1NRES 201 Lab4 Soil Structure and Aggregate Stability Test Soil structure or the internal configuration of the soil matrix is defined by the combination or arrangement of primary soil particles into compound elements, which are separated from adjoining structural elements by surfaces of weakness. There is no truly objective or universally applicable way to measure soil structure; therefore, the term soil structure expresses a qualitative concept rather than a directly quantifiable property. In general, three broad categories of soil structure are recognized: single grained, massive and aggregated. The last type of structure is generally the most desirable condition for plant growth, especially in the critical early stages of germination and seedling establishment. Soil structure is one of the main factors controlling plant growth by its influence on root penetration, soil temperature and gas diffusion, water transport and seedling emergence and therefore it is an important soil characteristic for farmers. Soil texture, soil structure, and the type of clay mineral, organic matter content and type, cementing agents and cropping history influence soil structure and aggregate stability. Formation and maintenance of stable aggregates is essential for soil as a media of plant growth. The aggregate stability of a soil is the resistance of soil structure against mechanical or physical-chemical destructive forces. Among the mechanical destructive forces are soil tillage, impact of heavy machinery, treading by animals and raindrop splash. Physical-chemical forces are e.g. slaking, swelling and shrinkage, dispersion and flocculation. Slaking is the process of structure breakdown under the influence of wetting of soil aggregates, due to swelling of clay minerals, dissolving of cementing agents, air explosion or reduction in pore water suction. Slaking may result in the formation of a superficial crust, reducing water infiltration and enhancing sediment loss by downward transportation with surface runoff water. The wet aggregate stability test is determined on the principle that unstable aggregates will break down more easily than stable aggregates when immerged into water. A bulk, air-dried soil sample is passed through a 2 mm sieve to collect the soil aggregates for testing. To determine aggregate stability, a certain amount of air-dried soil aggregates is placed on the top of a sieve with between 0.250 mm openings. This sieve is placed in a container filled with water under a wet -sieving machine, which will move up and downward for a fixed time. Unstable aggregates will fall apart and pass through the sieve and are collected in the water-filled container underneath the sieve. The testing procedure results in an index for aggregate stability (%). Applications of the wet sieving apparatus are the fields of agriculture and land conservation (research on soil erosion, land degradation/ conservation, salinization, agriculture, sustainable agriculture). Determining aggregate stability will give information on the sensitivity of soils to water and wind erosion, which might be prevented e.g. by mulching the soil surface. Information on soil aggregate stability can also improve tillage programs, adapted to the specific soil type and crop demands.2Materials needed to measure aggregate stability: • One 2-mm sieve (20 cm in diameter) • Two 0.250 mm sieves (20 cm in diameter) • Wet sieving machine with water bucket • Drying oven • Two unknown loess soil samples (free of sand greater than 0.250 mm) • scale (0.1 g precision) • Distilled water. Laboratory Directions Sieving of Unknown Soil Sample 1. 1. Pre-weight the mass of a 15 cm in diameter sieve with openings of 0.250 mm. Record the exact weight on the Soil Data worksheet (p. 4). 2. Place the 0.250-mm sieve into the wet sieving machine that has a container filled with distilled water, so that the water surface is just above the bottom of the sieve. 3. Weigh out approximately 25 grams of <2mm soil aggregates. Record the exact soil aggregate air-dry weight on the Soil Data worksheet. 4. Transfer approximately 25 g of less than 2-mm aggregates on to the sieve with 0.250 mm openings. Wet the soil aggregates for 10 minutes. 5. Then, turn on the wet-sieving machine which will move the sieve up and down in the water through a vertical distance of 1.5 cm at the rate of 30 oscillations per minute (one oscillation is an up and down stroke of 1.5 cm in length) for 5 minutes. It is important to make sure the aggregates remain immersed in water on the upstroke. 6. Put the sieve with the remaining soil aggregates into the drying oven for approximately 35 minutes or until dry. (Note: While unknown soil sample aggregates are drying class can start working on unknown soil sample 2). 7. Weigh dried soil aggregates and sieve and record its mass on the soil data worksheet. 8. Discard the dry soil aggregates. 9. Calculate the percent water stable aggregates using the data recorded in the data sheet. 10. Then answer the questions on page 5. Sieving of Unknown Soil Sample 2. 1. Pre-weight the mass of a 15 cm in diameter sieve with of 0.250 mm openings. Record the exact weight on the Soil Data worksheet. 2. Place the 0.250 mm sieve into the wet sieving machine that has a container filled with distilled water, so that the water surface is just above the bottom of the sieve.33. Weigh out approximately 25 grams of <2mm soil aggregates. Record the exact soil aggregate air-dry mass on the Soil Data worksheet. 4. Transfer approximately 25 g of less than 2-mm aggregates on to the sieve with openings of between 0.250 mm. Wet the soil aggregates for 10 minutes. 5. Then, turn on the wet sieving machine which will move the sieve up and down in the water through a vertical distance of 1.5 cm at the rate of 30 oscillations per minute (one oscillation is an up and down stroke of 1.5 cm in length) for 5 minutes. It is important to make sure the aggregates remain immersed in water on the upstroke. 6. Put the sieve with the remaining soil aggregates into the drying oven for approximately 35 minutes or until dry. 7. Weigh dried soil aggregates and sieve and record its mass on the soil data worksheet. 8. Discard the dry soil aggregates. 9. Calculate the percent water stable aggregates using the data recorded in the data sheet. 10. Then answer the questions on page 5. Turn in your data sheets, calculations and answers to the