Although this seems an easy question to ask, it is very hard to given a definitive answer.
Soils is the uppermost living layer (usually down to ~2 meters) of the earth’s crust. Soil is a dynamic, living resource (Doran et al., 1996) that was formed by weathering of rock material under a specific climate and a specific litter-producing vegetation over time in connection with decomposition and humification of organic material by soil organisms and physico-chemical processes. Arable soils develop further under the influence of man’s management.
Soil is an integral part of the environment. The pedosphere, hydrosphere, atmosphere, and biosphere are environmental compartments that overlap and are intimately associated in the ecosystem. Therefore, what happens in soil should have a profound impact on not only soil quality but also ecosystem health. Soils are an integral component of terrestrial ecosystems but it may not be as apparent that soils also play a key role in maintaining the health of air and water ecosystems (Huang et al., 1998).
An easier question to answer is "what are soils made of ?" If we were to 'deconstruct' soil into its constituent parts we would find minerals such as quartz, water, organic matter produced from living and dead plants and living organisms such small animals such as earthworms and possibly small mites and insects depending on where the soil was from. Making a very thin slice of soil may reveal that soil can be very porous with spaces between the particles – in fact ~50% of the soil volume is open pores. These spaces allow the flow of water and nutrients through soil and allow gases such as CO2, CH4, NOx and water vapor to exchange with the atmosphere. The different soil elements are combined forming aggregates of particles. The aggregates are held together at micro level by glomalin-related soil protein (GRSP), exudates of the hyphens of arbuscular mycorrhizal fungi (AMF) (Rillig, 2004). Soil carbon derived from AMF soil mycelium can range from 50 to 900 kg ha-1 (Zhu and Miller, 2003) and in tropical forest soils, it has been found that the amount of C and N in glomalin represented a sizeable amount (ca. 4–5%) of total soil C and N in the oldest soils (Rillig et al. 2001).
Soil structure is highly dynamic. Aggregate turnover dynamics probably represent a primary control of the relationships between soil organic matter occlusion and decomposition dynamics (Six et al. 2002), and a determinant of microbial community structure and processes. Mycorrhizal fungi play a central role in the regulation of aggregate turnover, but the extent to which they influence aggregate turnover rates has yet to be experimentally determined (Rillig and Mummey, 2006).
Soil structure is modified by soil management. Notably tillage has a profound effect on soil structure (Six et al, 1999; ) and should be minimized or even better completely avoided (FAO, 2001). Conservation Agriculture is a soil management system that avoids tilling the soil, while protecting and feeding its organisms with diverse crop residues.
We need the combined disciplines of geology, biology, chemistry and physics to study soil in detail, because of its complexity. Soils are really a living organism. The soil environment is likely the most complex biological community and soil organisms are extremely diverse (Barrios, 2007). A simple allegory demonstrates this. It is often said that without external inputs and under 'normal' conditions in Europe one hectare of land can feed a cow, weighing ~500 kg – this corresponds to a stocking rate of 1.0 ha-1. Underneath the cow, there is the about the same mass of organisms in one hectare of land that make sure, plants grow in order to feed the cow.