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Soil structure refers to the way sand, silt and clay particles are held together in the soil to form aggregates. Soil structure description is based on the shapes of the individual particles within the soil. More importantly it is about the pore space created between the aggregates.
When the sand, silt, and clay cluster together, they form aggregates called peds which provide structure to soil. Organic matter helps bind these particles together. A well-structured soil is like a sponge allowing water in but also enabling it to leave when there is too much of it.
Organic matter occurs in soil as a result of decomposition carried out by micro-organisms. As organic matter is broken down, small air pockets are created. The decomposed matter holds soil particles together. Plant roots and fungi also push soil particles together, creating clusters or ‘peds’. Organic matter provides the glue holding soil particles together.
The diagram to the left shows sand, silt and clay particles are bound together to create aggregates or peds of different sizes and shapes. Between these aggregates spaces are created. These are the macro pores and micro pores where water and air are held.
The Importance of Soil structure
Well-developed structures are essential for providing the necessary pore spaces where the soil organisms and plant roots live. In addition, in order to provide a suitable environment for cultivated plants, the soil must exhibit the following qualities:
· allow gaseous exchange (air)
· hold adequate reserves of available water and be free draining
· have a good infiltration rate (speed at which water soaks into the soil)
· allow root penetration
· have a stable structure that will not deteriorate and limit crop growth.
The structure of a soil has a huge impact on the way water moves through the soil which will be discussed later.
What types of soil structure are there?
Sand, silt and clay can bind together in many different patterns . With some, the bonding is very weak, in others very strong. The size of aggregates also plays a role, especially sand particles. The result can be that the aggregates can be dense containing few pores, or in others quite open with plenty of pores.
There are seven broad categories of soil structure:
There are seven broad categories of soil structure:
Good soil structure allows for improved root penetration by plants, increased soil fauna and flora, good drainage and good water retention and gas exchange. Soil structure can be easily damaged through over cultivation, compaction and excessive erosion. Cultivation can break up large peds to make soil more 'workable' but if a soil is excessively cultivated it can destroy the vital macro and micro pores. Compaction further squashes the aggregates closer together. When excessive cultivation and compaction occur at the same time, soil structure is lost.
The addition of organic matter is the best way to improve soil structure as it helps aggregate formation, increase pore space, cushion compaction. For an organic crop production system this is one of the main methods of soil management as discussed later.
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