Soils have many important functions. Perhaps the best appreciated is the function to support the growth of agricultural and horticultural crops. Soil is the mainstay of agriculture and horticulture, forming as it does the medium in which growth and ultimately the yield of food producing crops occurs. Farmers and gardeners have worked with their soils over many centuries to produce increasing amounts of food to keep pace with the needs of a burgeoning world population. The soil's natural cycles go a long way in ensuring that the soil can provide an adequate physical, chemical and biological medium for crop growth. The farmer and horticulturalist have also become skilled in managing soils so that these natural cycles can be added to as necessary to facilitate adequate soil support and increasing yield to enhance production.
Although it is difficult to rate the importance of the different soil functions, since all are vital to some extent, this function of supporting world agriculture and horticulture is a key one in the preservation and advancement of human life on this planet.
As well as this agricultural and horticultural remit, soil also supports a wide range of trees grown for commercial purposes, e.g. plantations such as forests of commercially grown conifers. Soil type is a fundamental factor in deciding what to grow and where to grow it - or rather optimising land use patterns.
In addition to supporting world agriculture and horticulture, soil is also essential for maintaining natural and semi-natural vegetation, our forests, our grasslands and the huge breadth of species that occur worldwide. Generally in these situations, the reliance is on the soil's natural cycles to provide the necessary medium for this huge range of vegetation worldwide, in conjunction with the prevailing climate, landscape and the length of time the soils have been in existence. One of the best examples of this soil function is the tropical rain forest where more and more is now being learned of the important soil cycles that maintain this huge ecosystem.
As well as being essential to agriculture, horticulture, forestry and natural and semi-natural systems, soil also plays an important role for our fauna. The soil itself contains million of organisms, the exact nature and role of which we are still trying to determine. Undoubtedly the soil flora and fauna play a vital role in cycles which are fundamental to the ability of the soil to support natural and semi-natural vegetation without additions of fertiliser and other support mechanisms. They breakdown plant debris, take in components from the atmosphere, aerate the soil together with many other functions that make the soil such an important medium.
It is quite staggering how much variety of life exists in soil. A recent key EU document on Thematic Soil Protection identifies that 1 gramme of soil in a good condition can contain as many as 600 million bacteria belonging to up to 20,000 species! Even a similar amount of apparently barren desert soils can contain 1 million bacteria from up to 8,000 species. As well as those soil organisms that spend their whole life in the soil, there are also larger organisms that spend a part of their existence in soil but depend on the soil for important parts of their daily life, e.g. badgers, rabbits, prairie gophers, reptiles. Although it is not as well recognised as many of the other soil functions, the soil plays an important role for many birds. Some birds nest on its surface, many birds rely on it for a food supply and a very large number of birds rely on the vegetation for which the soil is responsible. Many birds also nest in burrows in the soil.
Soil is increasingly being recognised as playing a fundamental role in the quality and distribution of our water supply. The soil, coupled with the landscape and its vegetation is responsible for the distribution of all rainwater falling upon it. The nature of the topsoil will influence greatly whether the rainwater will run away across its surface, where it can supplement surface bodies of water, e.g. lakes, rivers, and in extreme situations lead to flash flooding, whether it will infiltrate to become stored in the soil for use by vegetation growing on it and by the soil based organisms, or whether it will flow through the soil to the reach the groundwater and at what rate it will do this. The soil thus holds a key position with respect to our water supply cycle and is now seen as a key element as such by hydrologists.
Related to how water moves through the soil and the absorption properties of soils is the soil's ability to perform an important function in pollution control. The fate and behaviour of pesticides in soils has become a key issue in relation to human health, in particular around the world. Soils differ greatly in their ability to manage applied pesticides. Levels of organic matter in the topsoil, soil structure, particularly the presence of prismatic structure with large flow channels, and the texture and adsorption properties of the soils all affect the fate and behaviour (and 'attenuation') of pollutants in the soil. In recent decades the problem of managing nitrate levels in water has become a key issue as levels of nitrates in aquifers and surface water bodies continue to increase. Again the soil type and its inherent properties have become important in the attempts to regulate the movement downwards of the nitrate through the soil.
Soil has always been important for the foundation platform of buildings, roads and other communications and never more so. There is an ever increasing need to expand the number of dwellings and the supporting communication network. The problem with this is that buildings and engineering structures such as roads 'seal' the soil and negates the use of land for other purposes in the future. The nature of the soil, be it sandy, silty or clayey, has an important influence on the foundations to the structures and the measures that need to be taken to ensure stability to the structures.
Many people are interested in their origins and how earlier man lived. Soil plays an important part in the preservation of the earth's history. Archaeologists have come to recognise the importance of soils not only as a medium that has preserved evidence of the wares, properties and way of living of previous cultures and generations but also as a crucial component in determining the extent to which artefacts have been able to survive. Many village and town names today reflect the relationship between soil and society in the past - for instance 'Barton-le-Clay' in the UK.
Finally, soils have been recognised as having a key role in modifying and ameliorating the risks and effects of climate change. Soil organic matter is one of the major pools of carbon in the biosphere and is important both as a driver of climatic change and as a response variable to climate change, capable of acting both as a source and sink of carbon. There is significant interest in the fate of soil carbon, particularly the extent to which soils and land use may sequester carbon from the atmosphere, thus reducing CO2 levels in the atmosphere or lose organic carbon to the atmosphere, thus increasing already existing levels in the atmosphere. Soils can also act as a source and sink for nitrous oxides and methane, both also important in creating the greenhouse gas effect.