90 and 80 grower fields in Tokachi and Kamikawa districts, respectively, in the 2013 and 2014 growing seasons. The soil samples were analyzed for PAC, available phosphate by the Truog method, and total phosphate along with total carbon and acid-oxalate extractable aluminum. The tuber samples were collected at the harvesting stage for the calculation of yields. This study revealed (1) ongoing excessive phosphate fertilization regardless of high soil-available phosphate levels in potato-producing fields of Hokkaido, (2) no positive impacts of excess soil-available phosphate on tuber yields, and (3) no yield response from the applied phosphate fertilizer in excess of the recommended rate in two contrasting soil types. In conclusion, excess phosphate fertilizer was an unnecessary cost to the growers and could also be a potential harm to the environment, and changes are urgently needed to optimize the input-output balance of phosphate in this agricultural system.

Low or excessive soil fertility is a major constraint to potato production. The influence of each individual nutrient element on potato plants under field studies remains ambiguous due to the influence of environmental variations. Creating an in vitro model plant with deficient or excessive nutrient content will provide a more controlled study and allow for a better understanding of how the concentration of one element can affect the uptake of other elements. Here we designed a tissue culture-based nutrition control system to systematically analyze the effects of essential nutrients on potato plants. Insufficient or excessive nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) contents were created by modifying the Murashige and Skoog (MS) medium. Deficient to toxic plant nutrient statuses were successfully defined by the evaluation of dry biomass and morphological symptoms. The results showed that plant shoot growth, nutrient uptake and content, and nutrient interactions were all significantly impacted by the changes in the MS media nutrient concentrations. These tissue culture systems can be successfully used for further investigations of nutrient effects on potato production in response to biotic and abiotic stresses in vitro.

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