Soils, Weathering, and Nutrients

From our previous discourse we know that all chemical elements, with the exception of hydrogen and helium, came from the dying throes of large stars. Furthermore, we know that the amino acids needed to build animal proteins come from autotrophic plant life. Biological studies have shown how certain nutrients (e.g., nitrogen, phosphorus, calcium, magnesium, potassium, iron, etc.) are essential for the process of protein formation. All our amino acids and nutrients eventually come to us from plant life (sometimes via the meat of plant-eating animals). Plants synthesize amino acids from the combination of sunlight, water and soils.

Weathering
Since early geologic time, the atmosphere has interacted with the Earth's exposed crust though a process known as weathering. Weathering takes place through a combination of both mechanical and chemical means. We have all experienced the results of weathering first hand. Any visit to an old cemetery find us peering at the blurred inscriptions on old marble tombstones. These inscriptions were once perfectly legible, but with the passage of time, the small fractures and cracks in the rock have made it vulnerable to attack by aqueous solutions.

A dramatic example of weathering can be seen in these two images of the same 3000 year old Egyptian obelisk just before relocation to damp New York (at left) and 100 years after accelerated weathering in New York (at right).   The obelsik had survived quite well in the dry climate of Egypt, but deteriorated rapidly in the humid and polluted (acids) air of New York.  Weathering rates are obviously a strong function of climate.

Many of the original volcanic gases (e.g., carbon dioxide, sulphur-bearing gases, etc.) that formed the early atmosphere, were able to dissolve in water and produce acids. These acids, in turn, reacted with surface minerals. Later, oxygen in the atmosphere reacted with the exposed reduced materials, making the red beds discussed in an earlier lecture. Since the advent of land plants, soil and surface minerals have been exposed to relatively high concentrations of carbon dioxide maintained in soil pores as a result of decomposition and the metabolic activities of roots. The reaction of carbon dioxide with water in the soil produces carbonic acid (H2CO3) which determines the rate of rock weathering in most ecosystems.

CO2 (gas) + H2O (liquid) --> H2CO3 (solution)

Acid rain, produced by human effluents of nitrogen and sulphur-bearing gases will increase the rate of rock weathering in downwind areas. To understand why weathering occurs and why the rate of rock weathering is so dependent on climate, we need to discuss the chemistry of the process in a little more detail.

The result of the chemical breakdown of the original igneous rocks can be summarized as follows: 

"Igneous Rocks + Acid Volatiles = Sedimentary Rocks + Salty Oceans"

What we mean by this will become clearer if we look at the details of the weathering processes. Consider a boulder or rock containing Feldspars, the most common group of minerals in the Earth's crust.  Feldspars are weathered through a chemical process called hydrolysis, and it works in principle as outlined in the following equation:

2K Al Si₃O₈ + H₂O+H₂CO₃ --> K₂CO₃+Al₂Si₂O₅(OH)+4SiO_2aqu

In this chemical formula feldspar reacts with carbonic acid and water to form potassium carbonate (dissolved in water), kaolinite (a clay mineral, a solid), and dissolved silica (denoted by the subscript _aqu). Thus, during the hydrolysis reaction, soluble elements (potassium, silica) are leached out of the weathering horizon (soil), an alteration product is left behind (kaolinite), and acid is consumed (neutralized).    In this way, rocks containing feldspars are weakened though the conversion of rigid feldspar to more plastic clays which do not have anything like the same structural rigidity. The process occurs at exposed surfaces of the minerals making up the rock. The debris of weathering processes is the source material for all sedimentary rocks, and through geologic time large amounts of sedimentary rocks have been deposited as part of this process.

Rock weathering is critical for the release of biochemical elements that have no gaseous form - examples are calcium (Ca), Potassium (K), Iron (Fe), and Phosphorus (P). The latter element plays a key role in cell metabolism. Thus, we can say that weathering provides key nutrients for life through chemical breakdown of minerals in crustal rocks.

Chemical weathering reactions depend on a supply of acid, and in early Earth history the higher CO₂ content of the air may have helped to have sufficiently acid rain to drive the process forward.  Today, however, the CO₂ content is so low that the acidity of rainwater itself would fast be exhausted and neutralized.   The picture changes, however, when the land surface is colonized by organisms.   Today, for example, the roots of plants and soil bacteria produce abundant CO₂   through their respirative and metabolic processes, and it this constant resupply of CO₂ in combination with moisture that leads to formation of more acid and keeps hydrolysis humming along.  In the Proterozoic, colonization of the land surface by cyanobacterial communities may have served a similar purpose.

Soil
The addition of life the the weathering surface layer is of critical importance.   This combination of weathering blanket and life is what we call a soil, a dynamic natural body capable of supporting a vegetative cover. Today, where there is no soil, there is no plant life and we have barren rock and/or sand. Soil is composed primarily of weathered materials, along with water, oxygen and organic materials.

Soil is layered into sections called "horizons". The figure at left shows a typical soil profile developed on granite bedrock in a temperate region. The top horizon is composed of humus and contains most of the organic matter. This layer is often the darkest. The "A" horizon consists of tiny particles of decayed leaves, twigs and animal remains. The minerals in the A-horizon are mostly clays and other insoluble minerals. Minerals that dissolve in water are found at greater depths. The "B" horizon has relatively little organic material, but contains the soluble materials that are leached downwards from above. The "C" horizon is slightly broken-up bedrock, typically found 1-10 meters below the surface. While this is a typical soil profile, many other types exist, depending on climate, local rock conditions and the community of organisms living nearby. The U.S. Department of Agriculture has classified 10 orders and 47 suborders of soils. If you include other subsets, there are over 60,000 types of soil. The lunar surface, which has been produced by meteoroid impacts, is not classified as a soil, but is rather given the name "regolith" (derived from the Greek words meaning cover and stone). The layered nature of soil indicates its long evolution under the effects of atmospheric and biological processes.

Nutrients
Although living tissue is composed of carbon, hydrogen, and oxygen in the approximate proportion of formaldehyde (CH2O), as many as 23 other elements are necessary for biochemical reactions and for the growth of structural biomass.

Examples of important nutrients are:

Other important nutrients include magnesium, potassium, iron, sulphur, etc. We should note that, although carbon, nitrogen and sulphur can be obtained from the atmosphere, calcium, magnesium, potassium, iron, and phosphorus all come from rock weathering processes. The atmosphere has no store of these essential nutrients.  As mentioned earlier, one of the main purpose of plant root systems is to get access to nutrients stored in the soil. Some plants go to enormous lengths to do this. For example, Emiliani (1992) quotes that "A single plant of winter rye, 50 cm high, was found to have a root system consisting of 143 main roots, 35,600 secondary roots, 2.3 million tertiary roots, and 11.5 quaternary roots! The root system was found to have a total length of 600 km and a total surface of about 250 square meters".

Delivery of nutrients into plant root systems can occur by several pathways. In some cases, direct uptake in water solution occurs. Sometimes, plants actually have to protect themselves against too much nutrient intake. Too much of a good thing can prove poisonous. An example of this can be seen in the accumulations of calcium carbonate deposits that surround the roots of some desert shrubs.

In general, the availability of nutrients (deficit or surplus availability) often controls the form of the ecosystem, limiting the ability of a particular ecological niche to support a particular set of plants. Excess nitrogen, for example, can lead to the loss of fine root biomass and deficiencies in other nutrients. The pool of nutrients held in the soil and vegetation is many times larger than the annual receipt of nutrients from the atmosphere and rock weathering.  Much of the nutrients needed to support plant life are stored up in the humus of soils.