Aggregate stability and aggregate size distribution on soil surface that is impacted by rain drops affect soil erosion yet little is known about less weathered coarse textured soils. The objectives of the current study were to determine (i) the aggregate stability and associated aggregate fraction size distribution and (ii) the impact of the initial aggregate size on the aggregate stability and the resulting sediment fraction size distribution following rain drop impact in some quartz dominated coarse textured soils in the Eastern Cape Province. Soil samples for this experiment were collected from 14 ecotopes on the surface with a natural slope between 7.5 to 11% and at the depth between 0 to 0.2 m in the Eastern Cape Province. In each ecotope, twenty-five different spots were sampled using a spade at depth 0 to 0.2 m in other to eradicate biasness and ensure homogeneity. Thereafter, the soil samples were mixed to make a composite sample. The composited soil samples were then placed in rigid containers and taken to the soil science laboratory of the University of Fort Hare, Alice Campus where analyses were carried out. The soil properties were determined by passing the < 5 mm soil sample through a 2 mm sieve. The total Na, Ca and Mg contents in the soil samples were also determined using the wet digestion with sulphuric acid method. The total Soil organic matter content (SOM) was determined by the process known as weight loss on ignition. Thereafter, the fraction size distribution and aggregate stability was done by passing < 5 mm soil samples through a 3 mm sieve. The obtained calibrated aggregates between 3 and 5 mm were oven dried at 40o C. Thereafter, five gram (5g) of oven dried calibrated aggregates was immersed in a 50 mL deionized water in a 250 mL beaker for 10 minutes. The soil material left was transferred to a 0.053 mm sieve already immersed in ethanol and moved five times in the ethanol to separate < 0.053 mm from > 0.053 mm fragments. The remaining > 0.053 mm was re-immersed in ethanol and further oven dried at 40o C for 5 minutes. Thereafter, the > 0.053 mm fraction was transferred from 0.053 mm sieve, oven dried at 40o C, dry sieved using Digital Electromagnetic Shaker on a six column of sieves: 2 mm, 1 mm, 0.5 mm, 0.25 mm, 0.106 mm, and 0.053 mm. The aggregate stability was determined using the resulting size distribution in seven classes by calculating the mean weight diameter (MWD, mm). The soils were very stable, moderately stable or unstable. The presence of smectite and cultivation as opposed to pasture lowered aggregate stability. The studied soils showed three different aggregate size distributions. Unstable soils were dominated by 0.106 – 0.25 mm aggregate size and showed a positively skewed aggregate fraction size distribution. Aggregates finer than 0.106 mm were limited because of the coarse nature of the soil texture. Moderately stable soils broke down to both micro aggregates, 0.106 – 0.25 mm and macro aggregates, 2 – 5 mm giving a bimodal distribution. The aggregate size distribution in the very stable soils was dominated by the aggregate fraction size 2 – 5 mm and a negatively skewed aggregate fraction size distribution. The smaller the initial aggregate size the higher was the aggregate stability but the reverse was true for splash erosion. It was thought that the short 5 minutes duration of the rainfall might not have been enough to cause a total breakdown of the aggregates. Alternatively, ecotopes that were dominated by primary soil minerals such as quartz showed different breakdown behaviour compared to those containing secondary minerals such as kaolinite or smectite.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufh/vital:11963 |
| Date | January 2013 |
| Creators | Nebo, Godwin Iloabuchi |
| Publisher | University of Fort Hare, Faculty of Science & Agriculture |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Masters, MSc Agric (Soil Science) |
| Format | 51 leaves, pdf |
| Rights | University of Fort Hare |
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