Chemical Weathering And
The Formation Of Clay Minerals
By Professor Nils Nykvist
Plant nutrients in hard rocks are tied up in various forms of minerals that have a definite chemical composition, as opposed to rocks.
The most important minerals are silicate minerals, which make up to more than 90% of the earth’s crust. Silicate minerals can be considered as aggregates of negative oxygen ions, which are held together by positive ions. The structure’s stability depends mainly on how well the positive ions fit into the space between the oxygen ions.
Among the most common elements, silicon is the one that best fits between the oxygen ions and therefore provides the most stable structure. If the number of silicon ions in magma is insufficient, these are replaced by aluminum ions, which because of their size best fit within the structure after silicon. Various forms of aluminosilicates are therefore included in most silicates.
However, the aluminum ions have only three positive charges, as opposed to the silicon ions, which have four. This would lead to an excess of negative charges in the mineral, unless other positive ions such as calcium, potassium, magnesium, and iron are bound to the mineral. The greater the number of these ions in the mineral, which depends on the magma’s original composition, the more easily weathered it is.
Water, which is a prerequisite for the chemical weathering, is never pure, but contains many different ions and compounds.
Of these, the hydrogen ions are of particular importance, because they are small and constantly produced in the soil by living roots as a substitute for the positive ions that are taken up by the plants. Hydrogen ions penetrate into the mineral structure and replace other positive ions, primarily those with one positive charge, such as sodium and potassium, and thereafter those with higher charges, such as calcium and magnesium.
Silicon and aluminum compounds, which are among the most insoluble elements of the original minerals, combine with each other to form so-called secondary clay minerals that are more stable and in equilibrium with the prevailing soil conditions in terms of water, hydrogen ions, and other ions. These minerals, which are formed in the soil, consist of layers of silicon ions surrounded by four oxygen ions (Si-O layer), and aluminum ions, which are surrounded by oxygen ions and hydroxyl ions (Al-O-OH layer).
Depending on the soil conditions that existed when they were formed, there are several different types of clay minerals. One of their characteristics is that they are mostly negatively charged, a condition that is neutralized by hydrogen ions and other positive ions such as potassium, calcium, and magnesium adsorbed on the surface of the clay mineral. These ions are exchangeable and can be taken up by the plants.
Clay minerals also bind substantial amounts of water, which is the reason for the large cracks that often are formed in clay soils during dry periods. Some clay minerals can bind such large amounts of water that they are unsuitable for cultivation.
Hot climate in combination with high rainfall makes the silicon ions more soluble than the ions of aluminum and iron, and they are therefore in the long run leached out from the soil. Chemical weathering in the rainforest climate will therefore lead to the formation of minerals composed of oxides of aluminum and iron and clay minerals with low levels of silicon (kaolinite).
Hydrogen ions are tightly bound to oxides of aluminum and iron, which leads to decreases of the negative charges of the clay minerals and their capacity to adsorb positive plant nutrients.
On the other hand, these clay minerals with high contents of soluble aluminum provide soils with a very good structure and high infiltration capacity, which in humid areas at risk of excessive water-saturation probably are of greater importance than the capacity of soil to retain plant nutrients.
The availability of nutrients can always be improved by chemical fertilizers, but to improve the soil structure in rainforest climates is more difficult.
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