We also had a discussion on Soils by Kent Watson and Bill Chapman. Here is a summary of their discussion
The following table shows the soil orders, their horizons and the environments where they are found.
The top layer of soil (topsoil). This thin layer (usually less than 20 centimeters) is usually the most fertile because of the organic matter which has accumulated from plant and biological activity. In this layer, roots are most dense and exude nutrients which stimulate microorganisms which results in a high biological activity.
In Grassland soils the upper A horizon is usually the darkest layer in the profile. Since the A horizon is visible at the surface it is easy to see why we have called the grassland soil zones Brown, Dark Brown, Black etc.
Topsoil is very precious and can be easily eroded by water and wind and it is in this layer that the most leaching occurs. This downward movement of percolating water moves small mineral particles and salts giving rise to a process called "eluviation". More leaching commonly takes place in soils which are under a vegetative cover of trees, rather than grasses, since effective moisture (more water moves down through the profile) is higher.
Forested soils frequently show a lighter coloured A horizon or Ae (e = "eluviated") , as organic matter and iron compounds are leached downward whereas grassland soils are darker, resulting from an accumulation of more organic matter giving the characteristic Ah (h = "humic") horizon. A horizons are distinguished by adding a second letter after the capital "A" - for example Ah, Ap, Ae. We know already that Chernozemic soils from the grasslands have an Ah horizon - the characteristic dark organic or humic layer with a granular structure.
Where leaching is more intense in the wetter well drained areas, Luvisolic and Podsolic soils may form which have a distinguishing light-gray colored Ae horizon that shows losses of organic matter, clay and the dark colored iron compounds due to leaching.
The soil under the top layer is called subsoil. It is usually lighter in colour because it does not contain as much humus, making it less fertile. This layer can vary in thickness from a few centimetres to 30cm or more. The B horizon shows accumulations of mineral particles such as clay and salts due to leaching from the topsoil. This process is called "illuviation". The B horizon usually has a denser structure than the A horizon, making it more difficult for plants to extend their roots. B horizons are distinguished on the basis of colour, structure and the kind of material that has accumulated in by leaching the horizons above. A Bt horizon has blocky structure with an accumulation of clay. A Bnt horizon accumulates both clay and sodium from salty parent material and has an easy to recognize columnar structure. The reddish Bf horizon has a significant accumulation of iron and aluminum. Whereas, the only differences in a Bm horizon are perhaps colour and structure.
The soil profile we looked at in Churn Creek had a large amount of fine roots. This allows for the ease of water intake for the plants. Fungus was also found in the form of mycellium. They have a mutually beneficial association (symbiosis) with plants by producing mycorrhhiza on the roots that help the plants obtain nutrition from the soil.
This horizon lies under the subsoil and is called the "parent material". This is the original material from which the soil developed. This layer has deposits of sand, gravel, pebbles, boulders and rock in various mixtures. The original parent material could be deposits from glacial activity (Till), from sand and silt carried by the wind (Aolean), from sediments carried by flowing water (Fluvial), including water in flood plains, or from gravity moving material down a slope.
Another source of parent material forms at the bottom of lakes (Lacustrine). The lake bed is made of clay and fine silt deposited by slow moving water. After a long period of time, the lake is filled with soil and a very flat field is formed.
Chernozemic C horizons horizon enriched in calcium and magnesium carbonates designated Cca. C horizons that contain salts: sodium, magnesium, calcium sulphates and chlorides) are designated as Csa. Cg horizons are gleyed and show dull grey colours, sometimes with mottles - typical of poorly drained areas.
Grassland soils can be differentiated from forest or wetland soils most obviously by the way in which organic matter accumulates in the soil. In grassland soils, the organic matter is intimately mixed with mineral soil to form the dark coloured Ah horizon, but little organic matter accumulates at the surface. As the climate becomes cooler and/or wetter, more organic matter accumulates at the surface of the soil, forming increasingly deep litter layers in forests, until with increasingly cool/wet conditions, true organic soils are formed.
Three biomes can be defined in this area based on the way in which organic matter accumulates in the soil and the corresponding predominant type of mycorrhizal association that forms in that soil type. Mycorrhiza means fungus root and most terrestrial plants that we see are in fact symbiotic organisms that depend upon a fungus to take up moisture and nutrients in a natural environment. The three biomes are the ericoid mycorrhizal zone, the ectomycorrhizal zone and arbuscular mycorrhizal zone.
The ericoid zone is the highest elevation and/and or coldest and/or wetest zone. It is characterized by very deep accumulations of organic matter that has very high C/N ratios. The organic matter in this zone is very resistant to decay, but most nutrients on the site are tied up in organic matter. Ericoid mycorrhizae exude enzymes that break down the organic material and also are capable of taking up quite large organic molecules that contain nitrogen and other nutrients. Ericoid plants typically form a mat of fine roots and short hyphae near the bottom of the litter/organic layer.
The ectomycorrhizal zone is intermediate in elevation, moisture and/or temperature. It extends from the grassland with virtually no forest floor to the edges of the ericoid zone. Consequently, ectomycorrhizal fungi display a wide range of characteristics, with many hundreds of species of ectomycorrzhizal fungi. Some fungi are able to break down organic matter and directly take up organic molecules, while others are not. Many species are directly able to take up ammonium, which tends to have a longer residence time in forest soils than nitrate has. Soils in the ectomycorrhizal zone tend to be completely filled with hyphae in the forest floor and upper mineral soil. The ectomycorrhizal fungi are very adept at competing for nutrients, in part,by virtue of their almost complete physical domination of forest soils. Everybody associates woodlands with mushrooms, and many of the large forest mushrooms are mycorrhizal.
Soils in both the ericoid and ectomycorrhizal zones tend to be acidic. This is in part due to leaching of basic ions through the rooting zone by the higher levels of moisture associated with these zones. However, even more important is the fact that many of the cations on site in these zones, are tied up in the great accumulations of organic matter, either in the deep forest floors or the standing vegetation (trees). If trees are left to cycle naturally, most cations are eventually returned to the soil and neutrality is more or less maintained. With timber harvesting, the process of soil acidification is accelerated if the mineral soil cannot replace cations that are lost through the removal of the boles of the trees.
The arbuscular mycorrhizal zone, in contrast, is characterized by rapid decomposition of organic matter and cations are quickly returned to the soil. Grasses draw cations into their roots where they migrate to the above ground portion of the plant. The above ground portion dies, root turnover is relatively rapid and ions are quickly returned to the soil. This keeps the pH high and ultimately results in the zone of calcium accumulation discussed earlier (Cca horizon). The retention of cations on site is a good thing as they are plant nutrients, but it also presents a new set of problems. Phosphorus is only available through a relatively narrow range in pH and grassland soils, or parts of the profiles in grassland soils often have pHs higher than the levels where phosphorus is readily available. Also, the phosphorus combines with calcium and other cations to form compounds in which the phosphorus is very unavailable. One of the characteristics of arbuscular mycorrhizae is that they are able to scavenge phosphorus even in these very difficult conditions.
Another characteristic of Arbuscular mycorrhizae is that a relatively small number of species of fungi exist and it is common for the same and even different species of plants to be interconnected by the same fungal network. Carbon transfer has been demonstrated between plants of the same species and between plants of different species, but it is hard to say how this affects the overall plant community. It has been demonstrated that small seeded plants have a better chance of successfully producing a seedling if the seed germinates and quickly connects into an existing mycorrhizal network. Conversely, it is difficult for non-mycorrhizal plants to establish where there is an existing mycorrhizal network. In fact, weedy plants are often some of the few non-mycorrhizal plants that we have. They are good at establishing in disturbed areas, but as the mycorrhizal plants become established and the mycorrhizal network builds up, weedy plants are displaced.
So in summary, different soil types are not only characterised by different chemical and physical attributes. They are also defined by characteristic communities of organisms that live in the soil. These communities of organisms, in particular the mycorrhizal fungi, play fundamental roles in determining the composition and stability of the plant community. Soil is a complex living system and managing ecosystems, whether grasslands or forests, requires that you pay attention to that system.
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