According to a plot of pAlt' (-log of total inorganic Al concentration) as a function of pH in soil solution, aluminum species and their concentrations are summarized as follows. (1) Al^<3+> became the dominant Al species when the pH and pAlt' values were smaller than 4.5, respectively. (2) Al_6(OH)_<15>^<3+> became the dominant Al species when the pH values were around 4.5 to 6 and Alt' concentration in these pH values was relatively high. (3) Al(OH)_2^+ appears to become the dominant Al species when the pH value is higher than 5 and the pAlt' value is higher than 5.5. (4) The Al species which were present between (2) and (3) described above didn't exceed 50% of the Alt' independently. It was difficult to observe root systems in soil culture. Therefore, the degree of wheat response to soil solution aluminum may be represented by retention capacity of soil by the root system. The retention capacity of soil by a root system showed the lowest value in the Al^<3+> dominant area, such as (1) mentioned above, followed by the Al_6(OH)_<15>^<3+> dominant area, such as (2). However, even if the pH and pAlt' values of soil solution were in the Al_6(OH)_<15>^<3+> dominant area, the influence of Al_6(OH)_<15>^<3+> on the retention capacity of soil by the root system was negligible when the pH value was higher than 5. This is because the Alt' value is smaller than 10^<-5> mol L^<-1>. As the acidic aluminum in soil solution is neutralized, root elongation including root hair formation is promoted, resulting in the increase of the soil retention capacity of roots. Hence, a rhizosphere with an enlarged area of soil adhering to roots is created so that diffusion and mobility of water and nutrients from soil to plant can be facilitated favorably.

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We investigated the effects of drip irrigation and fertigation on the growth, yield and absorbed nitrogen of eggplant (scion: Solanum melongena L., rootstock: Solanum torvum) which was cultured in a forcing house. In addition, we evaluated the inorganic nitrogen supplied from soil, residual inorganic nitrogen in the soil at the end of cultivation and nitrogen removed from plow layer. The growth of eggplant cultured under the drip irrigation and fertigation way was better promoted than of that cultured under the customary way. Accordingly the yield of the former was increased. In the greenhouse, most nitrogen was supplied from the plow layer because of the accumulation of nutrients. Therefore the yields of eggplant cultured with drip tubes were equal whether the fertilizer was applied or not. Under the drip irrigation and fertigation way, the nitrogen removed from the plow layer was less than that under the customary way.

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The effects of nutrient uptake by plant roots on the solubility of Al in volcanic ash soil were studied. The soil used was classified as Alic Meranudand, which contains a large amount of active Al. Soybean plants were grown in a greenhouse for 84 d. The concentrations of K, Ca, Mg, Na and Al in plants and in soil solutions at different growth periods were measured. The mass flow of the elements from soil solution to the root surface was calculated from the concentrations in the soil solution and the daily transpiration of soybean. The concentrations of nutrient cations (Ca^<2+>, K^+ and Mg^<2+>) in the soil solution decreased as the soybean plants grew. On the other hand, the concentrations of Al^<3+> increased; the value of 10μmol(+) L^<-1> in the vegetative growth stage increased to 25μmol(+) L^<-1> in the podding stage. The concentrations of Al^<3+> negatively correlated with the concentrations of nutrient cations (i.e. Ca^<2+> and K^+). For soil solutions collected from unplanted pots, no correlations between the concentrations of Al and the concentrations of nutrient cations were observed. It was assumed that soybean roots released H^+ with the uptake of nutrient cations, and the H^+ caused the pH decrease near the roots, thus, the Al was desorbed from the soil solid phase. Although the pH values of the bulk soil were 6.0-6.2 during the soybean growth, exchangeable Al content increased after cultivation. Although the ratio of mass flow to actual uptake of Al was 1.1 at the vegetative growth stage, it increased to 2.1 at the podding stage. This shows that the mass flow of Al was in excess of what is absorbed by the plants, thus the Al accumulated near the roots at the podding stage. These results provide useful information on the mechanisms of Al toxicity and tolerance in plants.

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When bean curd is produced, a large amount of bean curd lees is turned out as a side product and is disposed of as a waste product. Carbide of the bean curd lees was prepared, and this was used for cultivation of Brassica campestris cv. komatsuna. Evaluation was made of the effects of the use of this carbide of bean curd lees on the cultivation of Brassica campestris cv. komatsuna, and its properties were assessed. 1) The carbide of bean curd lees contains phosphorus, potassium, calcium and magnesium, and we can expect much from its effects as fertilizer and as a substance to correct and adjust the acidity of soil. 2) When this was used for planter cultivation at a rate of 1-50 Mg ha^<-1>, growth of Brassica campestris cv. komatsuna could be promoted by 22-84%. Under farm conditions, growth of Brassica campestris cv. komatsuna could also be increased by 28-32% when this carbide was used at a rate of 5 Mg ha^<-1>. 3) The carbide of bean curd lees contains nitrogen in an amount of 37.6g kg^<-1>. When it was used in an amount of 1-5 Mg ha^<-1>, there was no change of nitrogen content in Brassica campestris cv. komatsuna cabbage. This may be attributed to the fact that this carbide contains very little nitrogen in the inorganic state, and that most of its nitrogen-related components are in a state stable to acids.

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The Japanese limit of cadmium (Cd) in brown rice is 1.0 mg kg^<-1>, and brown rice containing >0.4mg kg^<-1> has not been previously used for human consumption. The CODEX commission of FAO/WHO stipulates that the Cd content in cereal grains, such as rice and wheat, should be <0.4 and <0.2 mg kg^<-1>, respectively. Given this situation, the rehabilitation of polluted paddy fields is vital to Japanese agriculture. Phytoremediation is an emerging technology that employs plants to remove environmental pollutants such as heavy metals. Such efforts have proven cost-effective and are less disruptive to the environment compared to conventional soil clean-up methods. We could select kenaf (Hibiscus cannabinus) as a suitable plant for phytoremediation of cadmium-contaminated soils through pot-scale experiments. Then we conducted field trials using kenaf for 3 years (2001-2003) at a cadmium-contaminated paddy field in the southwest area of Japan. The kenaf showed large amounts of Cd-uptake each year. As fallen leaves contained large amounts of Cd, which was proven in the 2nd year trial, we managed to cultivate them in a proper space to avoid leaf fall in the 3rd year. Concerning with the production and Cd concentration, both of which are depending on width between plant rows and cultivation period, we proposed double cropping of kenaf in the southwest area of Japan for remediation of Cd. Our data clearly demonstrated that a significant level of Cd in the soil was taken up through the phytoremediation with kenaf.

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Potassium phosphate monobasic solution was applied to cabbage seedlings before they were transplanted into the field of andosol where no phosphorus was applied. In this way, even when an application rate of phosphorus was reduced to 20% of that of a conventional fertilizer system, the weight of cabbage heads was the same as that when using the conventional fertilizer system. Consequently, the phosphorus recovery rate was improved up to 43%. This high phosphorus recovery rate was due to high phosphorus absorption resulting in a high growth rate during the early growing stage, as well as less phosphorus fixation in the soil. In addition, it was important that the absorption rate of essential elements such as Mg and Ca tended to be increased by this fertilizer system. When we grew cabbage by this fertilizer system three times in succession, yields were always as the same level as those by the conventional system. The available phosphorus decreased slightly while that for the conventional system increased.

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[in Japanese]

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