Malaysian Society of Soil Science

Abstract
The objectives of this study were to determine the rate of aggregate breakdown of three aggregate sizes from three highly-weathered Malaysian soils and relate the aggregate breakdown rate with several aggregate constituents. Bungor, Munchong and Serdang soils (from orders Ultisol and Oxisol) were used. For each soil type, three macro-aggregate size fractions were separated: 3-5, 1-2 and 0.3-0.5 mm. Each aggregate size fraction was wet-sieved for six time durations to determine breakdown rate of aggregates with time. Each aggregate size fraction was also analysed for its texture, organic carbon, total nitrogen, cation exchange capacity, free iron oxides, humic acids, fulvic acids and polysaccharides. Results showed that the breakdown rate of aggregates followed an exponential relationship with time. This meant that aggregates would be particularly sensitive to disruptive forces during the initial periods of erosion by water. The breakdown rate of aggregates was primarily related to the amounts of clay, sand and fulvic acids. However, whether aggregates stability increased or decreased with aggregate size depended on both the soil type and aggregate size in question. This was because the distribution patterns of the aggregate constituents were dissimilar to each other, and their distribution patterns depended on both soil type and aggregate size. For a given soil type, the distribution of clay silt and sand differed among the macro- aggregate size fractions. Though the distributions of other aggregate constituents (organic carbon, total nitrogen, cation exchange capacity, free iron oxides, humic acids and polysaccharides) among the aggregate sizes were statistically similar, they differed according to soil type.

Abstract
Soil water characteristic (SWC) curves were established for various depths ranging between 0.05 and 1.5 m in a free draining sandy loam soil with a deep watertable the laboratory and between 0.2 and 2 m depth in the field. In the laboratory, drying curves were established applying incremental suctions and measuring corresponding water contents, while in the field, soil suction and water contents various depths were measured by tensiometers and neutron probes, respectively during infiltration and drainage. The SWC curves established in the laboratory were analysed using models proposed by Campbell (1974), van Genuchten (1980) and Fredlund and Xing (1994). In general, the three models fitted well to the measured data exceptCampbell’s model which overestimated the soil water content near saturation. The Campbell’s model followed a linear fit to the data when plotted on a semi-log scale, whereas van Genuchten’s and Fredlund and Xing’s models followed a classical non-linear curve silhouette. Both models seem to accurately predict the soil water content for the entire suction range applied in this study. Based on the results of study, it is suggested that the established SWC curves be used to estimate unsaturated soil functions in similar soil conditions in Ireland and elsewhere.

Abstract
The adsorption, desorption and mobility of metsulfruon methyl, a commonly used herbicide in Malaysia, was investigated in some selected agricultural soils namely Persit, Segamat, Kiol, Sedu and Penor as a means to understand the potential of the herbicide to contaminate the Malaysian soil environment. The range of organic matter content of the soil samples was 2.3 to 33.8, while the range of pH was 3.2 to 6.5. The adsorption of metsulfuron methyl to soils was found to best fit a Freundlich adsorption isotherm with distribution coefficients (K_d) ranging from 0.45 - 46.23. A positive correlation (r= 0.95) between organic matter content and adsorption was observed, while a significant negative correlation (r=-0.82 1) was seen between soil pH and adsorption. The extent of desorption of metsulfuron methyl was also observed to decrease with increasing organic matter content of the soil. Simulated rainfall following the application of metsulfuron methyl at a field application rate of 75 g/hectare in the Segamat soil resulted in the leaching of the herbicide to up to 15 cm of the soil column. Much of the chemical was found in the 10-15 cm zone of the packed column. Complete leaching of the chemical through the soil column was not observed under the conditions of the present study, as the chemical was not detected in the leachate. It was observed that the influence of organic matter content and soil pH was consistent with reports on soils from temperate countries. It was also noted that the relatively low pH, low organic matter content and high clay content of the Segamat soil resulted in limited leaching of the chemical.

Abstract
Land application of sewage sludge is considered the most economical: it also provides an opportunity to recycle beneficial plant nutrients and organic matter to soil for crop production. The application of sewage sludge as a source of N fertiliser for maize production was investigated in this study. Application of sewage sludge or inorganic N fertiliser ((NH₄)₂SO₄) produced significantly higher yields than the control (without N). The rate of 420 kg N ha^-1 sewage sludge gave the highest dry matter yield for the 1^st cycle and 746 kg N ha^-1 sewage sludge for the 3^rd cycle. It can be concluded that sewage sludge performs just as good as inorganic fertiliser as a source of N fertiliser. However, in the 2^nd cycle, mineralisation of sewage sludge was slow because of the dry period during this cycle. For sludge to perform as efficiently as fertiliser, good moisture conditions are necessary for N mineralisation to take place.

Abstract
Tillage treatments have been an integral part of many soil and crop management studies on the multifaceted concept of soil tilth. If soil tilth can be represented as a tillage index, it could be used to schedule farming operations and to improve soil management, which will consequently lead to sustainable, productive and profitable agriculture. This study was undertaken to investigate the effect of rotary tillage on some soil physical properties (bulk density, cone index, plasticity index, aggregate uniformity coefficient) and organic matter, and to develop and evaluate a soil tilth index based on changes of these soil properties. The tillage treatments were factorial combinations of forward speeds obtained with four selected tractor transmission gears (Gear 1 High, Gear 2 Low, Gear 3 Low and Gear 4 Low) and three rotary tilling speeds (140 rpm, 175 rpm and 200 rpm) of commonly used tillage implements in Malaysian paddy fields. Experimental results indicated an overall decrease in bulk density cone index, plasticity index and organic matter, after tillage treatments. Analysis of variance indicated significant differences (p<0.01) among the rice yield means. Bulk density, cone index and plasticity index were identified to have fairly high positive correlation with the yield (r>0.30). A tilth index was consequently developed with bulk density, cone index and plasticity index which gave a better linear relationship (R² = 0.56) with rice yield than when individual soil properties were considered. Results of the study suggest that tilth index may assist in yield prediction by comparing measured soil conditions in a paddy field.

Abstract
This research was carried out in a sand bed reactor for heterotrophic denitrification processes. A solution mixture of 24 mg nitrate-N/L and 64 mg methanol-CH₃OH/L was prepared for an influent of eterotrophic reactor. Influent pH level was maintained at 6.5 with the operating temperature in the range of 20 - 35°C. By adjusting nitrogen volumetric loading (NVL) and phosphorus concentrations at tropical operating conditions, it was found that the detention time in the reactor and the phosphorus concentrations of influent were the limiting factors for heterotrophic denitrification process to assure free-nitrite effluent. The maximum detention time was 1 hour and the phosphorus concentrations were 0.4 - 0.6 mg P/L for suppressed nitrite accumulation. The maximum NVL of 0.6 g N/day/L reactor achieved a maximum denitrification efficiency of 94%. The use of 64 mg glucose-C₆H₁₂O₆/L resulted in maximum denitrification efficiency of 98%, while 64 mg acetic acid-CH₃COOH/L only offered a maximum denitrification efficiency of 88%.