Soil organisms stabilise soil structure

Good soil structure is important for aeration, root proliferation (to access water and nutrients) and better drainage. Plant roots, bacteria, fungi, earthworms, and other soil biology help stabilise soil structure by binding soil into aggregates.  

• Soil organisms produce a range of sticky compounds (glycoproteins and polysaccharide gums) that help bind clay, silt and sand particles into microaggregates   
• Roots and fungi help to enmesh and bind these into larger aggregates or push tightly bound clay particles apart   
• Bacteria can form protective biofilms over aggregate surfaces   
• Plant roots also have a central role in structure development through drying and compression as they grow 
• Earthworms change the structure of soil by burrowing and generating new stable aggregates as worm casts. This creates new microhabitats for other soil organisms and plant roots 

The soil environment shows extreme variation in space (vertical and horizontal) and time. A wide range of surface types, pore sizes, microclimates and resources form the landscape in which soil organisms live and interact. 

Roots use pores of >100 µm as entry points, while root hairs, protists and fungi use pores of >10 µm. Bacteria can move in water films of only 1 µm depth.  

The amount and nature of the pore space in soil is dependent on soil texture and the way in which mineral particles and soil organic matter are aggregated together. 

Pore size distribution controls the balance of oxygen and water available to organisms. It also regulates the access of soil organisms to one another and their resources. 

Larger pores can drain freely, but water is held on particle surfaces and in smaller pores. The dung and organic compounds excreted by soil organisms are efficient at storing water in soil. It also acts as a sponge for many plant nutrients, retaining them in the soil in a form that plant roots can release.