Water management options for urban and rural Australia
Water as a limiting resource in dryland agriculture
Dr John Passioura FAA
Honorary Research Fellow
CSIRO Plant Industry
John Passioura is widely experienced in the environmental physiology of plants, especially in relation to the productive use of water by crops growing in dry environments. He led the Crop Adaptation Program in CSIRO Plant Industry for many years, and later spent several years as a consultant with the Grains R&D Corporation during which he oversaw their subprogram on Soil and Water Management which dealt with processes in soil and plants that affect productivity of farms and the sustainability of farming systems. He is currently an Honorary Research Fellow with CSIRO Plant Industry, where he is continuing to work on improving the performance of dryland crops and the soil in which they grow.
[SLIDE: Water as a limiting resource in dryland agriculture]
In fact, we have known it ever since the first settlement in Sydney. The First Fleet arrived at Botany Bay, rejected Captain Cook's suggestion that they settle there and moved to Port Jackson, where they settled in Sydney Cove because of the pleasant stream ‑ which they called the 'Tank Stream' ‑ coming through it.
They weren't aware that they were about to enter what Richard Grove, an environmental historian at the Australian National University, has cogently argued was the greatest El Nino event of the last millennium.
[SLIDE: Sydney Cove, March 1792]
Here is a picture painted in 1792 of the settlement with trees growing in the dry bed of the Tank Stream, which they had thought they could rely on for water.
The Tank Stream stopped flowing for three years. It started to rain again and the Tank Stream never dried up again.
So if you think we have been doing it tough for the past few years, spare a thought for our first settlement.
Australians generally, and even people in the business, such as myself and colleagues, thought for a long time that our crops were essentially limited by water in dryland circumstances. Often they are, but often they are not.
[SLIDE: Wheat yields in Wagga Wagga Shire 1949‑1983]
Here is a graph compiled by my colleague John Angus, which shows wheat yields in Wagga Wagga Shire for 35 years just after the nasty 1940s drought. A dispassionate eye would find it hard to discern that there was a relationship between the yield and the water supply during that time.
At about the same time, at least at the end of that time, there were a couple of agronomists in South Australia who started examining their research results in several dozen experimental plots and came up with a very remarkable graph that has been extremely influential, both on the farming community and on research scientists.
[SLIDE: Summary of yields in field experiments]
This is the graph. It showed that the cloud of points here represents the results of their field experiments over several years and at many sites. You see that they form a cloud. But it is a very odd cloud because it has a very distinct upper boundary. Nothing is above that upper boundary. The boundary intersects the water axis at about 100 millimetres. It is a sort of flag fall. Below that you don't get any yield at all.
This graph has proven to be very influential. What I want to do in this talk is to look at what our prospects are from now on for increasing the productivity of our dryland crops. But before doing that I would like to go back to see where we have come from, which gives us a good pointer to where we might go.
[SLIDE: Australian wheat yield from 1850]
This is where we have come from. This is the yield of wheat since 1850, again compiled by John Angus, which shows a remarkable series of steps. These were first pointed out ‑ at least up until 1960 ‑ by the great Australian agronomist Colin Donald. The period during the late 19th century showed a catastrophic decline in yields which was due to the exhaustion of nutrients. Nitrogen especially was just being run down in the soil because none was being put on. Similarly, no phosphorus was being put on.
But in 1900 there was a substantial change. Two things happened. One was that William Farrer, in his experiments with plant breeding at Lambrigg, revolutionised the wheat cultivars that were used in Australia by doing two things. He bred for resistance to one of the nastier diseases that wheat experiences, rust, and he ensured that the plants flowered at about the right time. Before that, plants flowered very late, say November, when there wasn't much water left in the soil for them to fill the grain that they might have set.
He brought the flowering date back to get a better balance between water use for vegetative growth and water used for reproduction, and he got rid of diseases.
At the same time farmers discovered fallowing, long fallows, which resulted in more nitrogen being released from the breakdown of the soil organic matter, and that, together with super phosphate being used, led to high yields,
In the 1950s there was a similar step change which was due to the introduction of improved pasture, based on subclover, which could fix nitrogen from the air. The phosphate was there, and the vigorous growth of subclover meant that there was a big increase in the amount of nitrogen that was available. That led to bigger yields.
Then there was the biggest improvement ever from about 1985 on until the millennium drought struck. So that's where we have been. There is a lot that we can learn from that. These step changes were, apart from Farrer's contribution, largely due to agronomic changes – that is, the way farmers managed the land.
[SLIDE: Yield of historical series of Australian wheat cultivars]
Meanwhile, the breeders were steadily increasing the potential yield of the varieties by about a percent a year, and are still doing so. Maybe not quite as fast as earlier, but half a percent a year is still happening.
Now I am going to show you another version of the previous graph, in which the data are portrayed as decadal averages.
[SLIDE: Average Australian wheat yields by decade]
You see the same jumps, perhaps more clearly. But what I want to show you in this graph is the remarkable increase in yield in several shires in south-east of New South Wales.
This is the area of the data from Wagga Wagga that I showed you earlier, which showed no response of yield to water supply. There was a huge increase in yield in the 80s and 90s. It is evident that all of the changes that we see [in this slide], and especially the one in pink, are not connected to water supply.
[SLIDE: Southeastern Australia Annual rainfall anomaly (base 1961 ‑ 90)]
Here is a graph from the Bureau of Meteorology which shows what they call 'the anomaly of rainfall', which is the rainfall compared with the average rainfall between 1961 and 1990.
The black line shows the 11‑year running mean. The pattern that you see, the ups and downs, bears no resemblance to the performance of the yield of the so‑called 'water limited crops'.
[SLIDE: Growing‑season rainfall at Harden]
Perhaps more pertinent to what we have been talking about is the growing season rainfall at Harden, in the south-east of New South Wales – for the growing season, that is, while the crops are in the ground. It is much more variable than the area as a whole, but you can see the ups and downs there, the very bad drought in 1982–83, and the very bad drought that we've had recently.
[SLIDE: Average Australian Wheat Yield by Decade]
So water isn't the driver of what we see there. It's a bit of a puzzle ‑ at least it was a puzzle as to what was going on.
[SLIDE: Picture of a canola field]
Well, what was going on? It was the introduction of canola into the farming system. Astute farmers noticed that wheat grew much better after canola than it had previously been growing. They talked with the agronomists, research agronomists and advisory agronomists – from CSIRO, New South Wales department of primary industry, and private agronomists – and realised that canola was inhibiting previously cryptic root diseases that were making the roots of the wheat crop grow poorly.
There are many other things you can do to beat those root diseases, one of which is to use specific herbicides for getting rid of grasses during the summer before you put a crop in; the grasses are similar to the wheat and they harbour the same root diseases.
[SLIDE: Canola plus grass‑specific herbicides]
Because they had discovered what was inhibiting growth, the farmers became more confident to apply more nutrients to aim for higher yields.
Less root disease resulted in a more reliable response, especially to nitrogen. That led to a large increase in the use of nitrogen fertiliser, which is necessary to get a high yield, and that is why the yield went up by such a large amount during that time.
[SLIDE: Wheat yields in Wagga Wagga Shire in 1949‑1983]
Let us return to wheat yields in Wagga Wagga during the 35 years after the 1940s drought. The reason that the yields were so low and not responsive to water is that the farmers were aiming for two‑ton yields. And that's what they got. They had learnt, sometimes through bitter experience, that if they tried to aim for higher yields their crops would crash. They would use too much water if they got highly vegetative and wouldn't leave enough water to enable the crops to set the grain and then to fill it.
When that happened, they not only did their money on putting the fertiliser on, they got even worse yields. That is the explanation ‑ perhaps a little simplistic, but a good explanation ‑ for why that graph is as it is.
This is not to say that water wasn't limiting at all there.
[SLIDE: Wheat yields in Wagga Wagga Shire 1949‑1983 (II)]
If you use a simulation model to estimate what a healthy crop might have done during that period, it is shown here in brown. It is responsive to water, but there are many circumstances in which you have a huge range in yield with the same amount of water. If, for example, you get nasty water stress at about the time of flowering you get a low yield. Or, if you get a frost at the time of flowering you get a low yield. So there are many other things that come into play.
But the take-home story is that the farmers were aiming for a low yield because they didn't have the confidence to aim for a high one.
[SLIDE: Water as a limiting resource]
Now I want to return to the graph that I showed you earlier, of yield in relation to water supply, and to explore the considerable insights that we have developed since the time that that graph was first produced. There are several insights there.
One is that when the farmers came to understand this graph it provided them with a benchmark for water limited yield. The upper boundary line we can think of as the water-limited potential yield – i.e. the yield if nothing else was limiting and if the water was evenly distributed, so there weren't any bad patches.
Having understood that, farmers then started measuring their own yields on a graph like this. If they were well below the line they would look for reasons why. It might be bad weather, it might be frost, or it could be inappropriate agronomy.
Because of that stimulus they had an incentive to improve the way they managed their crops. That resulted in a decreasing gap between actual yields and the water-limited potential.
Secondly, we also came to understand something about the flag fall – the loss of water by direct evaporation from the soil. That is what this intercept on the x-axis means [pointing to the 100]: It is the least loss of water by evaporation from the soil. What happens with a poorly growing crop that might be yielding well below the bounding line is not necessarily that it should be up here on the bounding line, but rather that the whole line moves to the right. Because, with a poorly growing crop, you might get, not 100, but 200mm of water evaporating directly from the soil.
And the third insight is that this region up here [pointing to where it says 'region beyond ...'], we now thoroughly understand is not an area that we can populate at all easily
It is very common for the media to talk about the breeding of drought‑resistant crops. The journalists might be thinking of getting something up here in this vacant space. But there is, as yet, no realistic basis to assume that.
There is, however, considerable optimism for moving the bounding line into this area [pointing to 'region of promise'] so that there is promise both of reducing the gap between actual yields and that attainable with the given water supply, and moving the attainable yield further into this region.
One of the things that came out of this graph is a conceptual framework, which I will now describe to you.
[SLIDE: How to improve water limited yield]
A very simple framework. If you think that water is limiting, you can do one or more of three things with your given water supply.
You can have the crops use more of that water supply – that is, to pass it through their leaves back to the atmosphere, which is called transpiration.
You can get them to trade that transpired water more effectively for carbon dioxide to produce biomass via photosynthesis. There are small holes in the leaves called stomata which let water out while they are letting carbon dioxide in. They can't let carbon dioxide in without letting water out.
And the third is that the greater is the proportion of the accumulated biomass that is converted into grain the greater will your yield be.
As an aside, I might say, that the green revolution in wheat and rice in the late fifties and sixties did two things. It reduced the height of the plants so that one could aim for a higher yield by putting on more nutrients without the crops falling over, which had been a severe limitation before, and it resulted serendipitously in about a 50 per cent increase in the proportion of the total dry matter above ground that you could get into the grain. It used to be a third. It was then possible to make it a half.
[SLIDE: Plant water use]
This little cartoon gives an example of part of that framework, in which rainfall may be dissipated. Run off; that's small. Drainage below the reach of the roots; also fairly small. Direct evaporation from bare soil is large, because in the normal winters here, the soil is usually wet and therefore evaporating, whether you have got a crop there or not. What is left is what goes through the plant.
[SLIDE: Soil water at sowing…]
More formally one can start to break down that framework, or elaborate it, so that you can think of various ways of improving the water supply. The growing season rainfall is, partly, as you accept it. Agronomists and breeders are also heading towards earlier sowing so that the crop is in the ground for longer. The growing season rain may thereby increase.
Quite importantly, you make sure that any summer rainfall that you do get is stored in the subsoil during the summer, because what is stored in the subsoil at the time of sowing can have an enormous impact on the final yield. As little as 20mm of that rainfall is worth a ton of yield at the end, in a country in which 2 tons is still the average yield.
So you increase the water used by the crop as much as you can. Trade the water for CO2 to give as much biomass as you can, and convert as much of that biomass into the grain. That involves making sure that the time of flowering is right, as Farrer recognised, so that there is a balance between the use of your limited water supply to produce vegetative growth and the use of water to set and fill grain.
There is a very large body of work that is now exploring all of those various possibilities and leading to substantial improvements.
[SLIDE: Environmental issues]
Now I want to get away from productivity for the time being and discuss some environmental issues that we have to take into account as well. Productivity in the short‑term is not of much use unless you can maintain it into the future.
We had severe problems with wind and water erosion from the earlier part of the last century through to the 1970s, becoming particularly bad in the 1970s; soil compaction, soil crusting. Major problems with soil degradation.
[SLIDE: Deep ploughing]
Deep ploughing, as it was then practiced.
[SLIDE: Followed by harrowing to get a fine seed bed]
Followed by a lot more tillage to get a seed bed, which made the soil sensitive to wind and water erosion.
[SLIDE: Dust storm, Melbourne, February 1983]
Leading to the infamous dust storm in Melbourne in February 1983, after the 1982 drought, where a lot of the top soil from the mallee and the Wimmera region came across Melbourne. That resulted in major changes of thought.
[SLIDE: The solution: Direct sowing without tillage ...]
At the same time solutions were starting to appear. Very effective herbicides became available, which meant that one of the reasons for tilling the soil so thoroughly, which was to remove the weeds, was no longer needed. Machinery became available with which you could directly sow the seed into the soil. Here is an early example. No ploughing, no tillage. The seed is going directly into the soil. The weeds that you see there would have been sprayed with herbicide and will die before the crop comes up.
[SLIDE: Now with 2cm autosteer machinery]
Fast forward 20 or 30 years, and this is what the machinery looks like now, with 2cm precision self‑steerage. With that technological innovation has come a lot of opportunities.
[SLIDE: Picture of a field]
This is the result of a machine like that. Sowing into undisturbed soil ‑ so undisturbed that the stalks from the previous crop are still there. And you can see the new crop perfectly centred between the lines of the old.
[SLIDE: Close up picture]
And again, a close up showing that. Not‑disturbing the soil surface and retaining plant cover, even dead, meant that there was much less wind erosion.
[SLIDE: Effect of management on wind erosion]
Here is a photograph from the mallee showing the traditional management against the modern management when a wind was moving from left to right. You can see no soil was lifting from the modern management. You can see it is starting to lift, perhaps even heading for Melbourne, with the traditional practice.
It is all very well to rely on herbicides, but one has to be very careful about the ecological consequences of using large amounts of herbicides.
[SLIDE: Autosteer and herbicides]
There is a lot of new technology available, for example this machinery, with which you can use fairly cheap knock‑down herbicides by having shields around the spray nozzles. You can spray the weeds between the rows without affecting the crop plants.
[SLIDE: Weedseeker technology]
Here’s another example, which is important when you are trying to get rid of weeds during the summer time. If you are trying to conserve water from summer rains, just 10 per cent of weed cover during the summer time will get rid of all of the water that infiltrates into the subsoil, because those weeds will get their roots down there.
This gadget looks at the soil, detects a weed, and as the machinery moves in this direction [left] it zaps the weed as it passes it. It brings the use of herbicide down by at least an order of magnitude.
[SLIDE: Trend in fuel used in producing grain]
This is a rough estimate from the Grains Council of Australia of the trend in fuel used in producing grain over the last 15 years. Partly as a result of the changes in the way machinery is used, and the lack of tillage, you get this very substantial fall in fuel use.
[SLIDE 41: Map of wheat yield across a farm]
Another innovation is the use of yield monitors on harvesting equipment which tells you what the yield is, using a GPS device, wherever you are in a paddock.
Farmers have been using these gadgets for some time. You can't buy a harvester now without one, just as you can't buy a car without a radio. Farmers produced these maps and didn't know what to do with them. But they became much more interested in them if you converted the yield into a profit or a loss, by taking the value of the grain and subtracting it from the cost of your inputs.
In this particular example you see that there is a paddock here which was predominantly giving you a loss and a paddock here – (the white patch was due to malfunction) – which was giving you a substantial profit.
Sometimes these patterns are stable across years and sometimes not. But if they are stable across years then that gives you another option for managing the land.
If this was stable across years there are two things you could do; you could either reduce the inputs, that is, you would aim for a much lower yield, just as the farmers in Wagga were doing in the fifties and sixties. Or you could set it aside and use it for something else ‑ trees, carbon sequestration, who knows.
Now back to the environment ‑ I will just go back to this slide of the rainfall anomaly and talk about managing the land.
[SLIDE: Will dryland salinity return as major problem?]
It is only 10 years ago ‑ for those of you with long memories you will remember ‑ that we were all going to be ruined because our landscapes were too wet. Dryland salinity was in the news, and there was concern that 17 million hectares of land across Australia would be at risk of dryland salinity. Now we are all going to be ruined because our landscapes are too dry. Who knows, they might remain dry, they might not.
But, if it starts to rain again, farmers have become so good at storing water from summer storms that they might exacerbate the risk of dryland salinity. Those summer storms which weren't being stored very well were sent by weeds back to the atmosphere. Efficient storage could lead to substantial deep drainage, which in turn could lead to salinity.
So I just wanted to remind you of how our concerns change over quite short times.
[SLIDE: Expected changes]
Here are some expected changes. We are certain that atmospheric CO2 will continue to rise, and pretty well certain that temperature will continue to rise. Rainfall, we don't know. The amount might change. Some of the general circulation models say that it will, and the pattern might change. We have seen a change in pattern in the last few years, in that spring rains have tended to migrate to the summer, and that is one of the reasons why the farmers have started storing summer rain. But it is unclear.
Input costs are certainly going to rise. Nitrogen fertiliser is produced by the Haber process, which consumes about 2 per cent of the world's energy and about 4 per cent of the world's natural gas. Phosphate is a worry because there is no way of getting it from anywhere else. And fuel costs are going to rise.
There may be legislation on greenhouse gas emissions, carbon sequestration, and the markets are likely to remain volatile.
[SLIDE: Free air CO2 enrichment]
I would like to spend a little time talking about the consequences of CO2 rising. The sort of experiment shown here is very expensive, but there are many around the world now where you have gadgets that emit carbon dioxide which fire whenever the wind is behind them, and you can increase the carbon dioxide experienced by the crop by, let's say, 200 parts per million above the current level of nearly 400.
[SLIDE: B. Elevated [CO2] 27.5C]
When you grow a crop in that way you can see what happens. The holes in the leaves, the stomata, close a bit, which means less water is evaporated. But the temperature increases in consequence. Nevertheless, because the CO2 is high, the photosynthetic rate goes up, and we might expect a 20 per cent increase in biomass from a 200 parts per million increase in CO2. And we have already had a hundred or more parts per million increase since the start of the industrial revolution.
[SLIDE: Peak phosphorus curve]
But the sleeper is phosphorus. A recent analysis by Dianna Cordell and colleagues have explored this and, like peak oil, considering what is available at the moment and the history of discoveries, reckon that we might expect to have a peak in about 20 years' time. Thereafter phosphorus will start to become less available at any price.
What are we going to do? Phosphorus is absolutely essential for the healthy growth of plants. If we go over that peak our chances of feeding the 9 or 10 billion population expected by 2050-60 will be remote. There may be technological solutions, but it is something we have to think very carefully about.
[SLIDE: Early sown crops need less phosphorus fertiliser].
There are tentative solutions that can be used in some circumstances. One is the remarkable observations by an agronomist in central New South Wales, Neil Fettell, who showed that the response of crops to fertiliser was enormously affected by when he sowed the crop. If he sowed the crop early in the season, when the soil was still warm and the roots could get growing fast, it didn't need much added phosphorus. But in June – cold soil, slow growth – you got very large responses to phosphorus.
Similarly, there are genetic possibilities in that we know some plants can release acids from their roots which dissolve phosphorus from the soil. There is a lot of phosphorus in the soil. But only a very minute part of it is immediately available to plants. That is the stuff that is in solution. Most of it is bound on a solid phase. But these acids can release it from the solid phase and give it to the plants. The banksias, the Proteaceae, are famous for that.
[SLIDE: Risk faced by farmers]
I want to finish off with a bit of bush sociology. Scientists tend to get enthusiastic about what they are doing and go to farmers saying, 'Do we have a deal for you'. But the farmers are not very impressed by the deal because they have got a dozen different risks that they have to worry about and have to juggle those. The good farmers are thinking two or three years ahead all the time, preparing their land for the future. This is a partial list of the sort of things that they have to worry about.
These days they have also become involved in finance markets, hedging, forward selling. An enormously difficult job. I've got time to give you a story about domestic risk management. There was a very good farmer in Western Australia who knew that if he planted lucerne on his farm he would get both greater productivity and greater environmental benefit. But he never would. The agronomists who worked with him were puzzled by this, until they got him well lubricated in a pub one evening and said, 'Why don't you grow lucerne?' He said that if he grew lucerne he would have to stay on the farm during the whole of the summer to manage the stock on it, and if he did that he would lose his wife.
[SLIDE: Some bush sociology]
Here’s a bit more bush sociology. The agricultural R&D game uses a lot of linear language. We talk about extension, technology transfer, delivering outcomes, as though it was a one‑way process. We find the truth, we dispense it to the farmer community. But my observations, from talking to farmers and watching skilled agronomists and breeders at work, is that this [in the slide] is a much more realistic representation of what actually happens.
You will recall that when canola was introduced into the farming system it was the farmers who noticed that their wheat was doing better afterwards.
The flow of information from their observations was like this [arrow to the right]. The agronomists who were in frequent conversation with the farmers started thinking about it, got the mechanistic understanding, saw new opportunities, created new options, such as the use of grass-specific herbicides to get rid of weeds during the summer, and away it goes.
So innovation involves problem or observation transfer as much as technology transfer. It seems to me a pity that we don't recognise that more explicitly, because, if we did, innovation would be much faster than it currently is.
[SLIDE: Water as a limiting resource]
I return to this graph and give you my optimistic view that we will be able to decrease the gap between these sorts of points down here [pointing to intersection been 400 and 2] and the potential yield [intersection between 400 and 6], providing that the differences are not due to bad weather or other matters outside our control.
And that breeders will continue to move the line up [pointing to the section between the diagonal line and the dotted line].
That we have very little chance of occupying this territory [pointing to the writing 'region beyond'], although in 30 or 40 years' time we might find, for example, ways of making photosynthesis more efficient in winter cereal crops. There is a lot of interest in doing that at the moment, but we are looking at a 30 or 40 or 50 year time horizon. And notice that irrespective of what the climate might do to us, if it is changing towards less water, this applies independent of your water supply.
I think it is quite remarkable that the yields farmers have been getting during these last few years of drought are so astonishingly high. Had this series of drought occurred 25 or 30 years ago, I think we would have had complete wipe‑outs in many parts of the country.
[SLIDE: And sometime we will get good rains again ...]
So I finish with the proposition that sometime we will get good rains again. Thank you.
Discussion
Sue Meek: Thank you, John. There are many thoughts embedded and many years’ worth of thinking there. I suspect that as the questions come the depth of some of those comments will come through. Any questions, please?
Question: John, first of all let me apologise. I now know why my wife left me. My snoring. I am sorry about that, everybody.
John Passioura: You were evidently very alert for the rest.
Question: If you could just put up that slide, the paddock scale slide of wheat yield, please. I think that illustrates a point to me, it illustrates it very strongly. (slide: Map of wheat yield across a farm)
I am a geologist and I specialise in soils. That, to me, is very, very obvious evidence that what is happening there is human imposed. It's not natural. It has absolutely no bearing on anything that is natural, otherwise that area of red in the north‑western corner would be continuing across into the adjacent paddock. It doesn't. It stops abruptly at the fence. So we are responsible for that.
Now, we go to the next point about phosphate and super phosphate. As you know, it is fraught with problems. It causes acidification of the soil, repeated usage you end up with soil pHs of about 3.5 or even lower. The more acidification of the soil there is the more leaching there is and degradation. The more breakdown of the soil structure, the more erosion, the poorer the performance of any plants.
I just wonder if you have looked at alternatives to super phosphate as a source of phosphorus. A friend of mine in Townsville, for example, has been involved with a material called Minplus which is powdered basalt. You can choose any basalt you like. Basalt is a very common rock type. It is a very easily quarried. It is very easily crushed up into a powder. It is not an expensive thing to do. It is, however, rather expensive to transport because it is rather dense. But it has the advantage that it breaks down very readily in soil profile.
It releases a lot of nutrients, including plant available nutrients, into the soil quite quickly. And it produces no unfortunate side effects whatsoever. You can tailor the type of rock to be used to whatever the problem your soil has. I would be interested in if you have looked at that kind of thing.
John Passioura: Well, coming back to the first point, it is clear that there is a cross‑fence comparison. But we have found in the last 20 years or so that there is a great deal of variability in the soil, that the subsoils are often saline, sodic, and that the top soil is variable in depth. And one can use electro‑magnetic gadgets to map the circumstances. They often correlate with what you see here. So there is a mixture. Some of it is human induced and some of it is in the landscape. You often get salt in the subsoil, and it is primary salinity. It is not secondary.
As far as phosphate is concerned, the story is a little more complicated than you mentioned. The main source of the acidification that we suffered from, which came from the improved pastures in the 1950s, is that the phosphate enabled very substantial growth of leguminous pastures, which fix a lot of nitrogen and that forms soil organic matter which will break down during the autumn rainfall, releasing a lot of nitrates well before the plants start growing to make use of them. The nitrates are leached. And with them they take potassium, calcium and magnesium. That is what acidifies. So the amount of phosphate going on certainly acidifies a little, but it is much less than that induced by leaching.
I would be very skeptical of the use of crushed rock as an economically and energetically viable alternative, but if we really run out then perhaps you are right. There was a lot of enthusiasm in Western Australia 10 years ago for the use of crushed granite as a soil ameliorant. It really wasn't doing anything at all, as you would have expected. I agree that basalt would be better.
Question: Two questions. Do you know of any way to direct drill and not use any herbicide or any gas burners. I have heard of pasture cropping, but when I try not weeding the veggie garden I don't get a crop. That's my first question.
The second one is, the phosphate that we put on, does it end up anywhere that we could cycle it back? Because it is an element it doesn’t just disappear?
John Passioura: The phosphate that we put on moves very slowly, unless in a very sandy soil, and it stays there. So whatever we have been putting on is still there. It slowly moves from an available form to a less available, to a substantially unavailable form. But some plants can get at that. I mentioned the proteas earlier, and some of the crop plants can do that. There is considerable scientific excitement at the possibility of breeding plants that might exude acids that can bring the phosphate back into solution.
So far as direct drilling without herbicides, I don't think you've got a hope. I think your observations are good: if you don't weed your garden you don't get any crop.
Question: John, I wonder if you would comment on the role of humus in soil fertility. As a school boy I remember that in preindustrial farming they used a rotation system, either seven‑year rotation or four‑year rotation, in which one of those years was fallow. They used legumes to introduce nitrogen and so on. There were no inorganic fertilisers. Animal manure and compost, which resulted in enrichment of the soil over many generations.
There is another very important point, I think, about humus because farmers are now thinking about re‑introducing humus and the link here is with climate change, that everybody talks about emissions reduction but not enough talk goes into carbon sequestration; in other words, extraction of carbon from the atmosphere by photosynthesis. The ocean is getting saturated with CO2. The soil is the biggest carbon sink we have. So by introducing humus we are doing two good jobs; one is for the climate and the other is the soil fertility. Instead of flogging a dead horse by using massive amounts of inorganic fertilisers and mono‑cultures we should perhaps go back to this preindustrial idea of crop rotation.
John Passioura: It is a current idea, as you would have heard from my remarks about the role of canola. It is a very attractive idea, but there are several difficulties. First of all, the composition of humus is fairly similar to that of the micro‑organisms that produce it when they are breaking down fresh organic materials that go on the soil. That composition requires that for every 10 units of carbon you have one of nitrogen. For every 30 units of carbon you have one of phosphorus, and about the same for sulphur. If you don't have adequate amounts of nitrogen, phosphorus and sulphur to add to the carbon that you are applying to the soil, the micro‑organisms use the carbon, send it back to the sky, and you are left with essentially nothing behind.
Humus breaks down at about 2 per cent a year, and in so doing releases nitrogen, phosphorus and sulphur. But if you want to sequester it you have to replace it each year. And if you have a very large amount of humus in the soil then to return to the first question, you will get galloping acidification because the breakdown of the humus releases a lot of nitrogen before the plants are able to take it up. And if it rains again that will be washed down and will take potassium, calcium and magnesium with it.
We have spent a bit of time doing spreadsheets on carbon, nitrogen, phosphorus and sulphur balance. At the current price of carbon you couldn't afford to provide enough nitrogen, phosphorus and sulphur to get a return. So if you are looking to put a ton per hectare of carbon on the soil in humus, you need 100 kilograms of nitrogen ‑ and that is expensive. And it is going to become more and more expensive because it depends on energy and natural gas.
If you are a home gardener that is no problem, you get Dynamic Lifter and so on. In fact I wrote an article for the Canberra Times a few years ago on this, which enraged a local distributor of Dynamic Lifter. But I persuaded him, I think, that the amount of Dynamic Lifter he would need across Australia's grain growing areas would require a thousand‑fold increase in the size of his business.
Question: John, I'm coming back to your early figures on wheat yield and the very rapid increase in yields in your eight districts in southeastern Australia. To what extent are those yields influenced by a migration where the crop is grown? When I drive to Melbourne these days I see wheat and canola all along the road and never observed it ‑ maybe a defective memory ‑ but never observed it in the past.
Do you get a significant graphic shift in where crops have been grown, and is that of sufficient magnitude of itself to affect the yields positively?
John Passioura: Certainly over the 150 year spread that was the case. Until Farrer solved the problem of rust and time of flowering the wheat was grown in the tablelands. But southeastern New South Wales has had wheat grown there for a century. I don't think that there has been a substantial artifact of the sort that you have mentioned.
Although, crop growing is moving back into the tablelands much more than it used to because of the realisation that we can grow long‑season crops more reliably there, now that we have the means to do so. And that is what you might be seeing. But the rapid rise in yield that I was talking about is in long‑standing cropped areas.
