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Understanding Soils and Nutrients

Part 1 - pH

Jim Barrow

I have chosen a brave heading.....It commits me to more than can be delivered in a single issue. I hope the editor will split the ramblings over a few issues (my pleasure....ed). What I hope to do is to discuss some of the things which decide how well a plant can grow in soil and what determines this.

So where do we start? Obviously with the most widely known property of soils - the pH. Everybody talks about it - from gardeners to garden advisers - so surely everyone understands the idea?

I wonder. You see, the problem is that the idea of pH of soil is basically a nonsense. This is because pH is a property of liquids. Strictly speaking, you just can't have a pH of a soil. It isn't, after all, a liquid.

So what do we mean by pH of a soil? We mean the pH of some liquid that is mixed with soil. When people first tackled this problem, they thought that the answer was pretty obvious. You just mixed some water with soil, stuck in indicator - later, an electrode - took a reading, and there it was.

Water Diagram

The first problem is that the answer you got depended on how much water you added. Then the next problem was that the answer varied by quite a bit from time to time. Another puzzling thing was that if you added a bit of a salt to the water you got a different answer again - usually a lower pH. That is really puzzling. If you add a salt to an ordinary solution, the pH shouldn't change. (Note I have written "a salt" - this includes not only common salt, sodium chloride, but other salts such as potassium nitrate or calcium chloride)

So what is going on?

The trick is that the particles that make up soil have an unusual property - they have an electric charge. Not the sort of charge that will light up a globe, a bit more like the charge you get when you rub a comb on a cloth. How it arises, we may discuss later, but take it for now that it has a charge. The other half of this problem is that salts in water break up to form another kind of charge. These are called "ions". What we measure when we measure pH is indeed hydrogen ions. So you have charge on the soil particles and charge on the ions in solution. These affect each other. If you change the salts in solution these interact with the soil particles and you get a change in the hydrogen ions and so a change in pH.

What does this mean to the measurement of soil pH? The answer you get for pH depends on the salt in the water. You can't avoid this by using salt-free water to measure pH because there is always some salt (using "salt" again in the broad meaning to indicate more than sodium chloride) in the soil itself.

So if you use a little bit of water, you get a rather more concentrated solution in which to measure pH. If you use a lot of water, you get a more dilute solution - and a different value for pH. And the salt content of the soil can vary quite a lot over a month or two, so you get a different value for pH even though the soil hasn't really changed.

What has happened is that scientists have come down to a couple of standard ways of measuring pH. One involves mixing the soil with a specified amount of water. This has been a widely used method for a long time and seems to be the method against which methods which use indicators have been standardised. The other widely used method uses a solution of calcium chloride. This is probably the most widely used method today because it is much more stable - the value doesn't change unless the soil has really changed.

"That would be all very well if both methods gave the same answers, but they don't."

That would be all very well if both methods gave the same answers, but they don't. Mostly the calcium chloride method gives a lower value - about 1 unit lower. But you can't even be general about this because it depends on the kind of charge on the soil.

So if someone says, "The pH of my soil is 5", the scientist has a dilemma. Do they mean a water pH - if so, that is pretty low, the sort of value at which you would find problems with a lot of plants in some soils. Or do they mean a calcium chloride value - in this case, that is not so low. Indeed not a bad pH for lots of plants. So next time someone tells you a pH value, try and look a bit quizzical - and perhaps go one up by asking what sort of pH they mean.

Why is pH important?

In many cases it isn't.

Many plants, indeed most plants, grow over a wide range of pH values. It is only important for some plants and then only at the extremes - when pH is very high or very low.

Oddly the reasons why the extremes are bad are related. Both are concerned with the availability of other things. In the case of low pH it is usually too much aluminium but sometimes too much manganese. For high pH it is usually not enough iron. For all three, the solubility decreases as you raise the pH and increases as you lower it. So you get toxicity at low pH, deficiency at high pH.

pH Scale

It would be nice if there was a simple relation between pH and the supply of aluminium and manganese so all you would need to do is to measure the pH (using a standard method!) and you would instantly know if there was a problem. However, how much aluminium or manganese you get at a particular pH depends on the kind of soil.

In many sandy Western Australian soils, a pH of say 4 (in calcium chloride - about 5 in water) isn't too bad. A higher pH would be a better but you can grow things at this pH. There just isn't all that much aluminium in these soils. In "better" soils, this would be a disastrous value. There would be too much aluminium and perhaps also too much manganese for most plants. Some plants are of course specialized for these kinds of soils: they find their niche there where they have the advantage that they have it to themselves because other plants can't grow.

If your soil is indeed too acid - or if you want to grow plants that don't like acid soils - you can always add lime to raise the pH. Lime doesn't dissolve very quickly so you need it to be finely ground and you need to mix it into your soil so there is good contact. But how much do you need? It doesn't take as much lime to raise the pH of a sandy soil as for a clay soil. And if you add too much, you could be into the opposite problem of too high a pH - which we will discuss next.

Soil scientists have derived methods for figuring how much lime is needed. The simplest of these is to add increasing amounts of lime to samples of soil, moisten them, seal them and then put it in a laboratory oven at say 60C. The trick is that this speeds up the reaction and you can compress the couple of months it would otherwise take into a day. All you need to do is measure the pH and work out how much lime gives the value you want.

What about high pH? Many soils are naturally alkaline. In Perth, for example, most of the coastal suburbs are on sand dunes which contain free lime coming from the shells originally present in the sand. On the youngest soils, there may still be lime in the surface soil; on older soils it may only be present in the sub-soil but the top soil will still be neutral to alkaline.

If you know your soil has lime, you don't really need to measure the pH.....it must be a bit above 8 if you use the water method or a bit above 7 if you use the calcium chloride method. These are the normal values for such soils and are fixed by the chemistry of lime. (You can get higher values in soils that are affected by sodium and these are usually nasty soils indeed with real problems. Here we will confine ourselves to the simpler problem of limey soils).

You can recognise free lime in soils by diluting some pool acid about 1 to 10 and carefully pouring it on some soil in a glass. If it fizzes, lime is present. In Perth, alkaline soils can be recognised by keen gardeners by a simple test: are there tuart trees about (Eucalyptus gomphocephala)? These trees mostly grew on soils with some lime either in the top soil or at depth. Sometimes, however, you may find another limey patch in your garden - where the bricklayers dumped some mortar or similar building material.

"What is wrong with high pH? For many plants, nothing. For others, the main problem is getting enough iron."

What is wrong with high pH? For many plants, nothing. For others, the main problem is getting enough iron. All soils, even sandy ones, contain quite a bit of iron. It is mostly there as oxides and its presence is one of the main reasons for the red and brown hues in many soils. And this is a clue.

If iron persists in soil, it must be pretty insoluble - otherwise it would have been leached out as the soil developed. Indeed it is so insoluble that plants have problems trying to get hold of it.

They have evolved differing strategies. Some acidify the region just behind the growing root tip and get their iron from this region. Remember just a small increase in acidity has a large effect on iron solubility. Others excrete special compounds which act as a sort of carrier system - these compounds go out from the root, grab some iron and bring it back.

Plants that are adapted to alkaline soils have these mechanisms well developed and so have no trouble; others have problems and you see a characteristic yellowing of the leaves due to the shortage of iron. The gardener has two choices. The obvious one is to only grow plants that like the limey soil. But gardeners seldom follow such a logical path. They prefer a challenge. If you must grow plants that are not good at getting iron, you have to make it easy for them by supplying it.

An effective way is to use iron chelate. The iron is in a form not unlike that which has reacted with the compounds excreted by the roots of some plants. Iron chelate is effective though fairly expensive to use on a broad scale and its effect doesn't always last so it has to be re-applied. You can also use iron sulphate. When this hits the soil it soon changes into a mixture of iron oxide and sulphuric acid and the local acidity keeps the iron available for a while - but again re-application is needed.

One thing that is seldom practical is to acidify a limey soil. You can drop the pH of a non-limey soil by adding sulphur. There are microbes which make a living out of oxidising sulphur to sulphuric acid - would you believe. Of course, you have to add a heavy dose, and the sulphur has to be finely divided so the microbes have a lot of surface area to work on. But even if you have as little as a couple of percent of lime, it is simply impractical to add enough sulphur to drop the pH of all the soil. You would have to add enough to dissolve all of the lime. While there is free lime there it will hold the pH at the standard value for limey soils - although of course, there may be pockets of soil which are acidified and the plants can get their iron from these parts.

Poor Availability of Iron
Symptom: Yellowing (chlorosis) of foliage
Possible Causes: High pH or high phosphorus concentration
Treatment: Add a soluble form of iron (eg. Iron Chelate)

Iron availability is the main problem in limey soils - but it is not the only one. Other trace elements can also be difficult for plants to get. Several of the trace elements are "heavy metals". For us, this just means that they have similar chemistry and are similarly affected by high soil pH. The elements are manganese, zinc and copper. All can be troublesome on alkaline soils and they are listed with the most often troublesome first.

There is an added problem with some native plants. Sometimes they can go quite yellow even on soils that are not particularly alkaline. This can be caused by too much phosphorus in the soil. The phosphate somehow interferes with iron uptake or use. This has been investigated by Kevin Handreck in South Australia (see "Australian Plants online" December 1997). Unfortunately, the only rule about the kinds of plants that are affected by this problem is that there are no rules. For example, members of the same genera could be found in both the most sensitive and least sensitive groups. You might think that Proteaceae (eg Banksia, Hakea, Grevillea, etc) would consistently be in the most sensitive group - but again not so. If you have this problem don't apply P fertilisers - but do apply iron. And apply some nitrogen, preferably as ammonium sulphate, to get the plant going again.

Jim Barrow is a member of the Wildflower Society of Western Australia and is a "semi-retired" CSIRO soil scientist.

This series of articles was first published in the Newsletter of the Wildflower Society of Western Australia during 1998-1999.

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Australian Plants online - March 1999
The Society for Growing Australian Plants