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.

Nutritional Concerns

A  second thing that stimulated my reentry into active involvement with soils and agriculture was the claim of declining nutritional values in our foodstuffs.

I know that such claims can be readily dismissed as cheap scare-mongering.  However, the facts seem to be there, staring us in the face if only we are prepared to look for them. There are apparently thousands of articles in medically related journals linking current disease incidence to nutritional causes. Dr. Arden Andersen, a medical doctor and agricultural consultant, cites figures from the FAO and British health organisations showing that average nutrient levels in many of our fruits and vegetables have declined by as much as 40% in the last 50 years.

Related to this is the invasion of "diseases of the elderly" on increasingly younger people. "The generation being born today," writes Dr. Andersen, "for the first time in American history, is predicted to have a shorter live expectancy than their parents." (http://www.biognz.com/articles/?a=5) He goes on to say, "The current generation's problem is not genetic. It is the lack of nutrition. Nutrition is the building block of every body, organ, tissue and cell. It is the foundation of the immune and repair system. It is the essence of our physical life."

If that's the case, and if the burgeoning occurrence of heart disease, stroke, Parkinson's, Alzheimer's diabetes, obesity and cancer are all related to the delcining nutritional value of our foodstuffs, then it makes sense to be concerned about our soils, the primary source of these foods and cause of their nutritional defectiveness.

World-wide Soil Degradation

One of the things that kick-started my reentry into active involvement in soil science was discovering the extent of the soil degradation problem world-wide.

This was brought home to me reading a National Geographic article (September 2008 issue) a year or so ago. The comment was made that as far back as 1991 it was reported that 750,000,000 (that's 750 million) square miles of soils on the earth's surface had been seriously degraded by such things as desertification, salinization, deforestation, nutrient depletion, and chemical pollution. That's apparently an area the combined size of the USA and Canada!!!

Wow! I don't think anyone can read something like that and remain indifferent. Nor is this a once-off guesstimate, an alarmist bit of reporting. A recent scientific paper put the extent of the global soil degradation problem at 8 billion hectares. I haven't done the conversion mathematics, but 8 billion sounds a terrifying number.

What makes this all the more serious is that the demand for food production keeps increasing all the time. It's estimated that by the year 2030 the world's population will top 8.3 billion people. To feed this number of people we are somehow going to have to produce at least 30% more grain than we do at present. Yet, with the area of usable arable land in a constant decline through misuse in one form or another, that task looms more and more difficult. Not impossible, but no doubt more difficult.

I don't write this as a "prophet of doom and gloom" as one well-meaning local community leader called me when I shared these statistics at a Lion's Club meeting. I'm simply mentioning well documented information that I'd previously been unaware of. It certainly jolted me into a fresh concern for soil use and a sustainable approach to agriculture. 

Kicking Off

When I left teaching soil science in 1980 to become a church pastor I thought I that would probably be the end of talking to people about podzols and cation exchange capacity and the like.

But how mistaken I was. Sure enough, for nearly thirty years my love of the soil was pretty much kept under wraps. I enjoyed dabbling in my vegetable garden, but beyond that seldom thought much about the world of soil science that had absorbed me for 12 years as a student and then researcher and teacher at Lincoln College (now Lincoln University).

All that changed a little over two years ago when I moved to Wyndham, a rural town in the south of New Zealand, to pastor the Wyndham Evangelical Church. I found myself again in a farming community, and it wasn't long before farmers started asking questions about their soils. I was quick to point out (a) the theoretical character of my background in the subject, and (b) the long time I had been away from the world of soils. That didn't deter them - they persisted in wanting me to help them. Terms like CEC and base saturation % on their soil test results befuddled them, and at least I could help them with that.

Before long I found the opportunities expanding. My research and teaching work at Lincoln had given me a good grasp of almost every aspect of soils, and with a bit of freshening up, I was soon at home helping farmers discover what soils they had on their properties and their natural strengths and weaknesses. Seeing the interest I was bold enough to advertise a couple of seminars - a one-off event featuring my farm coach/consultant brother Peter on Sustainable Agriculture, and a series of three talks I gave on "Knowing Your Soils."

The attendance at these events was a bit overwhelming. About 50 attended the Sustainability seminar, and more than that the Know Your Soils talks. Clearly there is a hunger among farmers to learn more about their soils - and especially from the angle that both my brother Pete and I come from.

We are both interested in soil health and sustainability, not just in soil productivity. Soils are living bodies with distinct characteristics and active processes. Using them well depends on knowing those characteristics and processes and working with them. Most of the time the information involved is not that complicated. Where it is, it can still be made pretty clear for anyone.

And that's what I'm keen to do - help farmers and other soil users understand the soils they are working with. The key to sustainable soil use, in my view, is to empower soil users themselves through education and information so they can treat their soils well. There are others who are trying to do this - probably far better than I can. But I'm aware that I've been given a talent (immersion in the world of soil science for more than 10 years) and a passion (a love for the soil and concern to see it wisely used and protected) that need to be expressed. Creating this blog site is a way I can do this.

Most of what I write will be gleaned from what I've learned in the past and read now. I'm happy to have people ask questions and raise issues. If I can help I will; if I can't, I'll be quick to let you know.

Join with me in the adventure of learning more about the wonderful soil that we depend so much upon.

Andrew Young (B.Ag.Sci.(Hons), PhD)