Many soils come from rocks which are weathered when exposed at the earth’s surface. Weathering involves breaking rocks down into small grains, which is caused by water from rain and changing temperatures. Some of the broken down rocks dissolve in rainwater and are washed away. Others remain as soil solids, or as new soil particles.  There are three main types of soil particle: sand, silt and clay, which are different sizes and give soils a distinctive feel, or texture.

Organic matter comes from living organisms, such as plant and animal remains. It is broken down in the soil and consumed by the millions of tiny microbes and animals which live in each teaspoon of soil! Their activities release nutrients for re-use by future plants and animals. Maintaining a regular source of organic matter is important to maintain and improve soil health.

Soil can store significant amounts of rainfall and engineers can use local soils to assess and control flooding risk. Poor soil management, such as soil compaction which can come from poor agricultural soil management, can lead to unexpected flooding.

It is estimated that there are 1,500 gigatonnes of carbon in the world’s soil; three times more than in all vegetation and forests! Deforestation, global warming and poor agricultural practices can lead to the release of soil carbon into the atmosphere, and in turn speeds up the climate warming process. 

To find out more about the importance of carbon in soils, how it behaves, and how it might be increased to help address the climate crisis, read our Science Note. 

Soil has a role in growing over 90% of the food that we eat and maintaining the health of our soil is essential to ensure the world is able to continue feeding its ever-growing population. In addition to food production, many parts of the world use soil, particularly clay, as a building material. Soil is also used to process waste: sewage sludge and slurry from intensive livestock production are spread on and ploughed into soils, which can render the previously harmful material harmless, and even productive. Soil caps are also used to restore contaminated and derelict areas left by industry and mining; covering, absorbing and mitigating effects of contaminants and revegetating sites. 

Leaves that fall to the ground are broken down into their constituent parts through a process called decomposition. Leaves that fall onto soil will break down faster than leaves which fall onto tarmac because the organisms which breakdown leaves are found in higher amounts in the soil environment. 

There are two pathways for the decomposition of leaves. The first is detritovore fragmentation where organisms consume the dead leaves to break them down, such as earthworms, ants, flies and millipedes. The second is saprotroph decomposition, where fungi and bacteria produce enzymes to break down the leaves. They are broken down into smaller constituents including simple sugars, water, nutrients and minerals. 

Decomposition is vitally important for replenishing stocks of nutrients into soils which are depleted when plants grow. Think of decomposition as nature’s recycling process, a circular system to sustain levels of important soil nutrients such at Nitrogen, Phosphorous and Potassium as well as contribute carbon for the promotion and maintenance of soil structure and functioning. Without decomposition, soils would become exhausted and unable to sustain life. 

Foreshore mud is anaerobic because close to the river, groundwater is near to the surface which means much of the soil is waterlogged and therefore the soil pores are filled with water rather than air, causing the anaerobic conditions. See the soilscapes map here for a very general description of these soils.