All farmers must understand quantitative productivity and the factors that are important to their farm fields for optimum crop management. This study attempted to estimate the soil quality index (SQI) of 61 winter wheat fields in Tokachi District, Hokkaido, over three years. It also examined the relationship between SQI and yield. SQI was estimated as a sum of weighted scores calculated from the value of each soil parameter (surface soil properties), which were adopted as soil diagnostic criteria by the Hokkaido government. A lower score was given when the parameter values deviated from the criteria. The principal component analysis parameters selected for SQI estimation were exchangeable calcium, solid ratio, exchangeable potassium, macropore, and hot-water extractable boron in descending order of spatial variation. Among them, exchangeable calcium and macropore had the greatest impact on low SQI. A significant positive correlation was detected between SQI and yield in 2018, when productivity was low due to heavy rain in May and July, especially in Brown Lowland soils and Andosols with alluvial subsoil. In contrast, a weaker correlation between SQI and yield was detected in Andosols with a thick effective soil layer. This implies that soil should be sampled on an effective soil layer to calculate the SQI. Additionally, in a case where the field contains much gravel in the surface layer, gravel content should be considered because of yield overestimation by SQI. Conclusively, this study implied that yield decreased as SQI decreased in a low yield environment with heavy rain only if the field has a thin effective soil layer and less gravel on the surface layer. However, soil quality improvement might stabilize the annual yield variation. Moreover, the consideration of meteorological and crop managemental conditions is required to better understand the relationship between SQI and productivity.

To achieve both stable yield and good quality in “Hatsushimo Gifu SL,” the main rice cultivar in the flat land of Gifu Prefecture, appropriate application rates of nitrogen fertilizer were estimated considering the amount of nitrogen mineralization from the topsoils of paddy fields.

The amount of nitrogen uptake at rice maturity (N-uptake) was correlated with grain yield and the number of proteins in brown rice. Based on these relationships, the ideal N-uptake of “Hatsushimo Gifu SL” was determined as 100-kg ha⁻¹．

A strong correlation was observed between the N-uptake and the amount of nitrogen supplied by nitrogen fertilizer (F-N) and that of nitrogen mineralized from paddy soils (S-N). The latter value was calculated from nitrogen mineralization when moist soil from paddy fields was incubated under submerged conditions at 30°C for 10 weeks (M10w). These results indicate that N-uptake was well predicted from N supply by F-N and S-N when M10w was used. However, the correlations were much stronger when the datasets were categorized based on the field use, i.e., continuously irrigated paddy rice fields or fields with a crop rotation of paddy rice and upland crops.

A formula for predicting N-uptake was developed using F-N, M10w, and the amount of nitrogen supplied from sources other than F-N and S-N. Using this formula, the ideal N-uptake and varying values of M10w, the appropriate nitrogen application rates could be predicted for the “Hatsushimo Gifu SL” cultivar.