Aluminium and Hydrogen are both naturally occurring and important elements within the soil system. Together they are the principal ‘acidic’ cations that are responsible for the vast majority of soil acidity. So whilst there inclusion within this basic section is potentially erroneous their importance to both pH and the effective cation exchange of soils is such that they demand being considered with respect to these properties.
Aluminium, the most abundant element on the Earth’s surface, is at the heart of the majority of clay crystals where it is bound by four oxygen molecules to form a tetrahedral crystal. It is this crystal structure that generates the electro-static charges that hold the soils cations in place but under very acidic conditions the mineral disintegrates and the aluminium ion is released where it adds to and exacerbates the soil acidity.
As a strongly charged cation the Al+3 ion becomes tightly bound to the remaining soil clays; displacing and reducing the availability of exchange sites to the weaker basic cations (Ca, Mg, K). A small amount of Aluminium is natural and beneficial however high levels lead to elevated Al ions in the soil solution which in turn elevate soil acidity causing damaged and stunted root growth. It is not only an extremely detrimental condition but more importantly is irreversible.
Under normal conditions Aluminium has an insignificant influence on the soil or plant and unless at elevated levels is supplied passively and sufficiently by the soil.
Whilst an important component of life, Hydrogen does not function as a nutrient directly within the soil system or within the plant but, as Carbon and Oxygen are supplied by the atmosphere, so Hydrogen is supplied by the water. These three elements bring the total elemental requirement of plants to 22.
Hydrogen whilst not a plant nutrient is an important basic cation that naturally exists within the system. Whilst part of and a consequence of many soil processes at elevated concentrations it is detrimental causing root damage to plants and the disintegration of the clay crystal leading to the release of Aluminium ions. Thus hydrogen functions neither in a nutritional nor a soil process capacity but is none the less an important factor in soil fertility.
Hydrogen concentrations are normally maintained by the action of the basic cations, and providing concentrations have not been allowed to accumulate and no damage to the clay crystals has occurred excess H+ can be removed or displaced by the addition of basic cations, usually Calcium or Magnesium. A practice and solution that has contributed to the confusion over the role of Calcium in plant nutrition and the purpose of the measure of pH.
Whilst elevated levels of Hydrogen ions are rare in natural situations they can and do occur in cultivated situations, particularly where Ammonia has been used as a fertiliser. For more on Ammonia visit
Free Cultural Works (CC-BY-NC-SA) Malcolm McEwen 2011