Determining the Effects of Nitrogen Fertilizer and Soil Chemical Properties on Rice Yield: Evidence from a Large-Scale Farm in Hokuriku Region, Japan

In order to contend with decreasing gross amounts and soaring costs, viable rice production in Japan requires high yield levels. This study assessed the effects of nitrogen fertilizer and soil chemical properties on rice yield. Data were recorded in 2014 and 2015 from 93 paddy fields at a farm larger than 30 ha in Hokuriku Region, Japan. Koshihikari, the most widely planted rice variety in Japan, was cultivated in the sampled fields. Soil chemical properties were quantified using 12 variables, namely pH, cation exchange capacity, phosphoric acid content, silicic acid content, content and saturation of the three exchangeable bases, and the equivalent ratios of calcium to magnesium and magnesium to potassium. Three principal components (PCs) were extracted explaining 76% of the total variation and comprised mainly the magnesium (PC₁), potassium (PC₂), and acidity–basicity (PC₃) variables. The multivariate regression model retained significant determinants, with the standardized principal components (SPCs) and the time trend explaining 66.6% of the total variation. The results showed that higher squared values of SPC₁ positively related to rice yield, while SPC₂ and SPC₃ increased rice yield up to a threshold, at which point increases plateaued. These findings were then confirmed through correlation analyses between each soil chemical property and both the PCs and rice yield. The effect of nitrogen fertilizer was insignificant, due to low variation among the paddy fields in which the special cultivation regime was adopted. Further investigations were conducted into countermeasures to improve soil chemical properties, especially those highly related to SPC₂ and SPC₃.