Soil functions

A mole on the surface.Click to enlarge
Soil is home to this creature. Soils contain much more than this.
Source: santia3 Fotolia.com

Soil carries out myriad functions that are free of charge and extremely powerful, and that are what make soil so valuable not only to us but also to natural flora and fauna. Clean water and healthy food are only obtainable if our soils are healthy too. And while the soil supports myriad functions, it also needs stewardship.

Table of Contents

 

Soil: the mainstay of our food supply

Although those of us who live in affluent Western countries take an ample supply of healthy food for granted, the availability of this food depends on the availability of uncompromised soils. Around half of Germany’s surface area is used as farmland, i.e. for crop cultivation, direct consumption and animal fattening. German farmers produce an average of more than 40 million tons of grain and ten million tons of potatoes annually. In recent years, increasing amounts of farmland have been used for energy plant cultivation that is in competition in Germany and elsewhere with crop plant cultivation.

Arable land is mainly found in places where the earth is inherently very fertile. Less fertile soils are found in forests or in meadows and pastures. Settlements and towns have traditionally sprung up in proximity to fertile land and desirable farmland, and rural areas still bear signs of this evolution today. Nonetheless, socioeconomic change has brought about changes in the use of and value attributed to arable land. Nowadays urban sprawl is a ubiquitous phenomenon entailing the use of arable land for purposes other than farming.

Soil fertility is determined by climate, as well as humus content, grain size and soil structure – which, in conjunction with tillage and cultivation yield growth conditions of varying quality for a given soil. In today’s agricultural sector, these growth conditions are artificially enhanced in order to increase crop yields. But these practices modify natural soil properties in ways that are not always beneficial, in that soil can in the long run lose its natural fertility for any of the following reasons: nutrient input resulting from the use of mineral fertilizers; mechanical tillage; pesticide use; and in some locations, the fact that the practice of tripartite crop rotation has been abandoned. It is for this reason that Germany’s Soil Protection Act (Gesetz zum Schutz der Böden) places such great emphasis on the precautionary principle.

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Soil: a multifarious habitat

Soils serve as habitats for flora and fauna. The former comprise fungi, algae and lichen, which do the heavy lifting for all breakdown processes. The breakdown of dead organic substances such as leaves and mulch makes nutrients available to plant roots. The number of organisms found in a handful of earth exceeds the world’s population.

Soil fauna range in size from tiny nematodes to medium sized mites and earthworms to voles and moles. Soil fauna create soil structures by digging and rooting around, and mix mineral soil particles of varying sizes with organic substances, thus creating air and water transport channels. Soil fauna ensure the availability of ramified soil structures, which in turn guarantee far higher quality soils than that provided by freshly tilled or dumped earth.

Earthworms are instrumental when it comes to the cementing of the organic and mineral elements in soil to each other – a process that is essential for plants to uptake water and nutrients. The optimal habitat for soil fauna is loose, well aerated soil with favourable temperature and moisture conditions.

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Soil is a repository of the history of civilization

Soil is an archive that we can read as we read a book. The vertical sequencing of soil strata bears testimony to the history of our natural and cultural terrain. Today’s soils are the product of a process that began after the Ice Age around 10,000 years ago and that reflects the environmental and usage conditions during this period. The structure of soil strata gives us insight into matters such as the climate during the formation period, and provides clear indications concerning human actions and farming techniques. Soils are repositories of archaeological artifacts and contain evidence of historical farming practices.

For example, clear-cutting of entire forests in medieval times and the consequent lack of ground covering resulted in severe erosion following heavy rains that is still visible in today’s terrain and that resulted in the formation of layered soil strata. Wind erosion resulted in the formation of inland dunes. Heathland comprises areas whose formation is attributable solely to plaggen management. Without human intervention, heathland would soon become forests. In Germany, farmland whose surface undulates slightly is referred to as Wölbäcker, which resulted from a medieval farming technique in which successions of ridges and furrows ensured that crops would grow during both wet and dry years. Traces of today’s settlement areas and the consequent nutrient surpluses and pollution will be discernible for eons in the archive constituted by the soil.

Photo of site plan of parc "Kalkriese".
Evidences from Roman Age are found in the soil on the grounds of the Battle of the Teutoburg Forest
Source: S. Marahrens / Umweltbundesamt
 

Soil water retention

Soil is composed of numerous mineral particles with gaps between them that are referred to as pores. Pore size varies according to soil structure and granulation, which is in turn mainly attributable to the activities of soil fauna. Pores, which contain either air or water, facilitate water transport at varying speeds (depending on pore size) into lower soil strata, or store percolated water as soil water. Predominantly sandy soil transports water rapidly and thus cannot store much soil water for plants. On the other hand, predominantly silty soil contains myriad medium sized pores that can store water for lengthy periods and that promotes good plant aeration, hydration and nutrition. The total water retention capacity of soil is also largely determined by soil depth – which is why deep and naturally developed soils are so important. Soil water retention slows down water discharges into rivers and streams, thus reducing the risk of flooding. However, this can only occur in non-built up areas, which are also indispensable for groundwater and our drinking water supply.

Scheme soil as water store
Non-cultivated soils store water. This secures water supply and restricts flood waves.
Source: S. Marahrens / Umweltbundesamt
 

Soil filters out impurities

The particle structure and physicochemical properties of soil enable it to filter out, neutralize and bind (a) chemical elements and compounds; (b) nutrients; and (c) any other toxic substances. Hence soil also prevents pollutants from percolating into the groundwater and thus, irreversibly, into our drinking water. The filtering capacity of soil varies according to grain size structure, humus content and pH. The manner in which the ground is used and the amount of pollutants that the soil comes into contact with are also key factors in this regard, since the soil is able to perform these herculean tasks to a limited degree only.

During the filtering process, pollutants and all chemical elements and compounds that are dissolved in soil water are bound by humus and clay particles. Hence, soils with a high clay particle content filter far more efficiently than do sandy soils. Changes in soil chemistry can potentially mobilize bound substances, an example of this being increasing soil acidification provoked by lower pH values.

Chemical compounds can be neutralized in the soil. During this buffering process, the compounds are changed by a chemical reaction and lose their original structure, an example of this being acid buffering of a substance such as nitric acid, which originates from atmospheric nitrogen compounds and is neutralized in the soil. This process continues so long as the soil has sufficient capacity to maintain the necessary chemical processes. Carbonates and clay minerals are particularly potent in this regard. Their depletion induces rapid soil acidification and a pH decrease to very low levels As this phenomenon is often observed on forest floors, the soils in question are artificially limed. Because crop field soil contains mineral fertilizer and is in any case regularly limed, the pH in such soils is conducive to plant growth.

Böden filtern Verunreinigungen
Soils filter and neutralize a lot of pollutants. Humans and the environment benefit.
Source: S. Marahrens / Umweltbundesamt
 

Soil and climate

Soil sequesters carbon highly efficiently, as do the world’s oceans and forests. Soil humus (i.e. the decomposed and converted organic-substance component of soil) contains carbon that has been removed from the atmosphere. Apart from soil’s beneficial effect on global warming, it also has a direct impact on the immediate environs.

The heat stored in soil, in conjunction with plant transpiration, affects local temperature and humidity – which is why temperatures are far higher in built-up areas than in green areas. Plant transpiration in well vegetated areas exerts a cooling effect, and such areas do not heat up as readily as their paved over counterparts. This phenomenon is readily apparent in parks or forests on hot summer days

Soils have an influence on the climate.
Soils have an influence on the climate on local and global levels.
Source: FG II 2.7 / Umweltbundesamt
 

Soils cover natural resources

The Federal Institute for Geosciences and Natural Resources (Bundesanstalt für Geowissenschaften und Rohstoffe) estimates that each German uses around 1,000 tons of raw materials over the course of their lifetime. These materials comprise varying proportions of mineral, energy and metallic resources, whose extraction entails visible interventions in natural landscapes and the destruction of naturally developed soils. It is for this reason that in Germany, recultivation during and after mining is an integral part of raw materials extraction.

Photo of wet mining
Soil is removed before sand can be extracted.
Source: S. Marahrens / Umweltbundesamt