Technology for Low-Productivity and Degraded Soils

2003-12-01

Population growth in Asia has resulted in more intensive exploitation of the resource base, including soils. Many farmers are being forced to cultivate marginal soils, for lack of any better alternatives. The challenge is to find food for an increasing population from finite land resources. Another problem is water, which has become another scarce resource. Many countries do not have enough water for agricultural uses, and have quality problems with the water that is available. Faced with expanding needs and finite resources, the only solution is to make better use of the resource base. The problem is to reconcile the needs for development with the constraints imposed by nature.

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Degraded Coastal Lowland Soils

Two kinds of lowland soils in Asia are particularly vulnerable to degradation. One supports mangrove swamps in brackish water, the other is the peat soil of freshwater swamp forest. Both are distinctive to the region. Mangroves are believed to have originated in Southeast Asia, which also has two-thirds of the world's tropical peat area.

Mangroves

Mangroves provide an important spawning ground for fish, as well as protecting the coast from erosion. Mangrove trees have arching roots that support the tree up in the air, like stilts. These roots are permeable to gasses, helping the tree to breathe, but protect the trees from salt.

The mud of mangrove swamps contains sulfide compounds, most of which are present as pyrite. When these are exposed to the air, oxidation takes place, and sulfuric acid is formed. This causes the soil to become extremely acid, and induces aluminum and iron toxicity. It is difficult to grow crops on such soils. They cannot even be used for fishponds, since the acid water kills the fish. Once these oxidation products are formed, it is not easy to get rid of them. Many attempts have been made, using lime applications and drainage, but the results are often poor. As a result, the land is abandoned.

Tropical Peat Forest

Deposits of tropical peat can be ten meters thick, but more than three-quarters of this layer is made up of water. Once forest peat soils are cleared and drained, they begin to subside as water is lost and the peat begins to decompose. It is common for reclaimed tropical peat soils to sink by several centimeters each year. Where peat layers are thin, the underlying acid clay may be exposed in only a few years. Even where the peat layer is thicker, peat soils lack basic elements and are strongly acid. It seems that as with mangrove soils, the agricultural use of tropical peat soils is often not sustainable.

Reclaiming Volcanic Lahar

The eruption of Mount Pinatubo, Philippines, in June 1991 covered thousands of fertile rice farms with a thick layer of ash and pumice, known as "lahar". The photo to the right, taken two years afterwards in 1993, shows a wasteland where almost every plant has been killed. Active programs by the Philippine government are restoring farmland, and now 80% of the lahar is under crops.

The lahar deposits are very sandy, and drain quickly. This means that applied fertilizer is quickly leached. A paper presented at the ESAFS conference showed how controlled-release fertilizers release the nutrients slowly, to match plant needs. The fertilizers have a semi-permeable coating of polymers through which dissolved nurients are diffused, at a rate governed by moisture and temperature.

Degraded Slopeland Soils

Although there are plenty of lowland areas with degraded soils, it is the region's uplands that are tending to deteriorate on a massive scale. It was generally agreed at the ESAFS conference that it is water erosion which is the major problem of Southeast Asia.

Millions of hectares of topsoil are being carried away by water. Some of this soil is carried away into the sea and permanently lost, the rest is deposited as unwanted sediments lower down in the watershed. In the rainfed cultivation of annual crops, soil losses of 20-100 tons per hectare are not uncommon. Even on gentler slopes under permanent crops, 3-10 tons per hectare are often washed away each year. Thus, the region is losing rapidly those soil resources which took generations to accumulate. As one participant at the ESAFS conference remarked:

"One centimeter of soil formation takes three hundred years; One centimeter of soil erosion takes one year. "

Agricultural practices such as contour tillage, mulching and cover crops to conserve soil and water in slopeland areas are well known. However, slopeland farmers are often resource poor. Their aim is to get food and cash for their families. They may not be interested in conservation practices, since these do not give an immediate return. Subsidies for conservation work have been successful in Sri Lanka and other Asian countries. Another successful incentive is to give traditional farmers title to their land if they practice conservation measures. The participatory approach is also being followed for soil conservation, often over an entire watershed.

Monitoring and assessment, to study the impact of agriculture on soil productivity, is the basis of sustainable agriculture. Another basic measure is land capability classification. Often steep slopes are not suitable for agricultural production, even though a lot of agricultural activity takes place there. Tackling the problem of rural poverty is an essential part of sustainable soil management.

Index of Images

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  Figure 1 Mangrove Trees

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  Figure 2 Fish Ponds Dug in Former Mangrove Swamp

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  Figure 3 Town in Tarlac, Philippines, Almost Buried by Lahar from MT. Pinatubo

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  Figure 4 Eroded Slopeland in Tropical Asia