Available Soil Nutrients and NPK Application Impacts on Yield, Quality, and Nutrient Composition of Potatoes Growing during the Main Season in Japan

Proper management of N, P, and K fertilizers is considered very important to maximize tuber yield and attain desirable processing quality. Generally, farmers apply NPK fertilizer rates based on the preplant soil tests. However, very limited information is available on the influence of available soil nutrients and in-season NPK application rates on yield and quality of tubers. In the present study, paired soil and tuber samples were evaluated from 170 commercial production sites to evaluate potential relationship between soil nutrients, tuber yield, and tuber quality. These paired samples gave us the opportunity to not only study yield response to fertilizer application rates but also study the relationship between soil nutrient availability and tuber nutrient concentrations. We found that about 50%, 80%, and 70% of farmers applied more NPK than recommended rates, respectively. Our results suggest that, in general available soil NPK increases tuber NPK concentrations and tuber N and K concentrations can reduce specific gravity. Interestingly, tuber P concentration had no effect on specific gravity. We also found some variations in these responses to nutrients (NPK) among cultivars suggesting some genetic control for the traits studied. Results also suggest that application of NPK fertilizers in excess of recommended rates on soils that are already rich in these nutrients does not increase tuber yields and may in fact increase potential environmental degradation associated with excessive fertilization and the cost of production.