Script 37.3


Some people wonder why it is necessary to use special conservation measures when they can reap crops from their fields without this extra effort. Why do these people think that all is well? They are unaware of the “silent thief” of our land – soil erosion.

You can stop that silent thief in your own fields. It will take a lot of work to begin with. But it will save your land from being washed away in the rainy season. If you do not do this work, it may be impossible to grow any crops on your land in a few years because all the good soil will have washed away.

To save your land, you can make a series of ridges and a trench that will keep the water from running down your field. You should do this before the rainy season. The ridges will stop the rainwater, and the trenches will carry it safely away without a lot of soil loss.

These ridges will run across the slope of your field. They will lead the water to a gully, ravine or other natural waterway. If there is no natural waterway then you will have to make one.

Once the natural waterway is located or established, you will want to build a large trench near the top of your field. This trench will catch all the water that flows from further up the slope. It is called a storm drain and you should dig it before you dig the other trenches.

First you will have to mark the line where the storm drain and the trenches will be dug. This is similar to making contour lines but there is one difference; the trenches should help the water get to the waterway. So you want the trenches to slope gently towards the waterway.

How do you make the trenches slope the right way? You need to make a simple change to the triangle with legs (also known as an A frame) that you usually use to make level lines.

What you must do is make one leg slightly longer than the other. The easiest way to do this is to tie a short stick to one of the legs so it is about six centimetres longer (about the length of your thumb) than the other leg. Tie it tightly so it doesn’t slip. When you are using the triangle with legs to stake the lines for your trenches, always point the longer leg towards the waterway. This will cause the line you make to slope gently towards the waterway.

You are now finished marking the first trench and are standing beside it.

To find the best location for the next trench, extend your arm straight in front of you and turn to face up the hill. Then walk backwards until the line of the storm drain you just dug is the same height as your extended arm. The level of your feet is where you should dig the next trench.

This technique is the same for all the small trenches.

Keep marking the lines for the trenches with stakes down the slope of your field, using your arm to tell you where they should be.

Now you are ready to dig the storm drain at the top of the field. It should be one metre wide and 50 centimetres deep. Pile the earth you take from the trench beside the trench just downhill of the trench. This ridge will help stop the water so that the storm drain can take it away.

Now you are ready to move down the slope and dig the other trenches. These trenches can be smaller. Make them about 50 centimetres wide and 50 centimetres deep.

If the soil is soft enough, use a hoe to dig the trenches. With this method, you can dig up to 20 metres a day. You can also use a plough to dig along the marked lines.

There is one more important thing. Make sure there is grass cover in the storm drain, trenches, and waterway. The grass cover helps to keep the soil from being washed away. If the grass dies, the contour ridges will be damaged and gullies will form. Cut the grass short so that the water will flow easily.

Check the trenches regularly and remove any obstacles that might block the flow of water. If the water is blocked, it will spill over the ridges and break them. Also, once a year you should dig out any sediment and litter in the trenches. Put the sediment back on the land.

This is how you can build and maintain contour ridges across the slope to reduce the loss of top soil from your fields. Remember it is practically impossible to replace the lost fertile soil. It is also costly in money and labour to repair the damaged land. Prevention is the best cure.


This script was produced at the Communications Workshop for participants of the Farm Radio Network, led by Ian Montagnes, 1993, Wensleydale Farmer Training Centre, Harare, Zimbabwe. It was written by:
James Gandari, Producer, Farming Programmes, Agritex, Zimbabwe
Richford Mombeyarara, Head, Agriculture Department, ZIMFEP College, Zimbabwe
Brian Mutete, Agriculture Teacher, Zimbabwe
Bernard Ragalese, Information Officer, Forum on Sustainable Agriculture, Botswana

It was reviewed by Vigneswaran Thievendaram, Agricultural Consultant, Cambridge, Canada, and Anne Weill, Agricultural Engineer, Macdonald College of McGill University, Montreal, Canada.

Information Sources

Soil conservation for small farmers in the humid tropics, T.C. Sheng, 1989, FAO Soils Bulletin No. 60. Food and Agriculture Organization (FAO), Via dell Terme di Caracalla, 00100, Rome, Italy.

“Our soil; is it forever?”, World Neighbors in Action, Volume 14, No. 4E. World Neighbors International Headquarters, 5116 North Portland Ave., Oklahoma City, OK 73112, U.S.A.