Effects of shading, soil moisture, fertilizations and sowing time on isoflavone content of soybean seed in Hokkaido

Aiming to produce soybean seeds with high yield and high isoflavone content in Hokkaido, which has cool weather, we evaluated the effects of shading, soil moisture, fertilizations (K,N,P,Mo) and sowing time on yield and seed isoflavone content using mainly a high-isoflavone variety, Yukipirika. Shading treatments of 50 % and 70 % during the seed filling stage in a greenhouse decreased both yield and isoflavone content. Applying four soil moisture treatments from pF 1.5 to pF 2.8 in the seed filling stage in a greenhouse gave a positive correlation between soil moisture and both yield and isoflavone content. K fertilization treatments with different amount and various types of fertilizers were conducted in fields at medium or high level of K2O in soils typical of Hokkaido uplands. The additional K treatments did not increase yield or isoflavone content significantly in the field experiments. N (urea foliar application) treatments in fields did not affect yield and isoflavone content significantly. And P (phosphite root application) and Mo (seed treatment) in fields did not change isoflavone content, either. Sowing two weeks and four weeks later than the standard sowing time in a field decreased the yield but did not change the isoflavone content. From a practical application perspective, for producing soybean seeds with high isoflavone contents and high yield in Hokkaido, it is effective to plant at a standard time with a normal amount of fertilizer in fields with minimum shade and to maintain the soil moisture at well-watered levels during the seed filling stage.