Unit C3 - Soils


Unit C3


The soil is one of the most important resources in any country, for it is the basis of all agriculture. All our important crop plants and trees require soil, as do the forests, grasslands and other kinds of natural vegetation that clothe our land. The nature and quality of the soils determine land productivity and development potential. Unfortunately, soils do not always stay the same. They can be improved, but usually if they are neglected or misused, they can deteriorate and lose their good qualities. It is therefore important to learn something about soils and how to care for them.

Origin of soils

The kind of soil depends partly on the origin of the underlying rock, which can be continental, volcanic or from ancient sea bottoms or coral reefs. Continental rocks are of many kinds, and produce many kinds of soil. Volcanic soils originate from molten rock (lava) coming up through the earth's crust; the lava either pours out and becomes rock, or is blown out and forms ash or loose sandy material. Coralline soils come from the accumulated skeletons of marine animals and plants. These skeletons, like our bones, are made of calcium carbonate which makes limestone.

When rock is exposed to the weather and to attack by plants and animals, it breaks down very slowly to make soil. The qualities of the soil depend on the rock it is made from. The quantity of soil depends on how old the land is and how long it has been exposed to weathering.

Good soil is not just of mineral origin. It also contains organic matter from dead plants and animals. Leaves and other dead things fall on the ground and decay, leaving tiny fragments that are mixed with the soil. Rich soil also contains many living things: microbes, moulds, worms, insects, algae, etc. This soil community keeps the ground aerated and mixed, breaks down dead material to release their nutrients, and sometimes even fixes nitrogen to make nitrate fertilizer. Forest soils are especially good because the dense cover of living things builds up a rich soil layer, but these good qualities may not remain if the forest is removed. Many tropical forests hold their nutrients in the plants; any nutrients lost from decaying material is immediately taken up again by the dense root systems at the surface. If the plants are removed, the nutrients may quickly wash away.

Qualities of good soils

A good soil for agriculture or other human uses should have a number of qualities regardless of its origin. Physically it should be a good rooting medium, neither so hard and dense that roots cannot grow through it easily, nor so loose that they pull out easily. It should have available adequate quantities of the basic plant nutrients: nitrogen, potassium and phosphorus. The trace minerals should also be present and well balanced, without any high amounts that might be poisonous. It should be neither too acid nor too alkaline. A good soil should also contain organic matter or humus, as this improves the texture and water-holding capacity of the soil, and assists in the availability of nutrients. Finally it should have a community of living organisms: bacteria and fungi to break down rotting matter and release the nutrients in it; fungi (mycorhyza) that help some plant roots take up materials; nitrogen fixers that take nitrogen from the air and convert it to chemical forms available to plants; worms and other animals that aerate and rework the soil; etc.

Problems of soil degradation

Even a good soil may not stay that way if the conditions under which it was formed are changed, such as by cutting down the forest. One of the most important aspects of agriculture is learning how to keep a good soil productive. The following are some of the problems that can destroy soils.

When plants or crops are grown in a soil, their roots take up nutrients and minerals needed for plant growth. If the plant dies in place and rots, these materials return to the soil, but if these plants of parts of them are harvested and taken away, those materials are lost to the soil. The more intensive the agriculture is, the more is lost from the soil. The organic matter in soil is also constantly being broken down or used up, and harvesting a crop takes away organic matter that would normally go to replace what has been lost. This is why farming exhausts the soil, and the soil needs to be "rested" or left in fallow to recover. A poor soil may be exhausted after a single crop.

Using fire to clear the land or burn weeds also damages the soil. At first things may seem to grow better because nutrients in the plants are quickly made available in the ashes. However, these nutrients lying on the surface are also more easily blown away or washed away by the rains, finally leaving the soil poorer than it was before. Burning also destroys the dead leaves and rotting wood that would go to replace the organic matter in the soil; it can even burn the organic matter and the useful soil animals in the top layers of the soil itself. The result is a soil that can no longer hold water or nutrients and that dries out quickly.

In nature soils are almost always covered by forest, grassland or other kinds of plant cover. If this protective layer is removed (by logging, clearing for agriculture, construction, forest fire, etc.), the soil is exposed to erosion, which is the removal of the soil by water or wind. When the soil is dry, it easily turns to dust, and the wind can pick up the soil particles and carry them away. The rain falling on the bare soil also loosens it, making mud or muddy water that runs off the land, carrying the soil away. The running water itself is no longer slowed by plants, and cuts into the land, making ravines and gullies. In extreme cases all the soil can be washed away, leaving bare rock, but more often only a little layer is lost each year, and people do not realize that their good land is disappearing.

Erosion can occur even on flat land, but it is made much worse when the land is sloping. The steeper the slope, the greater the danger of serious erosion, and the more difficult it is to apply effective control measures.

A little erosion is natural and normal, but it is usually very slow and the soil lost is replaced by the new soil that is slowly but constantly being made. When the natural plant cover is damaged or the land is developed, erosion goes much faster and this balance is lost.

Erosion not only damages the land, it hurts other resources as well. The mud and silt washed off the land become a pollutant, dirtying streams and water supplies, filling up dams and harbours, and smothering lagoons and coral reefs.

Certain kinds of tropical soils are also subject to laterization. The heat of the sun on the bare soil, combined with the rains washing out certain minerals, leads to chemical changes turning the soil into a hard rock-like pavement almost like concrete.

Inappropriate cultivation techniques can also damage certain soils. Where a thin volcanic ash soil rests on a coral limestone base, discing can mix the limestone with the soil, reducing its acidity and damaging its fertility.

Some low-lying coastal soils and agricultural areas are vulnerable to salinization or contamination by salt-water, either by flooding by the sea during storms or by sea-water infiltration into the ground water due to drought or over-extraction from wells.

Urbanization results in a significant loss of areas of productive soils, since many towns and villages are located in the best agricultural areas. Spreading roads, houses and industrial developments eat up land formerly used for agriculture.

Signs of soil damage

A close look at the soil will often show signs of what is happening to it. If there is still some well established forest near by, dig a little and look closely at the forest soil, and then compare it with the soil outside the forest. The first sign of soil degradation is the disappearance of the soft moist layers of rotting leaves or humus. As a soil loses its humus, it gets harder, often lighter in colour, and dries out more rapidly.

A soil that is exposed to rain will lose its smallest finest soil particles first. If erosion carries away the fine soil, a thin layer of sand and pebbles may be left on the surface. Such a layer of coarser grains, sometimes left in rows or streaks sorted by size, is a good sign of water or wind erosion. In areas of recent rapid erosion, such as newly cleared or worked land, rocks or pebbles in the soil may be raised up on little pedestals where they have protected the soil under them while the soil around them has washed away. The height of the pedestals will give some idea of how much soil has been lost.

As erosion gets heavier, especially on slopes, the running water may cut gullies or ravines in the soil. As the water washes soil out of the bottom of the gullies, the sides and top are gradually undercut until pieces break off and fall in. Such gullies can thus creep gradually up hill, eating into the land and destroying the vegetation and the land surface. Similar erosion can sometimes be seem where road cuts, other construction, or mining have cut into hills or slopes. Plants and trees with their roots exposed are a sign of this type of erosion.

Problems of soil erosion and loss of soil fertility occur in nearly every country. Since the natural processes of soil formation are very slow, this means that most soils are steadily losing their productivity. Soil productivity is essential for agriculture, so the ability of the land to provide food and other materials is decreasing at the same time that the needs, both for development and often for a growing population, are increasing. Unless the decline in soils is stopped, the future will not be very bright.

Imported agricultural techniques are not always well adapted to local conditions, especially in the tropics, and can thus unintentionally speed up soil damage. Since agriculture is an essential support for most societies, the trend towards continuing and sometimes increasing soil loss must be reversed. This will require both the better management of existing soils to prevent further degradation, and measures to restore degraded lands to productive use. It will not be possible to achieve instant results, but future generations will praise the foresightedness of those who make the effort today.

What can you do to improve soil?

Since much soil damage occurs when the soil is exposed to sun, wind and rain, keep the soil covered by plants as much as possible. This might involve planting in strips or mixing complementary crops; using appropriate ground covers, some of which actually improve the soil; planting along contours on slopes, with retaining walls, stone-lined channels and other works to prevent erosion in heavy rains; using wind-breaks and other shelter plantings to prevent wind and rain damage; agroforestry in which tree plantings are mixed with other crops, etc. Many of these techniques were already in use in traditional agriculture, where the importance of preserving the soil was widely appreciated.

Good soil is rich in organic matter, but this is quickly lost in the tropics unless it is continuously replaced as it is in the forest. Where land is developed for other uses, it is necessary to add organic matter to the soil. Leaves, trimmings and other plant material can be composted, or put in a pile with the proper conditions for its rapid conversion into humus, which can then be added to garden soil. Cut grass, straw, leaves and other materials can be left on the surface of the ground as a mulch to protect the soil; this can then be worked into the soil later when it is prepared for replanting. Even seaweed cast up on the beach can be added to soil once the salt is rinsed out.

Any crop will remove some things from the soil, but different crops have different requirements. Soil is exhausted much more rapidly by growing the same crop over again than by rotating crops with complementary requirements. Many soils need to be left in fallow to recover their fertility, or to be planted in a leguminous crop that fixes nitrogen. Chemical fertilizers can be used to maintain essential nutrients, but they are seldom a complete replacement for what is removed by a crop, and do not prevent some soil degradation.

Burning weeds or brush to clear the land should be avoided wherever possible for the bad effect it has on the soil. The heavy use of pesticides can also poison the soil flora and fauna which are important to maintaining its fertility and structure. The use of such toxic chemicals should be kept to a minimum, and the instructions of the manufacturer or agriculture department should be followed with care.

Good land use planning can also help to protect and maintain good soils. Soils vary greatly in their adaptability to different uses and different crops, and much soil damage comes from using the soil for the wrong purpose. A soil that is adequate for forest or tree crops may be destroyed by attempts to grow intensive cash crops or root crops. Sloping land may be best planted in tree crops, or terraced to control erosion. Important watersheds should be left in forest to protect water supplies. The different needs of the land need to be carefully balanced against the resources available in order to ensure a sustainable future.


Where did your local soil come from?

How was it made?

Was the land once covered by forest?

How much of the forest is left?

Is a long fallow with forest regeneration still important to keep gardens productive?

Is there anything important missing in your soils?

Are chemical fertilizers or supplements used on your soils?

Are organic materials like plant cuttings or compost used in local gardens?

Is the soil left bare when the land is cleared, or are cut weeds or leaves left to protect it?

Is the land burned to clear it?

Are there soil erosion problems?

How much has the soil changed since your grandparent's time?

What will the soil be like for your grandchildren if present trends continue?

How much land in your area is degraded (not good for anything)?

What are some of the things you can do to solve local soil problems?

Instructions for trainers in the use of this unit

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Last updated 15 November 2006