Aluminium in Acid Soils

It’s well known that the acid soils of the South Island of New Zealand contain high levels of aluminium. It’s also well known that aluminium ions in the soil solution can themselves act as acids – that is, as proton or H⁺ donors.

But what’s not so well known is how aluminium (as Al³⁺) acts as an acid. After all, you might say, it doesn’t have any hydrogen atoms associated with it to act as a proton donor. How does this work?

When aluminium ionizes, its small size and high charge mean that it attaches itself strongly to any negatively charged ligand with which it comes into contact. This includes water (H₂O) molecules. In the water molecule, there is a slight polarisation of negative and positive charges – the negative charge toward the oxygen atom, and the positive charge toward the hydrogen atoms. This means that positively charged ions (like Al³⁺) can attract the oxygen atom of water molecules to them, forming what is known as a hydrated ion.

In the case of aluminium, six water molecules are attracted to each Al³⁺ ion, resulting in a complex Al(H₂O)₆³⁺ ion. This is the form that Al³⁺ exists in under acid soil conditions. And it is this large, complex ion that acts as an acid or proton donor in the soil.

The way this happens can be represented by the following equations:

Al(H₂O)₆³⁺ + H₂O   ----  AlOH(H₂O)₅²⁺ + H⁺

AlOH(H₂O)₅²⁺ + H₂O  -----   Al(OH)₂(H₂O)₄⁺ + H⁺

Al(OH)₂(H₂O)₄⁺ + H₂O   ^-----  Al (OH)₃(H₂0)₃ + H⁺

Al (OH)₃(H₂0)₃ + H₂O ----  Al(OH)₄(H₂O)₂^- + H⁺

This series of protonation reactions occurs when soils containing Al³⁺ are limed. The hydroxyl (OH^-) ions formed by the reaction of lime (CaCO₃) with acidified water drive the above equations to the right. That is, they cause the complex Al(H₂O)₆³⁺ ion to donate protons to the soil solution. In doing so it acts as a weak acid.  Eventually the once soluble Al³⁺ ions form precipitates of aluminium hydroxide. When that happens, its ability to lower soil pH and cause Al toxicity problems in plants is greatly reduced.

Pugging Does Damage

This past weekend I came across the most severe case of animal pugging that I've seen in a long while.

I noticed it while visiting a farmer in the Catlins. He had a hilly winter feed block, most of which he grazed with hoggets. The lower part of it, however, he'd fenced off and used as a feed lot for a herd of cattle. In the process they had churned the heavy clay loam soils into a quagmire.

Looking closely I noticed a remarkable phenomenon. Many of the deep animal footprints were filled with blue-green water, and the sides of the pug marks were in many cases coated with a film of reddish-brown iron oxide. Pressure from cattle hooves had evidently created an anaerobic (oxygen depleted) condition in the soil leading to reduction in soil iron. Iron in its reduced ferrous ion state is soluble (and blue green in colour), and this had leached from the soil into the hoof marks. Once exposed to the air again, however, much of this had converted back to the oxidized or ferric form, producing the rust-coloured coatings on the sides of the hoof marks (see the photo).   

This was one of the more observable effects of pugging on the chemistry of the soil. The physical and biological state of the topsoil was also affected. Soil structure had been destroyed, porosity reduced and most of the biological life killed off. Pugging does damage the soil - there is no question about that. What can be done about it? That's another issue for another time.