Salinity and Sodicity -- How They Affect Trees
Professor, Soil Science, Faculty of Agriculture
E-mail: [email protected]
Organization:University of WA
70 Archdeacon Street
Nedlands WA 6009
Phone: +61 8-93802504
Many people are familiar with the meaning of salinity, but sodicity is another important factor to be considered in the rehabilitation of land.
Many people have talked about the magnitude of the problem, from a macroscopic sense, and from an economic sense. And the lead into my talk was given by Barrie Oldfield when he talked about the fact that the soil at the very surface can become a slurry. In other words, there is a problem of managing the surface soil during the process of reclamation and removing salt. What I want to talk about is the nexus between the salinity, or the salt levels, and the sodicity. I will explain each of those terms; you are probably very familiar with salinity, but perhaps not so familiar with sodicity.
The US Department of Agriculture, back in 1954, published a handbook. There is a tremendous amount of information in that handbook about all sorts of things relating to sodicity, that is, the sodium levels in the soil, and salinity. One of the diagrams they published is this one here in which you can see the concentration of electrolytes in the water expressed in milliequivalents per litre. These are in salt solutions, actually. The conductivity in millimhos per centimetre. These days, because of the changes in units from cgs to si units, we talk about decisiemens per metre. What I want you to note about this particular diagram, it has the range of salts that you find in soil solutions, sodium chloride, calcium chloride, magnesium chloride, etc. If you look at this log-log plot of concentration of the salt, whether it be sodium chloride, calcium chloride, concentration of the salt in milliequivalents per litre against decisiemens per litre. You will notice that one on this scale, the concentration is 10. At 10 on this scale, the concentration is about 100. In other words, it is approximate, but, nevertheless, a very good approximation between the conductivity of a solution and the electrolyte concentration. One can then take a soil, mix it into a slurry, extract some of the water from that soil, measure its conductivity, and you immediately have a fairly good measure of the amount of salt in the pores of that soil. The environment, so to speak, chemically, with respect to salinity, with respect to the range of nutrient demands, in the pores of the soil.
In this same handbook, they have a list of tolerances of cereal crops, forage crops, tree crops, vegetable crops, in which some have a high degree of tolerance to high levels of salt. For instance, the yield of barley is decreased by 50% at an electrolyte concentration of 160 milliequivalents per litre. To put this into focus, the concentration of salt in sea water is about 500 milliequivalents per litre. And we know that plants grow there--sea grasses, sea weeds, etc. There is a whole spectrum of things. The next slide shows just one aspect of the way in which salinity can affect plants. This shows you the conductivity of the saturated extract. You can convert this to milliequivalents per litre. You will see that germination of barley is not killed until you get up to very high concentrations. Whereas, at relatively low concentrations, the germination of bean plants is greatly restricted due to the osmotic effects of the salt, how it interacts with those particular seeds.
I want to now define sodicity. Within the soil, there are clay minerals, clay particles, and the only thing we need worry about clay particles is that they are negatively charged particles. They are crystalline, negatively charged particles. In order to balance the charge on the negatively charged particles, you have ions. In the main, these are calcium ions. When they are predominantly calcium ions, the soil structure, generally speaking, in this respect, is in a satisfactory condition. But there are certain circumstances when you have sodium ions there, and as the percentage of sodium ions balancing that charge increases, then the soil structure is likely to become more and more unfavourable, and particularly so when there is a low electrolyte level. You take a saline soil, you are reclaiming it. You leach the salt out, the sodium ions are still on the surface at low levels of salt or electrolyte in that residual water, the structure becomes very unstable. As Barrie Oldfield said, you have a slurry there. So, the nexus between sodicity and salinity is a very important one.
The ratio E of the amount of sodium balancing the charge on the clay particles to the amount of calcium balancing the charge on the clay particles, is given by a constant, which we needn't worry about. But in the solution you have a soil solution, the saturated extract in the soil pores, the water in the soil pores, you have so much sodium and so much calcium. What determines the ratio of sodium to calcium on those crystalline clay particles in the soil is the amount of sodium in the solution over the square root of the calcium in the solution. This has three consequences. If, for instance, we decided we wanted to keep that constant and we wanted to dilute the solution by a factor of two. We halve the sodium concentration; in order to keep the left-hand side constant, we would have to take a quarter of the calcium ion content out. Because the square root of the quarter is a half. A half squared is one on four. In other words, we would halve that and quarter that. There is a consequence of this in two respects with respect to salinity. It isn't often realised, as far as I can see, and the consequence is, if we simply dilute the soil solution, if we halve the sodium, and we only halve the calcium, instead of taking a quarter, then we have changed the balance such that it favours the calcium. If we are to maintain the sodium-calcium ratio the same, we would have to divide by a half there and a quarter there. If we only divide by a half here, we favour the calcium ion concentration. In other words, as we dilute, we tend to get more calcium on the surface, or if you like, we lower this ratio. Which is a favourable situation. The lower the sodium is, the better.
The interesting thing is, when you are dealing with a saline soil, when you go up the hill, so to speak, if you start at this point and you double the sodium ion concentration, you should quadruple the calcium ion concentration. Two and the square root of four is two, so you still have two over two. If, however, you say water moves to the surface, it evaporates, leaves the salt behind, as a result you will have a more concentrated solution. You double the sodium and you double the calcium, but if you want to keep this ratio the same, you should multiply the calcium by four times. But you are only multiplying it by two, which means as your solution becomes more concentrated, you are pushing this ratio of sodium to calcium up on the surfaces, the surfaces interact more strongly, they tend to swell more, and the situation becomes as Barrie Oldfield pointed out, a slurry condition, a very unsatisfactory circumstance.
If you are then talking about the reclamation of soils which are sodic, you have to allow for the fact that the soils which are saline, you have to allow for the fact that they will also have a lot of sodium there. If you want to keep that structure re the soil reasonably permeable and the structure stable, you have to introduce a certain amount of finesse in that situation.
I'm going to refer to an experiment which I did many years ago, but it is still pretty viable today. People still talk about it and quote it. I took a small volume of soil and I measured the flow of a solution through that soil, which is generally referred to as the permeability. On this axis, we have the permeability of the soil, the rate at which water flows through the soil, and on this axis we have the electrolyte concentration, which I was able to adjust in my experiments. If we look at the curve here, which says that 21% of the charge on the clay particles is balanced by sodium ions. You will immediately notice that if we have an electrolyte concentration up at this level, the permeability is high. As we dilute, and using that Gabon? equation we dilute so that the value of 21 remains constant, the soil becomes less and less permeable. In other words, the structure is starting to be destroyed. Permeability is a measure of the structural stability of the soil. If we look at this in a more general way, you will notice that when you have only 5% or 6% of the surface sites occupied by sodium ions, you only require about four of these units, milliequivalents per litre, if you like. Four of these units maintain the soil permeable. As we increase the sodium to 9, we require about 8 of the units to keep the soil permeable. As we go to 21, we require about 12 of the units to keep the soil permeable. In other words, the higher the amount of sodium balancing the charge, the higher the amount of electrolyte flowing through that soil, which is required to keep it permeable. If you go to 35, you finish up over here still higher. There is a connection between the degree of sodicity, nexus is the word I use, the amount of sodium balancing the charge on the clay particles and the structural stability or permeability of the soil.
I want to go back to this ESP21 curve, or Exchangeable Sodium Percentage, it is called, and draw your attention to two points. If we take point A, you will see that the permeability is starting to decrease. It has declined by about 15%. There is a boundary between satisfactory permeability at this top level, and as you go further down the hill, you are going to destroy your soil structure still further. If we take point A as a sort of a guideline, by the time we get down to point B, when we get that dilute, remember we have the soil sitting in a container with water flowing through, at this point, dispersed clay starts to flood out. In other words, not only is the structure declining, it is really disintegrating and becoming a tremendous mess.
You can take this one step further, if you come onto this sort of curve, we can plot ESP and sodium absorption ratio, they are very similar numbers, the differences are quite subtle so you needn't worry about that. That is the amount of sodium balancing the charge. Here is the electrolyte concentration. You will see that when we have 21% sodium, we come across here and it meets this line at about 12. So we would plot a point there. What this line does is divide the soil on this side of the line...if our electrolyte concentration flowing through the soil is on this side of this line, the permeability is stable, we are maintaining our structure. This line defines the amount of salt electrolyte in your percolating solution needed to maintain the structure. If, however, you get onto the other side of the line, and we take, say, 21% here, and we have only this amount of electrolyte, we are in a region of decreasing permeability which is a reflection of structural instability.
I once worked with a man who was the head of a big institute. He said, "I don't mind how far you people have your heads in the clouds as long as you have one foot on the ground." That has always stuck with me. So we are now getting our feet down on the ground. What a man named Davidson and I did, having great difficulty establishing pastures in the Riverina District of NSW, we got together and decided to see how the principle works in the field of maintaining permeability by adjusting electrolyte level. We took the Murrumbidgee irrigation water, which has 1 milliequivalent per litre in the water. We enhanced that by adding 10 milliequivalents per litre. That gave us a concentration equal to A on that earlier diagram. We flowed that solution through that soil. We knew the amount of sodium was about 20%, so it fit fairly closely with the 21%. We irrigated the soil at 4:00 in the afternoon with 3 inches of water with a concentration high enough to stop the 20% sodium having any effect. This was taken at 10:00 the next morning. You can notice the friable nature of the soil. No water left, the 3 inches has completely cleared, and you can say that the principle of maintaining the high electrolyte level was proved in that experiment. Furthermore, if we go to the point B, we irrigated with the water at about half the concentration of B, on this scale would be about 2. We irrigated with the water itself which had about 1 unit of electrolyte in it...that was irrigated at 3:00 and also taken at 10:00 the next morning...and you will see, not only is the water all standing there, but the clay is obviously dispersed. You have this white clay in the water. Quite clearly an unfavourable condition. As that soil dried over time, you got a large surface crust through which plants couldn't emerge.
How does this sort of thing fit the pattern of reclaiming the surface of a saline soil by removing the salt? We can go back to an experiment which was done in Arizona in the 1950s. The people who did the experiment didn't quite understand what was going on; it predated some of the things I am talking about. What they did was irrigate with Colorado River water, which became unavailable because there is a shortage of flow in the river, and they pumped underground water up for their irrigation of trees. They found this was very satisfactory. It had a high level of salt and a high level of sodium in the water, so there was more sodium on the soil colloids or clay. But the level of salt was fine to keep the soil permeable. It was to the right of that line. When the river water became available, they built up the sodium over about three seasons, the river water was quite low in electrolyte, relatively low. When they irrigated with this water, they said the soil froze up. In other words, the same description that Barrie Oldfield gave.
They could have avoided all this. Had the knowledge been there, they could have measured the amount of sodium using the sort of diagram I referred to here. If they wanted to stay to the right of this line, what they had to do was to keep the electrolyte concentration up; as the sodium became less, they could go down. If they had mixed river water and well water, three-quarters to one quarter, to start with, and then gradually decreased that, they could have avoided this unsatisfactory condition by always maintaining their water in relation to the sodium to the right of this line. That is an example that does work in practice.
In this situation, the problems with respect to reclamation of saline soils are quite massive. You wouldn't want to have the vast strategies that David Williamson so professionally and eloquently presented. You wouldn't want to have those things starting to come unstuck because there was a failure to manage the surface soil structure in these areas being claimed. This is why, when I started out to speak, I said what I am giving you is, in effect, a codicil to the big picture, or the big legal document, if you like, that David and others speak about.
I would like to touch on a couple of political issues. If you look back to the 1950s and earlier, and look to what CSIRO did up till about 1960, its research was almost exclusively agricultural research. To do with agricultural production. It was said after the war, we were all riding on the sheep's back. Agriculture has put in a fantastic amount to the development of Australia. Politically, we ought not to forget that. There is a tendency, because the environmental push starting in the '70s--no criticism of that, it is perfectly proper, perhaps it should have started earlier...there is a tendency to say the farmers are to blame for this and they should foot the bill. I take a different view to that. I take the view that we all benefited, generations before us, and the farmers had some pretty tough conditions, of course. Think of the erosion in the '30s and that sort of thing. Since the whole country benefited, the reclamation should be assigned to the whole country as well, and not just the finger pointed at the farmers. The reason why agricultural production was the predominant goal in policy was simply because the Commonwealth government, the state governments, decided that the primary thing was to produce food and sell it within the empire. We have reached the stage where we have to look at this within the historical perspective of Australia.
I want to say just one other thing. I was at a meeting about 12 months ago. The Murray-Darling Commission has been going for a long while. There was a man, very able and impressive in his way, who was saying that the state of the Murray-Darling is the fault of the soil scientists. They haven't provided us with the basic information. He made the comment, he said, "We want this measured, that measurement, some other measurement, and yet another measurement." It was early in the meeting, and I didn't want to start the ball rolling. I didn't ask the question, "Well, why haven't you got them already?" The next question was, "What are you going to do with them when you have got them?" The difficulty is, the state of soil science in Australia, was very advanced, very sophisticated, by the 1960s. What has been missing, of course, and it is being attended to with great vigour now, was the connection between that research and the broad acre situation, landscape, watershed, and so on. There is some really fine work going on in that area in Australia. We have to see it within the context of history, we have to see it within the context of our scientific history, particularly in relation to land management. And we have to see it in terms of the interaction between land and water, and at the centre of all this interaction between land and water is managing the soil structure.