Parameters of the bimodal van Genuchten (VG) hydraulic functions for two aggregated Andisols were in-versely determined using the evaporation method. Initial estimates of the water retention parameters were deter-mined from separate retention measurements, which fa-cilitated rapid convergence of the parameter optimization process regardless of the number of optimized parame-ters. When the bimodal water retention parameters were fixed according to the independently measured retention data from near saturation to very low pressure heads down to −105 cm, it was possible to estimate the unsaturated hydraulic conductivity, K(h), by optimizing only two con-ductivity parameters (Ks, ℓ). Since the flat region of the bi-modal retention curve at intermediate pressures is difficult to measure precisely, however, we still recommend opti-mizing all bimodal VG parameters to yield the best overall results. Including water retention data at very low pressure heads in the dry range extended the applicable range of the model predictions, at least down to pressure heads of ap-proximately −104 cm.
Stabilization of soybean cultivation in rotational paddy fields is important issue for efficient utilization of Japanese arable land. The present study aimed to clarify the suitable condition of soybean planting for stable es-tablishment in clayey paddy fields by taking account the particularity of physical properties of clayey paddy soils. The effects of four experimental factors: intensity of com-pression, days after tillage, seeding depth, and tilth on the germination and growth of soybean, were investigated by field experiments following the randomized block design. The seeding depth and the timing of seeding after tillage were revealed to be the most influential factor while com-pression had supplementary role. Mean dimension of ag-gregates did not affect the germination and growth under the poor tilth condition. The results suggest that seed-ing should be conducted soon after tillage but the optimal depth depends on the expected weather in the following days. However if drainage efficiency of the field can be improved, seeding deeply supposed to be less risky unless enough rain is surely expected.
Measurements of radiocaesium concentration profiles in a soil are very time-consuming and labor-intensive work. We developed a device for in-situ mea-surements of the vertical distribution of radioactivity in a soil using a photo diode sensor. Four photodiode sensors were separated by lead-shielded plates to increase direc-tivity at each depth. Because the directivity was relatively broad, we defined a leakage coefficient to compensate the broad directivity by introducing a matrix expression. With a field test of the device in an undisturbed rice field in Iitate Village, Fukushima, we confirmed that the vertical distri-bution of soil radioactivity was properly measured with the device developed.