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A greenhouse pot-experiment was conducted to evaluate the phytoremediation abilities of three tropical pasture grasses (signalgrass, napiergrass and atratum) in response to two types of soils (Kunigami-maji and Shimajiri-maji) contaminated with three levels of lead (0, 150 and 300 mg kg -1 ). The results demonstrated that the dry biomass, lead concentration and the accumulated amounts were different among the plant species and between the soil types. The amounts of dry matter in three tropical pasture grasses grown on Kunigamimaji soil were higher than that on Shimajiri-maji soil. On both soils, lead concentration of roots was higher than that of shoots, and it was suggested that transportation of lead from roots to shoots was restricted in these plants. The lead accumulated amounts per plant grown on Kunigami-maji soil were higher than that on Shimajiri-maji soil. And, on Kunigami-maji soil, accumulation of lead was relatively high in both shoots and roots of signalgrass and atratum, while, in napiergrass, the high level of lead was found only in roots. Amounts of lead extracted from the shoots of signalgrass, napiergrass and atratum grown on Kunigami-maji soil contaminated with the highest lead level of 300 mg kg -1 were 1.64, 0.17 and 0.92 mg plant -1 , respectively. As Kunigami-maji had lower soil pH than Shimajiri-maji, it can be suggested that lower soil pH may enhance lead bioavailability and uptake by the tropical pasture grasses. In conclusion, signalgrass and atratum could be useful for phytoremediation of lead contaminated soil, especially on Kunigami-maji soil.

Liming the contaminated soil is the most widely used remediation treatment to reduce the bioavailability of heavy metals. The objective of this study was to evaluate the effect of liming on the change of dry matter and lead uptake by three tropical pasture grasses from lead contaminated acidic soil. Lime at five rates of 0, 1, 2, 3 and 4 g kg−1 soil was amended to the Neubauer's pots filled with 500 g Kunigami‐maji soils and then the limed soil was contaminated with 150 mg kg−1 lead after it was maintained for 1 week. Addition of lime increased soil pH significantly from 4.43 to 5.40. The root and shoot dry matter of all three tropical pasture grasses increased with the increasing doses of lime. An elevation of soil pH induced by liming resulted in a significant reduction of lead concentrations in both roots and shoots of all experimental grasses. The effectiveness of liming on lead concentration and accumulation varied with the pH values of limed soil and grass species. The results of this study imply that napiergrass was the most effective tropical pasture grass in reducing lead concentration and accumulation of roots and shoots as a consequence of liming, and could be used for lead stabilization in moderately lead contaminated acidic soil. The shoot lead concentration of napiergrass in limed soils was within the critical level of lead tolerable to feeding domestic animals, and may act as low level lead toxicity in fodder for grazing livestock. However, lime application or soil pH had a little influence on the lead accumulated amount in roots and shoots of atratum and signalgrass. The high amounts of lead accumulated in shoots of atratum and signalgrass were found to be useful for lead phytoextraction.

This experiment was conducted to determine the effect of different fertilization rates on the cyanogen and yields of cassava foliage and tuber. Nine fertilization rates, three nitrogen and potassium levels (N: 0, 50, 100 kg/ha and K: 0, 100, 250 kg/ha, respectively) with constant phosphorus level (P: 50 kg/ha) (F-0:N0-P50-K0, F-1:N0-P50-K100, F-2:N0-P50-K250, F-3:N50-P50-K0, F-4:N50-P50-K100, F-5:N50-P50-K250, F-6:N100-P50-K0, F-7:N100-P50-K100, F-8:N100-P50-K250), were applied in the randomized completely block design. After one year experiment, cassava foliage and tuber were harvested, and determined the yields and cyanogen (HCNp) content. The lowest (P < 0.05) HCNp contents and the highest (P < 0.05) foliage, tuber and protein yields were observed in cassava applied with F-4 (N50-P50-K100) and F-5 (N50-P50-K250) in compare with other fertilization rates. Regarding growth characteristics, the plant height (P < 0.05) was also highest in cassava fertilized by F-4 (N50-P50-K100) and F-5 (N50-P50-K250), whereas the leaf numbers per plant and branches number per plant were highest in cassava applied with F-5 (N50-P50-K250) and F-7 (N100-P50-K100), respectively. It could be recommended that the nitrogen (N: 50 kg/ha) and potassium (K: 100-250 kg/ha) should be used to reduce cyanogen contents for safe utilization and increased cassava foliage and tuber yields.