IB Environmental systems and societies 5.1C Soil Texture Notes

Soil Texture Controls How Soil Holds Water and Nutrients

1. Texture is the proportion of sand, silt, and clay in a soil.
2. It governs drainage, aeration, nutrient-holding capacity, structure, and ease of cultivation.

The Components of Soil Texture

1. A loam blends sand, silt, clay, and humus in balanced proportions, which is why it is the best all-round agricultural soil.

Determining Soil Texture

Feel Test (Field Method)

1. Rub moist soil between your fingers: gritty means sand, smooth means silt, and sticky and plastic means clay.
2. It is quick and useful in the field but gives no precise percentages.

Sedimentation Test (Laboratory Method)

1. Shake soil in water in a measuring cylinder so the particles separate by size, then let them settle.
2. Sand settles first (minutes), then silt (1 to 2 hours), and clay last (24 to 48 hours).
3. Measure each layer to calculate the percentage of sand, silt, and clay.
4. Organic matter floats on top as a separate layer.

The Soil Texture Triangle

1. The soil texture triangle names a soil from its sand, silt, and clay percentages.
2. It shows classes such as loam, sandy clay, clay loam, and silty clay loam.
3. A soil can only be read off the triangle when the three percentages add up to 100%.

Texture Shapes Primary Productivity

1. Productivity depends on how well plants can draw water, minerals, and oxygen from the soil.
2. Texture sets the size of the pore spaces, which control aeration, infiltration, drainage, and nutrient retention.

Sandy Soils

1. Large particles create large pores, so water drains rapidly and retention is poor.
2. Sandy soils are well aerated but lose nutrients through leaching, giving low productivity.

Clay Soils

1. Tiny particles pack into minute pores, so water infiltrates slowly and can cause waterlogging.
2. Clay holds nutrients well but poor aeration and compaction often limit it to low to moderate productivity.

Silty Soils

1. Intermediate particles give moderate drainage and moderate retention.
2. Silt offers better moisture than sand and better drainage than clay, so productivity is moderate, especially with added organic matter.

The Role of Humus

1. Humus coats mineral particles and improves almost every soil property.
2. It boosts aggregation and aeration, holds water without waterlogging, and slowly releases nutrients.
3. It also cuts nutrient leaching and supports earthworms, fungi, and microbes.

Loam: the Ideal Texture

1. Loam combines medium drainage, good aeration, high water-holding capacity, and strong nutrient retention.
2. These balanced conditions let roots grow deeply and support high productivity.

Summary

1. Texture is fixed by the mix of sand, silt, clay, and humus, and is read from the texture triangle.
2. Because it controls water, air, and nutrients, texture is a key driver of primary productivity.