Soil structure refers to the way in which the sand, silt and clay particles are arranged relative to each other.
In soil with a good structure, the particles of sand and silt are held together in aggregates (small clumps) by clay, humus and calcium. The large empty spaces between the aggregates (macropores) allow water and air to circulate and plant roots to grow down into the soil. The small empty spaces (micropores) hold the water the plants need. This “ideal” structure is called granular, or crumbly.
Soil with a granular structure has a number of advantages:
- holds water and nutrients well
- good drainage
- good aeration
- good plant root system development
- soil is easy to work
- soil warms up quickly in spring
- good biological activity in the soil
- soil resists erosion and compaction
Clay, sandy and silty soils rarely have an ideal structure. They can be improved, however, by working in amendments.
You can improve the structure of sandy soils by regularly adding organic matter in the form of compost or composted manure. It is best to work these amendments into the soil in early spring, because working sandy soils in the fall promotes erosion. Adding basalt to these soils is one way of improving their ability to retain water and minerals.
You can improve the structure of clay soils by adding organic matter in the form of compost or composted manure. It is best to work these amendments into the soil in late fall. Poorly drained clay soils can also be amended by adding sand.
Some clay soils are very high in sodium, which prevents mineral particles from forming aggregates. The structure of such high-sodium soils can be improved by working in gypsum (if their pH is neutral or alkaline) or lime (if their pH is acidic).
You can improve the drainage and aeration of silty soils by adding large amounts of organic matter in the form of compost or composted manure. It is best to work these amendments into the soil in late fall.
Microorganisms in the soil break down organic matter, quickly releasing nutrients (mineralization) and forming humus (humification).
Mineralization: The process by which organic matter is decomposed into nutrients that can be directly assimilated by plants.
Humification: The process by which organic matter decomposes to form earthy-smelling dark brown to black material, called humus. Humus is mineralized in turn, but very slowly. This makes small amounts of nutrients available to plants over a very long time.
Other recommendations for improving or maintaining soil structure:
- Regularly add organic matter (compost or composted manure).
- Encourage biological activity in the soil.
- Correct the pH as necessary.
- Avoid overworking the soil. Hoe the soil or turn it over lightly.
- Use mulch.
Working the soil
To improve the structure of sandy soils, it is best to work them as little as possible. It is recommended that amendments and fertilizer be added to this type of soil in spring.
Compact clay soils can be turned over with a fork in fall. Over the course of the winter, the large clumps will be broken down by alternating freeze-thaw cycles. In spring, you can further break them down into small aggregates to obtain a granular structure. Avoid overworking clay soils that have a good structure.
Clay soils should never be worked when they are too dry or too wet. They become hard and cracked during dry spells, and are easily compacted during wet spells. The best time to work such soils is when clumps break apart when squeezed.