The purpose of this study was to evaluate the effect of planting depth (2,4, or 8 cm) on yield and the uptake of N and silicate by rice under low soil fertility in two years. (1) Regardless of the rate of application of basal N (0,3, or 6 gm^<-2>), the numbers of tillers and panicles per unit area were highest at a planting depth of 2 cm and lowest at 8 cm. (2) At 0 and 3 gNm^<-2>, the yield of brown rice tended to reduce as the planting depth increased. At 6 gNm^<-2>, it was highest at a planting depth of 4 cm and lowest at 8 cm. (3) At 0 and 3 gNm^<-2>, the uptake of N and silicate tended to reduce as the planting depth increased. (4) At 6 g m^<-2>, the uptake of N and silicate was highest at a planting depth of 4 cm and lowest at 8 cm. The results suggest that shallow planting increased the spikelet number per unit area (sink) and root mass (source of N and silicate).

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Natural forest soils promote sustainable production of the plant community by recycling ecosystem nutrients. To understand the nutritional characteristics of forest soils as a reference for conservation agriculture, which aims at reducing the use of fertilizer and enhancing biodiversity, we studied moderately moist Brown Forest Soils and Black soils on the footslopes of forested hills. We applied the criteria for desirable soil properties for crop and orchard lands as promoted by the Ministry of Agriculture, Forestry and Fisheries of Japan. Contents of organic matter and available nitrogen in the forest soils were 2 to 3 times those in agricultural soils. However, the pH in the forest soils was low, at about 5, and the concentration of exchangeable cations was also low, at around 10 cmolc kg-1. To understand how the nutrient characteristics of the forest soils form, we compared aspects of agricultural management, for example, the application of compost and lime, with the amount of litterfall and nutrients in the O layer. The productivity of the forest soil seems to be maintained by a lack of disturbance, which favors the predominance of the fungal community in the acidic soil in the absence of fertilizer use, the formation of soil aggregates by soil macrofauna, and the accumulation of nutrients in tree organs. To enable sustainable soil management on agricultural land with reduced fertilizer use, we will need to replicate the overall biological functions seen in the forest soils.

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The adequate ranges of soil N fertility and N fertilizer applications based on that fertility have not yet been clarified for open field production of vegetables in organic farming (OF) in Hokkaido, Japan. To resolve this, we grew pumpkin (Cucurbita maxima Duch.), maize (Zea mays, L.), and lettuce (Lactuca sativa L.) at three N fertilizer rates: none (0N), the standard rate (1N) used in conventional farming (CF), and twice the standard rate, supplied as fish meal or rapeseed meal. The crops were grown in conditions covering a wide range of soil N fertility under the Japanese agricultural standard for organic agricultural products. (1) The yields of OF at 1N exceeded the standard yields from CF, and the N uptake required to achieve these yields was considered to be the optimum for OF. (2) The soil N fertility based on hot-water extractable nitrogen (HN) needed for this optimum N uptake at 1N was 50-70 mg kg^<-1>. This range was established as the soil N diagnostic standard for OF, and is 20 mg-HN kg^<-1> greater than the standard for CF (30-50 mg-HN kg^<-1>). (3) For <50 and &ge;70 mg-HN kg^<-1>, the N fertilizer rates were calculated as (optimum N uptake - N uptake at 0N) / fertilizer N use rate. The calculated rates approximated the corresponding rates for <30 and &ge;50 mg-HN kg^<-1> in CF. Therefore, recommended N fertilizer rates for OF can be estimated as recommended rates for CF + 20 mg-HN kg^<-1>.

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We observed symptoms of excess boron (B) in tomato (Solanum lycopersicum Mill.) and cucumber (Cucumis sativus L.) grown in polyhouses covered year-round with plastic film. The polyhouses were heated by water from a hot spring during winter. The hot-water-soluble B content of the soil was high, often >1 mg kg^<-1>, with a maximum of 11.4 mg kg^<-1>. The B concentrations of the spring waters (15 mg L^<-1> on average) were significantly higher than those of stream water used for the irrigation of tomato and cucumber (<0.1 mg L^<-1>). Therefore, the spring water was the likely source of the excess B. Symptoms appeared in tomato at levels of hot-water-soluble B of about 8 mg kg^<-1> or higher, but the yield did not decrease with increased soil B levels. Symptoms appeared in cucumber at about 2 mg kg^<-1>, and the number of lateral branches was reduced at about 4 mg kg^<-1> or higher. Consequently, the fruit set of lateral branches, and therefore cucumber yield, was decreased. By removing the plastic film during winter, we allowed the B to be leached by snow to <4 mg kg^<-1>, at which content the number of lateral branches of cucumber was not reduced.

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