Effect of changes in the soil calcium-to-magnesium ratio by calcium application on soybeans, <i>Glycine max</i> (L.) Merr., growth

There have been few studies in Japan on the effects of the application of lime materials, especially in regard to the soil calcium-to-magnesium ratio (Ca:Mg). The aim of this study was to explore the effects of changes in the soil Ca:Mg on soybean growth via pot and field tests over two growing seasons: 2018–2019. The field tests were conducted in two different fields, one in each year, and the soil for the pot tests in both years was collected from the respective fields used in the field tests. In the pot test from 2018 to 2019, we adjusted the soil Ca:Mg ratio and soil pH using Ca(OH)2 and CaSO4, with three conditions: control (pH = 5.2, Ca:Mg = 1.3), CaMg (pH = 5.0, Ca:Mg = 2.6), and CaMg + pH (pH = 5.7, Ca:Mg = 2.4). Moreover, we added two further conditions for the pot test in 2019: CaCO3 (pH = 5.9, Ca:Mg = 1.8) and dolomite groups (pH = 6.4, Ca:Mg = 1.3). In the field test, we adjusted only the soil Ca:Mg ratio using phosphogypsum and had two conditions: control (pH = 5.0, Ca:Mg = 1.1) and gypsum (pH = 4.7, Ca:Mg = 1.9). In both tests, the application of Ca except for dolomite contributed to the increase in the concentration and uptake of Ca both in the whole plants and some parts of the soybean plants. Likewise, application of Ca except for dolomite contributed to a significant increase in the total number of pods. The weight of seeds and the total dry weight in the CaMg + pH and CaCO3 groups and in the gypsum plot were significantly higher than were those in each control group. Thus, the study suggests that the Ca:Mg ratio in soils with a low Ca:Mg ratio may limit soybean growth to a greater extent than does the soil pH. However, the effect of the interaction between the soil Ca:Mg ratio and soil pH on dry-matter production of soybeans was different between the pot and field tests, and further studies were needed to clarify the discrepancy.