The Basic Nutrients of The Clay Mineral Group
As the biotic group is made up of the three major non-metals of Nitrogen, Phosphorus and Sulphur; so the basic group is composed of the four major metals or essential cations of Calcium, Magnesium, Potassium and Sodium: The Basic Nutrients.
In the UM-BBD spiral periodic table these four are grouped together as the major cations. Collectively they occupy more than 99% of the basic cation exchange capacity of the soil. A capacity that is controlled by the clay mineralogy.
The Basic Nutrients:
The Four Major Cations; Calcium, Magnesium, Potassium and Sodium
Calcium and magnesium are the two most dominant cations of the basic nutrient group. They are both divalent cations having a valency of two (2+) and have similar ionic radius (0.412nm, 0.428nm). With Calcium occupying between 60-80% of the cation exchange capacity (CEC) and Magnesium 10-15%, these two collectively occupy between 70-95% of the total cation exchange capacity of the soil.
From a practical perspective this charge (+2) relates to the degree of availability of the element in a soil environment. The higher the charge the stronger the element is bound to the soil and the less soluble and available to the plant it becomes. Thus both calcium and magnesium, with a valency of 2+, are more strongly bound than the other two major cations and basic nutrients Potassium and Sodium.
First observed over 3 hundred years ago to improve plant growth, the practice of marling (adding Calcareous clay) became widespread and pre-dates any concept of plant nutrition or soil chemistry by over 100 years. The later advent of the pH measurement enabled the opportunity to observed and measure the influence of Calcium on acidic soils. The conclusion then, and still widely held today, was that lime ‘sweetened the soil’ making it less acidic; and whilst in practice this occurs it is an incorrect conclusion. One that fosters the belief that it is the pH that is being ‘corrected’ rather than the pH being used to determine the Calcium requirements.
Calcium in many respects is not only the dominant basic nutrient but arguably the dominant plant nutrient. An extremely important element in cell structural calcium also coordinates other metabolic activity. However few ever regard Calcium as a nutrient in its own right and as a consequence it has become the forgotten nutrient. As a basic nutrient Calcium is required in large quantities to both satisfy plant requirements and to compensate for its low solubility. Thus for sufficient concentrations to enter solution and satisfy plant needs large quantities are needed within the soil matrix.
Within the plant Magnesium plays a major regulatory role in maintaining pH and anion/cation balance. It is also important in the chlorophyll molecule where it forms the central atom. However in the soil phase Magnesium is a rather shy element and can easily be depressed by excesses of other cations. In particular Calcium can depress Magnesium and result in deficiencies.
Potassium and Sodium
As with Calcium and Magnesium, Potassium and Sodium are very similar elements. Both are univalent cations (K+, Na+) and have very similar ionic radius (0.331nm, 0.351nm). They are found in relatively similar concentrations in the soil with Potassium occupying 1-5% of the cation exchange capacity. Sodium which is a beneficial rather than an essential nutrient can become harmful at high concentrations and so should not occupy more than 15% of the CEC. Both Potassium and Sodium fulfil very similar roles within the plant and are capable of substituting each other (a property that equally applies to animal (human) nutrition).
Within the plant Potassium plays a functional role in regulating metabolic activity and is particularly important in regulating osmotic potential. In this respect Potassium fulfils a very similar function to Magnesium.
Whilst these four elements are the essential cations for life only three are essential to all plants, with Sodium instead being a beneficial nutrient. In many respects Sodium acts as a ‘higher element’ to the function of Potassium. For Sodium can and does get substituted for the functions of Potassium within the plant, a substitution that can be extremely beneficial and resulting in far more efficient growth and functioning of the plant. Equally too much Sodium is detrimental both directly, where it can impair function and indirectly where the osmotic properties can result in arresting the flow or solutes from the soil to the plant, a condition known as physiological drought.
next: The Minor Nutrients
the practice of marling (adding Calcareous clay)
Free Cultural Works (CC-BY-NC-SA) Malcolm McEwen 2011