Soil color originates mainly from organic matter, iron mineralogy and moisture content. We aimed to find a suitable method to measure soil color sensitively and to evaluate the extent to which the color parameters can be useful for statistically estimating total carbon (C), total nitrogen (N) and active iron (Fe) contents in Japanese agricultural soils. A soil color reader (SPAD-503) was applied to two sample sets: (1) 100 surface soils collected throughout a 0.5-ha paddy field (field scale) and (2) 147 surface soils collected from agricultural fields in Japan (national scale). For analysis with this instrument, about 2 g of air-dried, finely-ground samples were packed firmly in a plastic cell, and their colors as they appeared on windows in both sides of the cell were measured. A CIE 1976 (L*, a*, b*) color space was used for color description. For the field-scale samples, the values of the coefficient of variation were around 15% for total C, total N and acid oxalate extractable iron (Fe o ). The L* value (lightness) was negatively correlated with the content of total C and total N (R 2 = 0.18** and 0.26**, respectively), and the b* value (yellowness) was positively correlated with the Fe o content (R 2 = 0.59**). For the national-scale samples, the values of the coefficient of variation were around 60% for total C, total N and Fe o . The L* value was negatively correlated with the content of total C and total N (R 2 = 0.70** and 0.59**, respectively), but the b* value was not correlated with the content of Fe o (R 2 = 0.00). When the analysis was limited to 65 samples frequently used for paddy fields, the b* value was positively correlated with the Fe o content (R 2 = 0.52**). In conclusion, the proposed method enabled us to measure soil color sensitively with a small sample size. The L* and b* values obtained can be useful for rapid estimation of total C, total N and Fe o contents in agricultural surface soils in Japan.

No abstract

Nonexchangeable potassium (K-ne), i.e. 1 M NH 4 0Ac-nonexchangeable K, often contributes significantly to plant nutrition. However conventional extraction methods often extract much more K-ne than plants even after intensive cropping, suggesting the difficulty in evaluating the amount of readily available soil K-ne. In this study, we used a milder extraction method (0.01 M Hel method) to examine its applicability to evaluate the amount of readily available K-ne in soil. In the first experiment, the concentration of K-ne in twenty surface soils sampled from agricultural fields in Japan and K-bearing minerals was determined by the 0.01 M Hel method, i.e. sequential extraction with 0.01 M Hel over a period of 10 d after removal of exchangeable K, and by conventional methods. The average percentage of the soil K-ne extracted by the 0.01 M Hel method amounted to 0.66% of the total K amount, and was much lower than that by a single extraction with 1 M HN0 3 (2.0%) or with 0.2 M sodium tetraphenylboron for 2 d (22%). In the second experiment, the amount of K-ne removed by chemical extractions was compared with that of K-ne removed by maize plants grown for 29 d in five of the above soils. The amount of the K-ne evaluated by the 0.01 M Hel method gave the highest correlation (p < 0.05) with that of the K-ne utilized by plants among the extraction methods applied. The amount of soil K-ne extracted by the 0.01 M Hel method could therefore become a suitable index of the amount of readily available K-ne in soil. Extraction of K-ne in soils after maize planting further indicated that plants had removed K-ne more intensively than the 0.01 < Hel method probably only from the rhizosphere, although a high correlation was observed between the amount of K-ne removed by the 0.01 M Hel method and that by plants. This implies that the estimation of the amount of K-ne utilized by plants requires not only soil chemical analysis but also the evaluation of the percentage of the soil volume where the plant-induced release of K-ne actually occurs.

The Codex Alimentarius Commission has recently adopted maximum levels for inorganic arsenic (As; in 2014) and total cadmium (Cd; in 2006) in polished rice grains to maintain food safety and to decrease the risk to human health. As rice is a staple crop in Japan and monsoon Asian countries, reducing concentrations of As and Cd in rice is an urgent matter. In flooded conditions, Cd concentration in soil solution decreases whereas As concentration increases. Therefore, we aimed to evaluate the efficiency of iron-bearing materials to decrease As concentration in soil solution and rice (Oryza sativa L.) grain under submerged cultivation, while also considering Cd concentration. In experiments conducted in paddy fields in six regions, As concentrations in the soil solution during the cultivation period decreased in the following order: control (REF) > steel converter furnace slag (SCS) > non-crystalline iron hydroxide (FH) > zero-valent iron (ZVI). The concentrations of As in brown rice were in the same order, with ZVI achieving particularly strong reduction. Cadmium concentrations were low, probably owing to submerged cultivation conditions. Application of iron-bearing materials slightly and insignificantly reduced the yields of brown rice and straw. Application of these materials did not have a significant negative impact on the quality of rice. Our data indicate that the application of iron-bearing materials effectively reduces As concentrations in soil solution and rice grains without negative effects on yield and quality, with a particularly powerful effect of ZVI which is possibly explained by arsenic sulfide formation.

The relationship between the amount of organic matter determined using various extraction methods and the nitrogen mineralization potential (N 0 ) in soil was examined with reference to land use (upland and paddy soils) and soil type (non-volcanic and volcanic soils). Surface soils were collected from agricultural lands all over Japan (n = 72). Seven methods of extracting labile organic matter were used: water extraction (WE), hot water extraction (HE), extraction using an autoclave (AU), phosphate buffer extraction (PB), sulfuric acid extraction (SA), separation of light fraction (LF) and permanganate oxidation (PM). Significant relationships were observed between N 0 and the content of organic N (OrgN) when the WE, AU, PB and LF methods were applied for all soil samples. Higher correlation coefficients were obtained for groups of similar soil samples in terms of land use and soil type. Namely, for the upland non-volcanic soils, PB-OrgN (r = 0.66**, **P < 0.01) and SA-OrgN (r = 0.69**, **P < 0.01) showed significant relationships and close values to N 0 . For the upland volcanic soils, PB-OrgN (r = 0.72**, **P < 0.01) showed significant relationships and the closest value to N 0 . Therefore, for the upland soils, PB was considered to be the most suitable method for predicting N mineralization potential. For the paddy non-volcanic soils, AU was considered to be the most suitable method for predicting the mineralization potential because of the significant relationship and the result was closest to N 0 . The content of labile organic N regulated the soil N mineralization pattern. In the upland soils, the source of N mineralization differed between short-term and long-term mineralization and, in general, mineralization was regulated by the amount of labile organic N to be mineralized because there was a limited amount of labile organic N. In the paddy soils, the source of N mineralization was similar, regardless of the duration of mineralization, and mineralization could be regulated by a high amount of labile organic C instead of N because of the high content of labile organic N. These findings may provide useful information not only for investigations of the source of mineralized organic N, but also for enhancing the productivity of agricultural lands.