Toxic elements in the soil

Some important toxic elements are Fluorine and heavy metals like Lead and Cadmium, but the one most people have heard of is Aluminium. In fact, even the essential nutrients can sometimes become toxic elements, but the elements mentioned here have no nutritional value to plants.

Unfortunately, the role of Aluminium in the soil has somehow developed something of an “Urban-myth” status and so requires special attention.

Research has clearly shown a close relationship between plant available Aluminium and soil acidity (measured by pH). It has also shown that Aluminium is present in the soil mainly in one of two forms – either Al2+ or Al3+. The first is available for uptake by plant roots and so can easily become a toxic element, but the second is unavailable and so has no effect on plants. The bit that some people find it convenient to overlook was that Aluminium switches from one form to another depending on the soil pH. In other words, it becomes a toxic element in acid soil – it does not make the soil acidic.

Plant available Aluminium is toxic to plant roots (some plants more than others), but it disappears quickly if someone applies Lime and increases the soil pH.

But wait, wasn’t that a ‘plus’ sign we saw? Surely that would make Aluminium a soil cation wouldn’t it?

Actually, some people use the term "Acid Cation" (as opposed to the other "Base Cations"), but this is very misleading. This doesn’t actually say that Aluminium makes soil acidic, but it sure lets people think so.

The truth about Aluminium

Yes, Aluminium is a toxic element, but tests have shown it plays no part in the cation exchange system and seldom has any impact on soil friability – it is just toxic to plant roots when in its soluble form. Even more importantly, it is a problem that is easily fixed. Here is how you do it:

Step 1: Get a soil test that measures exchangeable cations – Calcium, Magnesium, Sodium, Potassium and Hydrogen. People tend to ‘Poo-Hoo’ the last one, but it is important so we will look at it in a bit more detail later.

It is also important to make sure the test you get actually measures exchangeable Hydrogen (rather than estimating it). Many soils – and Australian soils in particular – tend to be rather unco-operative when it comes to agreeing with an estimate.

Step 2: Work out the amount of Lime you need to get the Calcium (as a percentage of the Adjusted CEC) above 60% and the Hydrogen below 20% (you may also need to use some Dolomite, if both Calcium and Magnesium are low). Having achieved this, you will find plant available Aluminium virtually non-existent in your soil.

Step 3: Plant you crop or pasture and ignore the hype about Aluminium!

The Truth about Exchangeable Hydrogen

There are those who would have you believe that if there were any Hydrogen about, it would probably make your soil up and float away. Not so! Clay and organic matter in the soil carry a static charge and so attract anything with the opposite charge – things like Calcium, Magnesium, Sodium and Potassium (the so-called ‘Base Cations’). However, it is impossible to have a charged particle in the soil and nothing attached to it – to balance out that charge. If there is not enough of Calcium and other base cations to go round, then the ‘left-over’ charges in the soil will attract Hydrogen. This will come from water in the soil, plant roots, excessive applications of chemical fertilizers and the soil organic matter.

Put simply, you cannot work out the cation percentages properly if you do not know exactly how much exchangeable hydrogen is present; and if you can’t do that, then you cannot work out the right amount of Lime to use.

There is, however, one last wrinkle in the story. Organic matter is intrinsically high in exchangeable Hydrogen and will break down more rapidly if too much of this Hydrogen is replaced with Calcium (or other base cations).

To account for this, a simple adjustment was developed to reduce the amount of exchangeable Hydrogen found in a test, according to the Organic Matter percentage. This is called the “Adjusted Hydrogen” and when combined with the base cation results, gives a figure for the "Adjusted CEC" and produces more reliable calculations of Lime requirement than any other method.

In soils with low organic matter percentages, this makes little difference, but as the organic matter percentage increases, the amount of Lime you seem to need (without applying this adjustment) would quickly becomes excessive.

The Bottom Line

Plant available (soluble) Aluminium can be present as a toxic element in soil because of acid conditions in the soil – it is not the cause of the problem. With a good soil test and the proper application of Lime (& perhaps some Dolomite), it will simply switch back to its insoluble form and the problem will disappear.