Classification of common rocks and soils and more
by Dr J Floor Anthoni (2000)

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Properties of soil
Soil is the collection of natural bodies in the earth's surface, in places modified or even made by man, of earthy materials, containing living matter and supporting or capable of supporting plants out-of-doors. Its upper limit is air or shallow water. At its margins it grades to deep water or to barren areas of rock or ice. Its lower limit to the not-soil beneath is perhaps the most difficult to define. Soil includes the horizons near the surface that differ from the underlying rock material as a result of interactions, through time, of climate, living organisms, parent materials and relief. In the few places where it contains thin cemented horizons that are impermeable to roots, soil is as deep as the deepest horizon. More commonly, soil grades at its lower margin to hard rock or to earthy materials, virtually devoid of roots, animals or marks of other biologic activity. The lower limit of soil, therefore, is normally the lower limit of biologic activity, which generally coincides with the common rooting depth of native perennial plants. (US Soil Survey staff, 1975)
soil type CEC
of minerals
sandy soils
sandy loam
silt loam
clay/ clay loam
organic soil
2 - 4
2 - 17
7 - 16
9 - 30
4 - 60
50 - 300
fast to moderate
very low
very low
very low


very low

Soil degradation
This systematic classification of the many ways that soil can be lost, is not only interesting but also shows that sustainable farming is like walking a tight-rope. Managing agricultural soil can be improved considerably by paying attention to each of the factors detailed below.

soil timescales
Soil is perhaps the only resource that is not directly consumed. Water and air are principally inexhaustible renewable resources, but they are used. Water is used. The used water does not return directly, but recycles through the atmosphere at high rates. So does air (carbon dioxide). But soil forms a noticeable exception. Every time it is used for a new crop, it is still there, afterwards. But soil degrades and is lost gradually. Here are some timescales to remind you of its uniqueness.

Important rock and soil chemistry

Acidic -  intermediate - Basic = grouping according to the ratio of metal to oxygen atoms. Basic= high ratio (less than 50% silica). Acidic = low (more than 50% silica). It is also the order of mineral formation from a magma melt.

Solid solution = composed of various components such that the chemical formula of the rock is not unique and any combination is possible. Two or more elements can substitute for each other completely. For example, the anions Mg++ and Ca++ , which are similar in size and function, can combine CaSiO3 and MgSiO3 to (Ca,Mg)SiO3 as if the rock components were dissolved into one another. The fact that silicate rocks allow for substitution makes them easy to take apart through weathering.

Bowen Series: the Bowen Series orders igneous minerals by how soon they condensate out of a magma melt, as it cools. First 'ultra-basic' minerals are formed. These have a high content of heavy elements and are correspondingly low in silica content. Likewise, the last minerals to condensate are 'acidic', having high silica content and low heavy elements. A very rough rule is that the darker or denser the rock type, the more basic it is. Crustal minerals and rocks tend to be siliceous. Erupted lavas tend to be basic, and deep-seated minerals and rocks tend to be ultrabasic. The bowen series is:

Lyotropic replacement series = the relative capacity for cations to replace one another if present in equivalent quantities. The order of preference is: Al+++ > H+ > Ca++ > Mg++ > K+ > Na+. Thus Al has the weakest bond whereas Na the strongest.

Law of mass action: adding large amounts of one cation will replace others, regardless of their relative replacement. Since H+ will replace Ca++, an excess in Ca++ must be added to soil in order to raise the pH (make it less acidic).

Cation Exchange Capacity = a measure of soil, particularly the charged clay particles, to attract, hold and eschange cations Ca++, K+, Mg++, NH4+, H+ and Na+. The more negatively charged sites a clay contains, the more cations it can hold and the higher its CEC. A high CEC increases the soil's buffering capacity (its resistance to changes in pH or changes in nutrient concentrations). A high CEC enhances nutrient retention in soils so that they can hold more. Soil CEC ranges from 0 to over 1 mole/kg. Note that 'organic' soil (the soil biota) has a CEC of nearly two orders of magnitude (100x) larger than soils without soil biota.
(See CEC table above)

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