This study developed a simple, rapid calibration method to determine a calibration function using one dataset volumetric water content (θ) and the raw sensor output of two soil water sensor models 5TE and 5TM. The method was based on the slope–intercept relationship, determined by several calibration functions similar to the linear function previously proposed by the authors for calibrating the EC-5 soil water sensor. The volumetric water content θ0 and the dielectric permittivity εa0 at the singular point of the calibration functions for the 5TE and 5TM sensors were compared with their values at the singular point of the EC-5 calibration function. An analysis of the relationship between the slope (a) and intercept (b) of the calibration functions for the 5TE and 5TM sensors, obtained using soil water monitoring data from agricultural fields in the Kochi and Kumamoto Prefectures, Japan, showed that b was linearly correlated with a. A slope–intercept relationship similar to the EC-5 sensor was obtained for the 5TE and 5TM sensors, and its variables, equivalent to the singular point of the calibration function for the 5TE and 5TM sensors were identified considering the a and b values reported in the literature. The root mean square error between the estimated volumetric water content and the thermogravimetrically determined content was smaller using this newly developed calibration method than the conventional Topp’s equation. The θ0 and εa0 values at the singular points for the 5TE and 5TM sensors were inconsistent with their values at the singular point regarded as the residual volumetric water content in the van Genuchten equation in the previous EC-5 sensor study.
Adjusting soil pH to weakly alkaline (approximately 7.5) can inhibit filamentous fungi, such as Plasmodiophora brassicae, which causes Brassicaceae clubroot disease. Since pH increase in response to the addition of alkali materials depends on soil composition, a simple method is needed to estimate the quantity of alkali material added. In this study, simple sigmoidal soil pH buffer curves were drawn from the data obtained from adding converter furnace slag or lime hydrate to approximately 230 paddy soil samples from eastern Japan and Hokkaido. The constant part of the curve, which is an indicator of soil pH buffering capacity, was expressed as a function of the clay and total carbon content. The estimated formula-derived value was linearly correlated with the actual value of the constant parts of the curves (R2=0.333 for converter furnace slag and 0.429 for lime hydrate). The linearity of the calculated values derived from the formula with respect to the measured pH was examined after inputting five material volume points to verify the practicality of the equation. The measured and estimated pH after the converter furnace slag added were well fitted when the material input was 0.025–0.25 g⋅10 g−1 of dry soil as alkaline content (R2=0.81–0.61), which corresponded to an acidic to weakly alkaline pH; whereas for the lime hydrate they were well fitted when the material input was 0.025–0.05 g⋅10 g−1 of dry soil as alkaline content (R2=0.55–0.70), also corresponding to an acidic to weakly alkaline pH. Thus, the present equation can be used for acidification and soil pH adjustment to weak alkalinity.
Increased soil carbon content exhibits an ameliorating effect on global warming. Grassland soil has a high soil carbon content and functions as a carbon dioxide absorption source. Therefore, we investigated the long-term effects of grassland renewal and fertilization on soil carbon content. The survey area included 14 grasslands in Nemuro, Hokkaido, Japan. The vegetation in each grassland was investigated, and soil samples were collected for physical and chemical analyses. The soil A0 layer thickness was measured, and the chemical properties and eukaryotic microorganism community characteristics were examined. The data, analyzed considering grassland management implications, showed that the total soil carbon content was positively correlated with the A0 layer thickness, whereas it was negatively correlated with the soil pH (H2O). The A0 layer thickness increased with the number of years after grassland renewal; however, it decreased with increased nitrogen input. Reduced nitrogen input and increased number of years after grassland renewal were associated with increased soil carbon content. Increased soil exchangeable calcium increased the soil pH (H2O) and reduced the soil carbon content, while increased soil pH (H2O) increased the grass canopy coverage and soil eukaryotic microbiota diversity. These results suggested the possibility of identifying the optimum quantity and application method of calcium material, which fosters a high diversity of eukaryotic microflora while maintaining a high soil carbon content.
Due to low nitrogen fertility, the crop yield of land recently engaged in organic farming is often reduced. This study aimed to develop methods to attain the desired nitrogen fertility of 50–70 mg kg−1 hot-water extractable nitrogen for organic farming by cultivating legume green manure during the two-year conversion period without chemical fertilizers or synthetic pesticides. When the hairy vetch (Vicia villosa) green manure or red clover (Trifolium pratense) fallow green manure was cultivated during the two-year conversion period, the hot-water extractable nitrogen increased by approximately 5 to 15 mg kg−1. This result suggested that green manure was an effective source of easily decomposable organic nitrogen, which is the main component of hot-water extractable nitrogen. In addition, the vegetable crop yield increased by approximately 10 to 30％ in the third and fourth years (the two years post-conversion to organic farming), and the α-glucosidase activity was positively correlated with the green manure dry-matter yield. Based on these results, we proposed a model to increase soil nitrogen fertility by introducing green manure during the organic farming conversion period.