Soil physical properties

Dry, cracked soil. Photo: Ricky Thakrar shared under CC BY-NC-ND 2.0

Understanding the physical properties of your soil gives you valuable information into what your plants need, how often to water, how best to work your soil, how much to feed and which plants will do well.

Soil structure

Soil structure describes the layers in soil. Organic material is added from above as plant debris, creating an organic layer near the surface (the O-horizon). Underneath the O-horizon is the A-horizon – the topsoil; a mix of organic and mineral components, full of soil life. Next, the B-horizon - the subsoil; mainly a mineral soil but broken up by weathering. The deepest layer, below the soil, is the bedrock (R-horizon) - a solid layer of rock. Together, these layers are called a soil profile.

Soil texture

Soil texture refers to the relative amounts of three particle sizes: sand (the largest), silt, and clay (the smallest). It influences water drainage, nutrient levels and susceptibility to erosion.

Sandy soils have large particles and gaps between them so they dry out quickly and nutrients are washed out. However they do warm up quickly and are easy to dig.

Loams are ideal for growing food as they have a good balance of smaller and larger particles. Water does drain in loam soils, but not too quickly and they hold nutrients well.

Clay soils can get waterlogged and are vulnerable to compaction. Clay soils take longer to warm up in spring and can be difficult to dig but usually have high nutrient levels.

Soil texture can be difficult to change except by adding sand, so choose crops that grow well in the soil you have. Root vegetables grow well in sandy soils and nutrient-hungry plants like brassicas grow better in clay. Both sandy and clay soils can be improved by adding bulky organic matter.

Porosity and bulk density

Soil porosity refers to the number of gaps (or pores) between soil particles. Pores can be created by physical soil processes like drought or frost, and biological soil processes like  plant roots and earthworm burrows.

Bulk density is the mass of soil particles in a given volume and depends on soil type, porosity and the level of soil compaction. If soils become compacted the spaces between particles are compressed so porosity decreases and bulk density increases. This can cause problems for growing.

You can carry out a range of simple tests at home to assess your soil structure, soil texture, bulk density and porosity.

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This content is derived from work created as part of the GROW Observatory, which received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 690199.