Soil Mineral Element Reserves
Within the soil matrix the soil mineral elements that plants need for growth exist as reserves or “pools” . These pools hold the vast majority of the soil mineral reserve of the soil and can be considerable in size. However the reserves are in forms that are not accessible to the plant and must be converted by chemical or biological processes into soluble forms which the plant can then access.
Thus within the soil matrix exists the majority of the soil mineral elements that a plant needs for growth. However these elements are maintained in pools which whilst often vast are similarly inaccessible to the plant. The soil mineral reserve must therefore be transformed by chemical and biological action into the soluble forms, the nutrients, that a plant can access.
The process is ongoing so that as soluble nutrients are removed by plants and bacteria, or leached by precipitation into ground and surface waters, fresh nutrients are steadily liberate back into solution.
Whilst different soil minerals exist in different pools in varying quantities and with different dissolution rates, the system is to some degree balanced and self regulating; with an equilibrium between the reserves and the soil solution being maintained.
Anions and Cations
A common distinction between the soil mineral elements and the ‘pools’ which maintain them in the soil is derived from the ionic charge: whether the soil mineral element is an Anion (-) and negatively charged or a Cation and positive charged (+), and the degree of strength of that charge.
Anions Within the nutrient suite Nitrogen is the singularly most significant Anion. However with most soils having little capacity to retain anions, Nitrogen, in it’s soluble forms of Nitrate and Nitrite are easily lost. Hence the availability of Nitrogen to the plant is often or appears lacking. However whilst the soil lacks the ability to retain anions and in particular Nitrogen, the soils organic matter reserves and the bacterial communities that maintain them do not.
Thus it is within the organic matter and the resident bacterial populations of the soil that the principle reserves of Nitrogen are maintained. Transformation and release of this reserve as Nitrate and Nitrite is then achieved through the action of the microbial biomass: it is therefore a biological rather than a chemical process and so covered in more detail in the next chapter: The soils biological properties.
Cations: Soils do have a cation capacity, one that is a consequence of the properties of the clay mineral. Clays, of which there are six main types, possess a charge as a consequence of an imperfection in the mineral.
Composed of four Oxygen ions surrounding an Aluminium or Silica ion the clay mineral is tetrahedral in shape. However the (-) charge on the oxygen ions is not balanced by the (+) charge on the central Aluminium or Silica ion and so the mineral has a slight negative charge.
These clay minerals form interlocking latices and sheets, the arrangement of which is what largely differentiates the mineral types and further the degree of charge ‘strength’ or Cation Exchange Capacity.
The ‘edges’ of these sheets are ‘broken’ and along these broken edges the excess negative charge of the clay mineral accumulates sufficiently to attract positively charged Cations. These ions are not chemically bound but attracted to the negatively charged sites on clays.
The process is dynamic, with ions constantly jumping in and out of solution as they jostle with other ions at the exchange sites. Akin to a game of musical chairs the soil solution and the soil mineral reserve are maintained in balance by a constant state of flux.
Whilst the charge on each individual clay mineral is minute and negligible the the mineral is similarly very small and extremely numerous; so what it lacks in individual strength it makes up for in collective. In clay rich soils the total number of broken edges can be quite considerable and as a consequence such soils can hold a similarly considerable reserve of cation nutrients, one that is known as their cation exchange capacity
Thus it is this clay mineralogy that is responsible for maintaining and supplying the soil solution with the principal cations and basic nutrients of Calcium , Magnesium and Potassium. In addition the mineralogy retains the acidic cations, Aluminium (Al3+) and Hydrogen (H+) and it is the ration between the acid and basic cations rather than the quantities of either or both that is responsible for maintaining the soil pH.
The Minor Nutrients (Fe, Mn, Cu, Zn, B)
So whilst in a strict sense the soils chemical properties include the fate and transformation of Nitrogen, Phosphorus and Sulphur, these elements are regulated more by the biological condition of the soil than the chemical properties. The same is not though true for the minor nutrients, all of which, with the exception of Boron, are regulated by the clay mineralogy.
The Minor Nutrients are, despite their essentialness, required in relatively insignificant amounts by the plant. They similarly occupy an insignificant number of cation exchange sites within the clay minerology and so whilst technically part of the CEC are, from a practical perspective considered separately. A case could be made for placing all the remaining 12 soil mineral elements required by plants in this final group. However they are not all cations, they are not all used in a regulatory capacity and more importantly they are not all prone to limitation.
Thus the minor nutrients contains the four alkaline metals and the transitional element Boron. These being the five most likely elements to be found limiting in soil systems and ones best corrected by direct chemical additions.
The Meaning and Value of Soil pH
As it is the ratio between the basic (Ca+2, Mg+2, K+) and the acidic (Al+3 and H+) cations that determines soil pH, rather than the reserve of the basic nutrients themselves the measure of soil pH is not indicative of the soils calcium reserve or requirements. Thus on its own pH tells us little.
To make good use of pH one must know more about the soil, ideally both the effective cation exchange capacity and the Percentage base saturation, however these are relatively complex operations that need a laboratory environment. In the absence of this environment the measurement of pH is still though a very useful tool in helping to understand and calculate Calcium needs and buffer capacity, as well as in identifying or qualifying other soil properties.
Cation Exchange Capacity (CEC) and Percentage Base Saturation (PBS)
The measure of the cation exchange capacity (CEC) and the percentage base saturation (PBS) are moderately complex processes which at this point in our model are premature to consider.That said an understanding of these concepts, if no means to measure them, is still useful and so they are covered in more detail in the 3-4-5 Dynamic Nutrient Model pages: CEC & PBS
Free Cultural Works (CC BY-NC-SA) : Malcolm McEwen (2011)