Pablo Tittonell: Dishing up the dirt
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Pablo Tittonell: Dishing up the dirt02 mars 2016
Pablo Tittonell, Professor and Chair of the Farming Systems Ecology Group at Wageningen University
It is estimated that about 25 per cent of the soils in the world are degraded, are in a severely degraded state, and we are talking about agricultural soils in particular, soils that we are using for cultivation. So that means that on the one hand, what we are doing is expanding the footprint of agriculture by bringing more land from nature into agriculture and, at the same time, we are not taking care of the land that we are already using. This is obviously a major problem, especially in a changing world, but it is also a major opportunity because if we are able to actually restore fertility of the soils, restore their productivity, we can actually produce 25 per cent more land and at the same time sink a lot of carbon. Why? Well, because what it takes to restore the soils is to increase the organic matter content and organic matter, as you may know, is 50 per cent carbon. That means that the many tons of organic matter that you are going to need from there are going to be withdrawn from the air, from the carbon that is now causing climate warming. So it is actually a 5 per cent deal to move from this situation to a situation like this.
Why do I say a 5 per cent deal? Well, because in most soils, organic matter occupies no more than 5 per cent, in natural soils, of the volume of the soil. Here I have some samples where we can see that, we can see some elements of organic matter are going to be part of the soil later on. Normally, soils have what we call pores; only half of the volume of the soil is solid, the other part is pore, porosity. That porosity is very important, it is the one that hosts water and air. Without that there is no life in the soil, there is no life for plants, there is nothing. Anything important that happens in the soil happens in the pores. It does not happen in the solid phase.
If we look now at the solid phase, most of it is mineral and the minerals that form the soil are actually depending on the type of geological material that gave origin to those soils. If we look at this map we see all those red areas; there are areas where we had very old rocks, very old what we call pre-Cambrian rocks. They are very old, older than the internet, for instance! And those soils have been weathered for a long time. That means they have lost a lot of nutrients so the soils that form of that rock are inherently poorer in nutrients, are acidic, and there is not much we can do about that. We cannot change the mineral face of the soil. What we can do is work on the organic part of the soil, this 5 per cent, and in this 5 per cent, what we find is humus – and here I would like to open a parenthesis for the soil scientists in the audience because I know it is more complicated than that but let us keep it simple, like this – that is most of it, then we also have the roots of the plants, like we see in that picture; all these roots are going to die down and then become organic matter in the soil. Then we have plant litter, like this, which is going to decompose and form organic matter. Actually, if you take a little bit of soil from here you see some plant material still decomposing.
Then what we have also in this soil is organisms. Those organisms, some of them are really tiny, we cannot see them, we need a microscope. Not for nothing they are called microorganisms. And then there are many others that we can see, that we can actually see and study, see their movements and activity. All these organisms are very important and they need organic matter. They need organic matter as food for themselves. Without organic matter they are not able to grow, they are not able to reproduce, they are not able to comply with all the functions that we need from those organisms, like helping plants to acquire nutrients. Another important function of organic matter is to act like a cement, keeping particles together. What we have here is what we call soil aggregates, actually the spatial organization of the particles in the soil that are cemented thanks to organic matter.
Now, the existence of these aggregates is very important because this is what creates porosity in the soil and that is, as I said before, anything important that happens in the soil happens in the pores. If we destroy that porosity by compacting this, then nothing else happens in the soil. We can put a lot of nutrients there but they are not going to be available for the plants. The existence of a good structure, it is called soil structure, can be also evidenced when we put these soils in water. What we have here is a glass of water where we put this soil, which has a lot of organic matter, and the water looks clean. You feel like taking a sip, even! And here we put other soil that has less organic matter and the water gets dark, gets turbid, and that is because there is less cohesion between the particles of the soil. So when it rains and this happens, then this water carries away a lot of nutrients.
So this important to know because when we soils into production the first thing we do is we destroy that structure, right, by working it but we also change the vegetation cover and that means that we have exposed the soil to sunshine, to rainfall, which change the conditions of the soil. What we are doing there is creating what we call oxidation of organic matter. Oxidation means that the organic matter is being decomposed and turns into CO2, contributing to global warming. That is how agriculture contributes to global warming. Interestingly, when I teach this to my students I normally use this analogy of the reservoir. If you have a reservoir and the soil is a reservoir and the water is a lot of organic matter, the amount of organic matter that you can store in the soil is a balance between what comes in and what goes out. If you open the outlet and reduce the inlet then you are going to have less water, less organic matter. This is exactly what we did in the last century. We have been opening the outlet by ploughing the soils and exposing them to oxidation and we have been reducing the inlet by using less organic fertilizers and also varieties that are smaller and produce less biomass than before. So we have been doing this and also, next to that, there is the problem of erosion that is also accounting for a lot of losses of organic matter.
When we think about the solutions to soil problems, especially in the case of Africa, we think about fertilizers, obviously, because nutrients are limiting productivity in Africa, and we think that by making fertilizers available we are going to solve the problem. Now, when we talk to farmers, as I did for almost ten years now, the situation is a bit different. In the first place, because indeed fertilizers are very expensive to farmers, farmers in Africa may pay ten times more what a European farmers pays for fertilizers, but on the other hand, what they tell you is, well, if I use fertilizers that will make my soil hungry and that, for a scientist, is a very challenging statement. How is that? Well, if you use fertilizer once you have to keep using it. Well, that sounds very strange but there may be a reason for that.
Actually, when we look back in history, we can go back to the father of the fertilizer industry or the father of modern agriculture and he was also the inventor of these cubes so he made a lot of money with that but he was a brilliant mind and he was the one who discovered nutrients, in a way. He made nutrient solutions, applied them to plants and saw the plants going better. That is the creation of fertilizers, in a way. That was written in this book in 1840 but 20 years later – people say that when you get older you become greener – he said something like this: adding chemical mixtures to soil – he did not call them fertilizers yet – without organic matter has, in the long term, the same effect that alcohol has on uneducated people: he said laziness, inefficiency. When you tell this to farmers, that you need just 5 per cent of organic matter in your soil, farmers sometimes say yes, that is easy to say but if I need to apply organic matter, if I need to apply animal manure, first I need a cow. If I do not have a cow, so… This is true. If you look at the density of cattle in Africa there are few places, only the red spots there, where you have enough cows to be able to maintain soil fertility only with animal manure. The rest of the places there are not enough cows so we cannot count on this resource. Only the wealthier farmers have cows.
So here is where conservation agriculture may help. Conservation agriculture may help by doing two things: reducing the outlet of organic matter, reducing the losses, and increasing the input of organic matter in the soil. Conservation agriculture is this combination of principles of minimum or no soil tillage – just do not move the soil, just keep it as it is, keep the structure – and open a little furrow and put your seed there. The second one is keep the soil covered, as in nature – in nature the soils are always covered, either by plants or by mulch, by litter; and third, increase the diversity of plants. Next to your crop grow something else that is also giving organic matter to the soil. This other crop that you can include can also be a legume. Legumes are plants of a certain family, a large family that can form a symbiotic relationship with small bacteria. The bacteria make their houses there, in the roots, in their nodules, they make these nodules that we see here; they live down there and they take nitrogen from the air and they transform this – we have 14 per cent nitrogen in the air – and they transform it into a form that plants can take. That is exactly what the fertilizer industry does, they use a lot of fossil energy to do that, they use energy that comes from photosynthesis of plants and that is why it is a symbiosis. They use sugars to do that.
Now, when you do this, when you, for instance, you have your maize – this is an example, an experiment in Mozambique that shows a maize yield of 1 tonne per hectare, which is, as you know, a common yield in many areas, when you combine your maize crop with a legume – in this case the pigeon pea, a species that produces a lot of biomass and also produces peas that you can eat and fixes nitrogen – what you get after inter-cropping, rotating, an increase in maize yield but also those peas, which also contribute to a diversified diet. So there is potential and we are underutilizing the potential of nitrogen fixation in agriculture.
In some situations it is much more difficult, much more challenging, even if we want to practice conservation agriculture here, this is a picture from Burkina Faso, from the Sahel, of degraded soil. This is agricultural soil that has been degraded and when we tried to do that to produce biomass, to keep in mulch, we had lots of challenges there because it is difficult to grow a crop in the first place. If you try to generate organic matter first you need to have a healthy crop with a lot of biomass. And if you put that biomass on top of the soil during the dry season it will quickly disappear because of the composition, because termites are going to eat it or because may be roaming around and eat it. But when you start doing things that farmers did in the past, like creating these micro-depressions where you can concentrate water and concentrate nutrients and organic matter and biological activity, then you start having – after some time – a recolonization of the area with natural vegetation. Once you have those plants there, all that natural biomass, you can use it as a source of mulch. Now your nutrient inputs are going to respond much better.
All over the world we see farmers coming up with very inventive ways of managing soils and keeping organic matter. Farmers in the Andes – 3,000-4,000 metres – they come together to work in these very harsh conditions on very steep slopes and, of course, they know that the way to maintain soil is to avoid erosion and for that they come up with very innovative ways of cultivating the land. Very often, we go there to those places and we propose farmers to adopt technology that was developed for the flat lands, not for those soils. We come with our tractors and everything else. You can actually develop a machine that does that, these interesting furrows that slow down the water and reduce erosion. So it is not that we do not need tractors or any other technologies. What we need is a combination of both. Most of the technologies that we have in agriculture are technologies that were developed for this kind of systems. There is a lot of investment, there is a lot of technology, there is a lot of knowledge and, of course, the GDP of this sector is very important. But if we look at who is feeding the world and if we look at who are the ones who are hungry when we say these numbers like 800 million people are going hungry well actually lots of them are farmers, they have land. When we think about this agriculture, we need to think about how many calories we can produce per hectare and how many people therefore we can feed. But actually we should think about how many people can make a living out of one hectare of land because those are family famers. They need to make more than just calories. That is why we need to put more science, more technologies, and more knowledge into this kind of systems.
Comparatively to the other system, we have invested less in understanding and trying to develop technologies which are adapted to this kind of systems. This kind of very diverse systems, we call them agricological systems, are actually not only feeding people, making a living for people but also delivering ecosystem services of global importance. They are contributing to sequestered carbon, they are contributing to preserving diversity, and they are contributing to regulating water. So they are contributing to life on the planet. They should be rewarded for that as well.
It is important to realize that it is a 5 per cent field, the difference between this and this is just 5 per cent and of course it is a challenge. Of course organic matter is not something you can pack and transport and deliver and of course there are no recipes. You cannot just say that all farmers should do this or this or that. We need to find locally adapted solutions and then the best way to do that is to actually include farmers in the technology development. What we need is a dialogue, a dialogue of wisdom; scientific wisdom on the one hand and farmer wisdom on the other. We are not going to solve the problem if we think that we have the solution and we want to take it down there.