| Soil profile |
| What is soil structure? |
| Why is structure important? |
| Grading soil structure |
| How to improve soil structure |
| Functions of organic matter |
structure
Soil is an essential natural resource, formed spontaneously from rock over an extensive period of time, and made up of soil solids (mineral particles and organic matter) and pore spaces filled with water and air.
Soil is made up of several distinct layers or horizons. These layers form what is known as the soil profile.
The top layer of soil or topsoil is the richest, having the most amount of humus (partially decomposed organic soil material). This phenomenon is largely due to the presence of decomposers (predominantly bacteria, fungi, and earthworms) that recycle dead organic matter (plants and animals) into humus. The subsoil is below the topsoil and is low in humus. However, this is where most soil nutrients are found. Below the subsoil is weathered parent material, which is full of rock particles and minerals with no humus. Parent material is the initial state of soil and can be bedrock, organic material or deposits from water, glaciers, volcanoes, or wind. Physical weathering over millions of years has broken down these materials into fine particles, and soil inherits physical and chemical properties from this parent material. Bedrock sits underneath the weathered parent material and is made up of solid rock. This solid rock will stay hidden and undisturbed until an earthquake or erosion expose it to the surface where some of it will be weathered to make way for the next batch of parent material, which starts the soil forming process all over again. However, soil is not the end product of weathering rock, it is simply a stage in the mineral cycle, and the process by which nutrients such as carbon, nitrogen, and calcium cycle between living things, and the atmosphere and soils.
The form that the soil takes based on its physical and chemical properties is termed soil structure. Mineral particles that make up soil ranges in size from fine to coarse and are categorised accordingly as clay, silt, and sand. The amount of clay and organic matter in a soil plays an important role in determining soil structure. Clays carry a negative electric charge and can attract positively charged cations and water molecules, forming small aggregates. Sand and silt do not have any charge, but are combined into these aggregates when their surfaces become coated with clay or organic matter. These small aggregates can then form larger aggregates with the help of fungal hyphae. The structure of the soil depends on the size, shape and arrangement of these aggregates, and on the pores between these aggregates.
Why is structure important
in your garden?
A ‘well structured’ soil will hold large amounts of water and dissolved nutrients. The aggregates will withstand cultivation and will not ‘puddle’ when wet or become dusty or set hard when dry. The network of pores will ensure adequate drainage and aeration, which are essential for the health of plant roots. Additionally, good structure will provide an excellent medium from which seedlings can emerge and through which roots can explore for moisture and nutrients.
Grading soil structure in your garden
| Structureless soil | No aggregates have formed and the soil consists of either individual separate grains, as in a sand, or a densely packed mass of particles without many pore spaces |
| Weakly developed soil structure | Poorly formed aggregates that are hard to distinguish from the rest of the soil |
| Moderately developed soil structure | Mainly well formed aggregates that, when disturbed, will break down to whole and broken aggregates and only a little unaggregated soil |
| Strongly developed soil | Almost all of the soil particles are in clearly identifiable aggregates |
How to improve soil structure?
The most preferred soil structure for growing crops contains a ratio of 2:2:1 of sand, silt and clay, and is referred to as loam or balanced soil. However, a good soil structure can be obtained by adding organic matter to the soil.
Clay and hardpan type soils can be improved by adding gypsum to the soil. Gypsum tends to improve soil structure by loosening the soil, improving drainage and aeration and reducing crusting. However, you still must add organic material to the soil to add nutrients and assist good soil structure.
Note: Gypsum use on sandy soils can aggressively leach out nutrients.
- Serves as a source of plant nutrients (especially nitrogen and phosphorus)
- Helps the formation of soil aggregates with the help of microorganisms (especially fungi), improving soil structure, aeration and water infiltration and resisting erosion
- Increases buffering of soils
- Provides sources of energy that affect the activities of both macro and microfaunal organisms
- Improves nutrient holding capacity (cation exchange capacity)
- Improves soil colour
From: www.uvm.edu