Script .8

Notes to broadcasters

We at this radio station are part of a world wide information network that gathers farming information from developing countries all over the world. It’s the Developing Countries Farm Radio Network, sponsored by the Canadian International Development Agency, Massey Ferguson and the University of Guelph.

Through this Network we bring you information on ways to increase food supplies for your family, or to sell ways that other farmers have used successfully.

Today our topic is soil fertility. Here’s George Atkins.


Much of the land we use for growing crops was formerly covered with trees. When clearing that land, a lot of wood, brush and leaves were burned and crops grew very well at first. You know, however, that as time goes on, crop yields on that land decrease.
Now you’ll probably be glad to hear that a way has been developed to make crop yields increase on that land without using costly fertilizer. Before we talk about this, though, we should know why yields start to decrease soon after the trees have been cut. Well, one person who knows the reason is Dr. Bede Okigbo, the Deputy Director General of the International Institute of Tropical Agriculture (IITA) in Africa. Here’s what he says about soil nutrients in a forest:

Tropical soils are very deep. When it rains, a lot of the nutrients in the top decomposing litter (dead leaves rotting on the ground) are carried far deep into the soil. In a forest where the trees have deep tap roots, a lot of these nutrients which are leached down into the lower layers of the subsoil are again more or less pumped up by the roots and pumped into the leaves. These leaves then drop onto the surface of the soil, so that you have a continuous recycling of nutrients which drop in the leaves, go into the soil, are again pumped up through the roots up into the leaves so you have a continuous recycling process. This is why a tropical forest looks so lush, so very luxuriant; but actually a lot of the nutrients that we are talking about are to be found right in the vegetation.
Where we run into problems is not with trees but as soon as we remove the trees. When we start growing cultivated crops that are not very deep rooted, we run into problems for two reasons: (1) their roots don’t grow down deep enough to be able to bring those nutrients back up like tree roots do; and (2) they don’t deposit a lot of litter on the surface like trees do.
Under very high temperatures the litter or organic matter that’s in the soil decomposes very rapidly. We must not only maintain a good amount of organic matter in the soil, but we must also supply nutrients that have been removed from the soil by crops.

While some people have been doing both by carefully tending the soil over many years, others have done it with “slash and burn” or bush fallow farming. They’ve done it by cutting and burning the trees and brush, and then growing a few crops on the land the trees were on. After that, deep rooted shrubs and trees are allowed to grow again while another part of the bush is cleared for cropping. Then, many years later, that same land can be cleared again for growing a few crops, and so on.
But let’s think some more now about continually using that land that has been cleared. How can we keep the soil fertile without buying a lot of fertilizer? And where can we get a lot of organic matter to improve the soil we cultivate? After clearing the land of those deep rooted trees, is there still a way we can bring back those plant nutrients that get leached down deep into the soil?
There is indeed and it doesn’t cost a lot of money either. A new method has been developed that allows us to continue cropping the land, using the recycling process Dr. Okigbo was talking about.
The method is called Alley Cropping or Avenue Cropping.
Let me tell you what your field might look like if you try out this method of growing crops. All along one side of the field you’ll have a row of legume shrubs or small legume trees growing very close together. Then over in the field, 3 or 4 metres (yards) from that first row, you’ll have another row of these shrubs growing parallel to the first row. Then farther over, another row, and so on.
Now, see what you have? Long rows of legume shrubs with strips of land between them, you could call these strips of land “alleys” or “avenues”; and it’s on these alleys that you’ll be growing your farm crops; crops like maize, upland rice, and other cereal grains, cowpeas, or soybeans crops like that. If you’re farming on a hillside, even on gently sloping land, these alleys of course should be established on the contour, not up and down the slope. If they’re on the contour, soil erosion would be greatly reduced or even totally eliminated.
Now if you cut off the legume shrubs just above the ground, they’ll keep growing up again. So by cutting down these shrubs two or three times a year and spreading the leaves and stems on the land in the alleys, you’ll be adding organic matter and plant nutrients to the soil for your crops to use.
Doug Couper is the Farm Manager at Dr. Okigbo’s Institute. He explained Alley Cropping to me.

When trees are cut down, there are no more deep roots. When good plant nutrients leach down into the soil they are lost so they can’t be used by plants any more. This is why so many nutrients are lost after the forest is cut down.
Because of this, we are trying to do something to recreate forest conditions. We are doing this by planting leguminous shrubs with very deep roots. Their roots then will intercept (bring back) nutrients that have leached down into the soil. These nutrients are then taken up by the shrubs which will then produce new twigs and leaves. They are then returned to the soil for the use of the crops we grow.

How often do you cut down the shrubs and then return the leaves and stems to the soil?

Well we can’t allow the shrubs to grow too high, otherwise they will shade the crops from the sun and that would be bad for the crop.
So before each crop is planted, you do your pruning, spreading the leaves and twigs over the space between the rows of shrubs.

Now let’s talk about different types of leguminous shrubs. Depending on what part of the world we’re talking about, I suppose a farmer would use whatever leguminous trees or shrubs are available.
Yes indeed, but we have found in this area that Leucaena (Leucaena leucocephala) (ipil ipil) (lompora) or another shrub called Gliricidia (Gliricidia sepium) are very satisfactory for this system of farming as you can see in this field we’re standing in now.

For a farmer who is going to try out this system, to see how it would work for him or her, what would be a fairly optimum width between these rows of leguminous shrubs that you’re planting?

I would think 3 metres (10 feet). This will probably be optimal.

There’s another aspect of this. You’re changing your method of farming. By mulching like this, you’re actually putting mulch with every crop that you grow. What about that? Is that going to change the amount of fertility in the soil?

The value of these leguminous shrubs will be to produce nitrogen and organic matter. In addition of course, these deep rooted shrubs are intercepting plant nutrients which have been leached downwards in the soil and these are returned to the soil surface for use by the plants.

Is all this mulch going to assist in maintaining moisture supply?

Well, we try not to incorporate the mulch into the soil. We try to keep the mulch on the soil surface. In this way, of course, you’re protecting the soil surface from raindrop impact which is very destructive. So you’re retaining more moisture; you’re retaining more nutrients.

Now there seems also to be an additional advantage when the shrubs become rather woody.

Yes the woody parts of the shrubs can be used as firewood. They can be used as stakes for climbing vine crops, and in areas where yams are grown, they can also be used as yamsticks.
One of the additional benefits of alley cropping is the use of some of the twigs and leaves for feeding animals, cattle, goats and sheep. And there’s no reason at all why these leaves, which are very high in protein, can’t be dried and used as part of the protein supplement for poultry, for example. These would be the main other uses.
So by maintaining what we have in the soil by recycling, we’re going a long way to extending the time that cropping can be carried out on these soils without having to purchase large amounts of fertilizer.
This system has got many many things to recommend it, so I’m sure that, in the future, alley cropping will become a major method of farming in the tropics.

Thank you very much, Doug Couper, the Farm Manager here at the International Institute of Tropical Agriculture (IITA) in Ibadan, Nigeria.
Serving Agriculture, the Basic Industry, this is George Atkins.

Information Sources

1. Leucaena: Promising Forage and Tree Crop for the Tropics, Second Edition 1984 (100 pages), by National Research Council. It is available from BOSTID (JH 217D), National Research Council, 2101 Constitution Ave., Washington, D.C. 20418, U.S.A. Available for download at