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A series of study was carried out to evaluate the impact of Eucalyptus plantation on soil properties and subsequent cropping under the environmental conditions of northeast Thailand. The main frame of the research project was splitted into 7 separated sub-projects including the study programs specifically planned out :
- To evaluated the impact of Eucalyptus plantation on ground water table beneath the plantation in comparison to the level monitored near by but outside the plantation area.
- To assess the impact of Eucalyptus plantation on soil physico-chemical properties.
- To monitor litter fall in 4 year-old Eucalyptus plantation with different planting spaces.
- To compare the decomposition rate of Eucalyptus leaves with organic materials from other sources.
- To evaluate the effect of organic matter from Eucalyptus on germination, growth and yield of essential food and fodder crops commonly grown in northeast Thailand.
The results from these studies indicated that :-
- Eucalyptus plantation could lower the ground water table provided the size of the planting area is greater than 100 rai (= 16 hectares).
- Of the 13 soil properties investigated including soil texture, total acidity, pH, organic matter content, cation exchange capacity (CEC), N, P, K, Ca, Mg, Na and exchangeable Al, there was a trend for organic matter content, CEC and P to increase in the top soil (0-15 cm) in the plantation compared with the soil in the adjacent area; however, none of the change was significant statistically.
- Litter fall was affected markedly by the planting space. The amounts of 382.12 g/sq.m and 271.92 g/sq.m were recorded over a period of 11 months from 2x4 and 2x8 m planting spaces from 4 year-old respectively. Eucalyptus plantation. Maximum and minimum litter fall were recorded from December to January and from may to June.
- Similar to rice straw, mango and Acacia leaves, decomposition of Eucalyptus leaf litter occurred at the highest rate within the first month. However, decomposition seemed to continue in rice straw, mango and Acacia leaves whereas little change was recorded over a period of 7 months for Eucalyptus leaves. Dry weights of the remaining residues were 45.6, 15.75, 11.25 and 7.75 percent for Eucalyptus, rice straw, mango and Acacia leaves respectively.
Regarding seed germination,- our results also indicated that except for kenaf and cowpea, seed germination percentage of maize, sorghum, sesame, soybean, mungbean, peanut and leucaena decreased significantly in the soil amended with ground Eucalyptus leaves at the rate of 5 g/Kg soil. The reduction of seed germination of these crops was worsened at higher rates of addition. Greater rate of decrease in seed germination was noted when sand was used. Even though addition of ground Eucalyptus leaves at rate of 10g/Kg soil significantly enhanced shoot growth of such crops as maize sorghum kenaf and cassava, nodulation of soybean, mungbean, peanut, leucaena and stylo ceased, and the shortgrowth and yeilds of all these leguminous crops and maize, sorghum, Kenaf and cassava decreased significantly when the rates was raised to 25 g and 50 g/Kg soil. Four to eight folds reduction of shoot growth was recorded in soybean, mungbean, peanut, maize and sorghum and as much as 150 and 800 folds decreases were recorded in leucaena and stylo respectively at the rate of 100g/Kg soil.
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