Formation of soil aggregates

This topic contains 4 questions:

  1. What are soil aggregates and soil pores?
  2. What is a polysaccharide?
  3. How could bacteria be involved in the formation of a soil aggregate?
  4. How could fungal hyphae be involved in the formation of a soil aggregate?


Question 1. What are soil aggregates and soil pores?

Soil aggregates are clumps of soil particles that are held together by moist clay, organic matter (like roots), gums (from bacteria and fungi) and by fungal hyphae. The aggregates are relatively stable. Aggregates vary in size from about 2 thousandths of a millimetre across, up to about 2 millimetres across. Soil pores are the spaces between soil particles and between soil aggregates. They can be full of air or they can have water in them. Soils with lots of aggregates are called "well-aggregated" and this condition is thought to be very desirable, for a number of reasons. The aggregates are made up of particles of different sizes and some of these particles fit closely together. Some do not. This means that there are spaces of many different sizes in the soil and these spaces are essential for storing air, water, microbes, nutrients and organic matter.

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Question 2. What is a polysaccharide?

A polysaccharide is an organic compound composed of many simple carbon compounds, identical to eachother, joined together in a long chain. They are released into the soil by microorganisms and roots. Polysaccharides are long and flexible and they can make points of contact with many soil particles. In this way they have been associated with the formation of aggregates in soil.

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Question 3. How could bacteria be involved in the formation of a soil aggregate?

There are two ways that bacteria could be involved in the formation of soil aggragates. Firstly, bacteria produce polysaccharides which can make many points of contact with soil particles producing aggregates. Unlike the polysaccharides produced by plant roots, bacterial polysaccharides resist decomposition long enough to be involved in holding soil particles together in aggregates. Secondly, bacteria develop a small electrostatic charge which attracts to the electrostatic charge on clay surfaces, bringing together small aggregates of soil.

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Question 4. How could fungal hyphae be involved in the formation of a soil aggregate?

Fungi grow in long thin strands called hyphae. The length of fungal hyphae in the soil has been found to be related to the amount of aggregates in the soil. Fungi help to form aggragates in the soil hen their hyphae enmesh soil particles and when they form cross links between soil particles. Mycorrhizal fungi and fungi that colonise fresh organic matter and are believed to be the most important for assisting aggregation.

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