5.1C Soil texture Notes

1. Soil texture influences key soil characteristics such as drainage, aeration, nutrient-holding capacity, structure, and ease of cultivation.
2. The classification of soil texture relies on identifying the proportions of these particle types and determining how they interact to affect soil function.

Components of Soil Texture

Methods of Determining Soil Texture

Feel Test (Field Method)

1. Soil is rubbed between fingers to determine whether it feels gritty (sand-rich), smooth (silt-rich), or sticky and plastic (clay-rich).
2. This method is quick and useful for fieldwork but does not provide precise percentages.

Soil Texture Keys

1. Standardized soil texture keys guide users through physical observations and hand tests to identify the most likely soil class.
2. Such keys combine visual cues, moisture behaviour, and ribbon-making to assign the soil to a texture group.

Sedimentation / Water-Settling Method (Laboratory Method)

1. Soil is suspended in water in a measuring cylinder and shaken thoroughly to separate particles by size.
2. Sand settles first within minutes, followed by silt (1–2 hours), and finally clay after 24–48 hours.
3. Layer thicknesses are measured to calculate the percentage of each particle type.
4. These percentages are then plotted on a soil texture triangle to determine the soil’s classification

Observe layers

1. Bottom layer = Sand (heaviest, settles first).
2. Middle layer = Silt (lighter, settles next).
3. Top layer = Clay (finest, remains suspended longest).
4. Floating material = Organic matter (humus).

The Soil Texture Triangle

1. The soil texture triangle classifies soil types based on their sand-silt-clay composition.
2. It shows soil categories such as loam, sandy clay, silty clay loam, clay loam, and others.
3. It allows large datasets to be displayed clearly, making it easy to identify soil groups.
4. It highlights the dominant particle type, helping determine likely soil behaviour.
5. A soil can be classified accurately only when the three percentages total 100%.

Importance of Soil Texture

1. Agriculture: Affects root growth, water retention, and nutrient availability.
2. Water Management: Determines drainage capacity (sand drains fast, clay retains water).
3. Construction: Clay-rich soils shrink and swell, causing structural damage.
4. Erosion Control: Fine-textured soils (clay) are prone to erosion when disturbed.

Texture and Primary Productivity

1. Primary productivity depends on how effectively plants can obtain water, minerals, and oxygen from the soil.
2. Soil texture determines the size and distribution of pore spaces, which govern aeration, infiltration, drainage, and nutrient retention.
3. Different proportions of sand, silt, clay, and humus lead to significant differences in soil fertility and ability to support plant growth.

Sandy Soil: Characteristics and Productivity

1. Sandy soils contain large particles that create large pore spaces, allowing water to drain rapidly through the soil.
2. This rapid drainage results in poor water retention, causing plants to experience drought stress unless watering occurs frequently.
3. Sandy soils are well-aerated, ensuring roots have access to oxygen, but they lose nutrients easily due to high leaching.
4. Because they hold limited water and nutrients, sandy soils typically have low primary productivity.

Clay Soil: Characteristics and Productivity

1. Clay soils consist of extremely small particles that pack closely together, creating tiny pore spaces.
2. Water infiltrates clay slowly and tends to accumulate, causing waterlogging and limiting oxygen availability for root respiration.
3. Clay holds nutrients effectively due to its high cation exchange capacity, but roots may struggle to penetrate densely compacted clay.
4. Clay soils may support plant growth only when well-managed, and they generally have low to moderate productivity.

Silt Soil: Characteristics and Productivity

1. Silt particles are intermediate in size, resulting in moderate drainage and moderate water retention.
2. Silt soils are smooth to the touch but may erode easily due to weak cohesion between particles.
3. Silt provides better moisture availability than sand and better drainage than clay, leading to moderate productivity, especially when enriched with organic matter.

Role of Humus in Soil Productivity

1. Humus is the dark, decomposed organic material that coats mineral particles and improves overall soil characteristics.
2. Humus increases aggregation, forming larger clumps that improve aeration and create stable pore networks.
3. It enhances water retention without causing waterlogging, reducing the extremes of both sandy and clay soils.
4. Humus supplies a slow release of minerals as it decomposes and significantly reduces nutrient leaching.
5. Humus fosters high biological activity, supporting organisms such as earthworms, fungi, and microbes.

Loam as the Ideal Soil

1. Loam contains a balanced proportion of sand, silt, clay, and humus, producing optimal conditions for plant growth.
2. Loam provides medium drainage, high nutrient retention, adequate aeration, and high water-holding capacity, all of which support high productivity.
3. Loam supports diverse soil organisms and allows roots to grow deeply and efficiently.

Texture and Ecosystem Productivity

1. Pore sizes larger than 0.1 mm allow oxygen diffusion, root growth, and water movement, supporting healthy plant development.
2. Smaller pores below 0.05 mm retain water, providing sustained moisture during dry periods but increasing the risk of poor aeration when excessive.
3. Soils with moderate pore sizes and balanced texture support higher primary productivity due to optimal water and air exchange.