Soil structure

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Soil structure is determined by the types of particles that make up the soil, and the way they stick together. Soil is often described on the basis of the size distribution of the mineral particles: sand, silt and clay.

 

Diagram of U.S.D.A. soil textural classes

From the plantís perspective, the size of soil particles is less important than the size of the spaces between them. Relatively large spaces are needed, to allow water to infiltrate easily, and to drain freely allowing air to re-enter after wetting. Smaller spaces are also needed to hold water, and to provide plenty of contact between soil particles and soil water so that nutrients on the particles can dissolve and become available to plant roots.

Generally, large particles (sand and stones) pack loosely with large spaces between. Very fine particles (clay) pack very densely with little space. A well structured soil has small particles clumped together in aggregates, so that there are both small spaces (between the particles within the aggregates) and larger spaces (between the aggregates).

If soil aggregates are broken down, for example by plowing when the soil is too wet, the soil becomes dense and hard for roots to penetrate. Water inside the heavy clods is barely accessible to roots, and the large spaces around the clods dry out rapidly.

The stability of soil aggregates is promoted by having appropriate ratios of Calcium to Potassium and Sodium. If too much Calcium is displaced by Sodium or Potassium, the bonds between the clay particles are loosened so that aggregates disperse when wet, and the soil feels heavy and sticky. This can happen over a period of time as a result of irrigation with slightly saline water (salt contains sodium), or persistent use of Potassium fertilizer without Calcium additions. Application of Calcium is needed to restore the soil structure. Calcium is contained in superphosphate fertilizers, and many farmers have observed that these fertilizes reduce soil stickiness and improve its manageability. However, a much cheaper source of Calcium is Gypsum. Superphosphate should only be applied in amounts needed to meet the cropís phosphorus requirement, because surplus phosphate can leach into waterways and cause pollution. Farmers with sticky soil should be encouraged to try gypsum, or lime if the soil is also acidic.

Soil organic matter is also very effective in stabilizing soil structure. Microscopic fibres from decomposed plants, as well as the living filaments of soil fungi, can bind mineral particles together in stable aggregates. Larger pieces of partially decomposed plant material act as aggregates themselves, holding water and nutrients inside but allowing free drainage around them. Even after plant material is completely decomposed, there are organic chemicals, such as tannins, which decay more slowly, and which can help to bind clay particles together.

Contributed by: Jane O'Sullivan 

Further topics on Soil Management:

Soil management

Soil organic matter

Plant nutrients

Fertilisation

Causes of nutritional disorders

Diagnosing nutritional disorders

Correcting nutritional disorders

Other topics on Crop Management:

Land preparation

Production of planting materials

Planting

Soil fertility management

Vine lifting

Integrated pest management

Harvesting

Postharvest practices

Examining the soil profile in a pit dug alongside a sweetpotato field in PNG (J. O'Sullivan).