The Courses of Study for Elementary Schools in the Home Economics section emphasizes that “students should understand that rice is a staple food integral to the Japanese diet.” Furthermore, the 5th grade social studies section asserts the importance of “rice cultivation, which plays a crucial role in securing the nation’s staple food.” However, the necessity of soil for rice cultivation is not explicitly mentioned in either the home economics and social studies sections or the 4th grade science section that discusses soil particles. This study aimed to visualize the relationship among rice grains, rice plants, rice fields, water, and soil. It quantified the number of grains of polished rice and rice plants required for one bowl of curry rice (150 g cooked rice) and the area and mass of topsoil required to produce it. According to the calculations, approximately 3,571 grains of polished rice per bowl of curry rice are produced by 2.25, 2.22, and 3.85 rice plants of the composted, chemical fertilized, and no-fertilized areas, respectively. These rice plants require 30.6 kg, 30.6 kg, and 50.8 kg of moist soil (with 16.5, 15.9, and 26.3 kg moisture content) to grow. The water required to produce one bowl of curry rice includes the water used to cook the rice grains and the water in the soil of rice fields. However, only the former is mentioned in elementary school home economics classes. The water retention and permeability functions of soils, particularly those generated under forests, are crucial for maintaining moist soil. As a result, the role of forests as a water source is included in 5th grade social studies. Incorporating soil studies into the school curriculum can help students understand the connection between agriculture, their daily lives, and society. This approach can provide students with developmental and exploratory learning experiences that are relevant to home economics, social studies, and science.

This study aimed to assess areas of poor water retention within agricultural fields by examining the relationship between satellite images, topography, and soil water retention in the Tokachi and Okhotsk districts of Hokkaido, Japan. Areas with a lower normalized difference vegetation index (NDVI) during both the young panicle formation and maturation periods of winter wheat fields were found to have a higher sand content, a shallower gravel horizon, and lower levels of easily available water and autoclaved nitrogen contents (NAC) compared to the control area. These conditions, indicative of poor water retention and low NAC, could hinder initial growth and accelerate the maturation of winter wheat, resulting in a lower NDVI. However, NAC levels were found to vary spatially in some fields that had different topographies or management histories, and this variability influenced the NDVI. Additionally, the presence of weeds could potentially affect the NDVI within a field, even if the water retention was consistent. In conclusion, areas of poor water retention can potentially be identified by areas with a lower NDVI during both the young panicle formation and maturation periods of winter wheat. This is applicable if the field is free from weeds, pests, and diseases, has a consistent management history, and is located on unique topography.

This study investigated the relationship between satellite images, topography, and soil drainage in agricultural fields in the Tokachi district of Hokkaido, Japan, to identify areas of poor drainage. The normalized difference vegetation index (NDVI) of winter wheat was found to be lower during the young panicle formation period but increased before harvesting in areas with poor drainage. These areas were characterized by a shallower appearance of the upper boundary of redoximorphic or gleyic features, a shallower groundwater level, and lower saturated hydraulic conductivity compared to the control area. Poor drainage in these areas likely resulted in delayed initial wheat growth but pronounced later growth due to extensive nitrogen and water uptake from subsoils before harvest, or in the death of wheat and subsequent weed growth due to water logging caused by heavy rain. Significant positive correlations were observed between elevation and NDVI in the early growth stage in each field (P<0.001), suggesting that water accumulation at relatively low elevations could be another cause of poor drainage. Time-series analysis of NDVI under different meteorological conditions revealed that the delay of NDVI in poorly drained areas was not observed in years of low precipitation. In conclusion, areas of lower elevation, lower NDVI in the early growth stage, and higher NDVI before harvesting likely indicate poor soil drainage, provided the management history is consistent within the agricultural fields.

There are limited studies that investigate the competitive adsorption between arsenate and phosphate and the impact of pH on arsenate adsorption on Gray Lowland soil, a common paddy soil in Japan. Furthermore, the effect of the native soil phosphorus on arsenate adsorption warrants evaluation. This study aims to 1) assess the characteristics of arsenate adsorption on Gray Lowland soil by comparing it with that on Andosols, which was reported in a previous study and 2) assess the dissolution of native soil phosphorus and its impact on arsenate adsorption. The findings revealed that the amount of arsenate adsorption on Gray Lowland soil was approximately 10% of that on Andosols. The maximum arsenate adsorption on Andosols and Gray Lowland soil was observed at an equilibrium pH of ~3.8 and ~3.0, respectively. Moreover, competitive adsorption between arsenate and phosphate was observed on Gray Lowland soil. The fraction of weakly adsorbed arsenate accounted for 32% of the total adsorbed arsenate, despite adding only arsenate to the soil. It is suggested that arsenate competed with phosphorus, which was released from phosphorous bound to calcium and/or unstable iron contained in soil, in the adsorption on Gray Lowland soil. Native soil phosphorus in Gray Lowland soil continued to be released following repeated input of distilled water. These findings demonstrate that arsenate is not retained for extended periods by the topsoil, which has a low adsorption capacity of arsenate and whose adsorption sites are already saturated with phosphorous. This is because the added arsenate is either weakly or hardly adsorbed to the soil.