Rehabilitation Aceh's soil

Fahmuddin Agus, Bogor

Beyond the tragedy of the tsunami disaster in Nanggroe Aceh Darussalam, North Sumatra, and other parts of Asia, there is hope for recovery.

National and international aid are pouring into the devastated areas, and most survivors, despite their grief, will begin their lives anew within a few months.

What is needed now is a coordinated effort for short- and medium-term rehabilitation to assist local stakeholders in investing prudently, particularly in the agriculture sector. In Aceh province, for example, about half of its 147,000 ha of irrigated paddy fields and 683,000 ha of rain-fed food crop areas have sustained damage of varying degrees, according to the Directorate General of Food Crops.

The tsunami caused three major forms of damage to agricultural lands, with some areas sustaining a combination of the three: one, an accumulated deposit of mud, silt and sand -- or coarse materials; two, salinity, and three, desurfacing and compaction of topsoil.

As the powerful waves flooded coastal and inland areas, they transported mud from the sea and coastline. Fine clay was transported furthest inland, to the extent of the water's path. When the water receded, the materials sunk and capped the soil surface.

Unlike normal surface soil particles, which is bound by organic matter and other cementing agents to form aggregates, mud is dispersed -- or "structureless" -- as single particles. Mud thus has a very low capacity to adsorb plant nutrients and absorb or retain water.

Fertilization is ineffective on such land, as the nutrients are easily leached by percolating water deeper into the earth. The land may also be vulnerable to more frequent droughts because of its low capacity to retain water. Furthermore, root anchorage into such soil is very weak, and thus crops are easily uprooted. An accumulation of 5 to 10 centimeters of mud may start to affect plant growth.

The salinity problem is easily understood, because of the high salt content of seawater. The level of damage caused by salinity is dependent on the amount of saltwater that infiltrated the soil. If the soil's water content was high when the tsunami struck, then only a relatively small amount of saltwater infiltrated it, and vice versa.

A high salt content may disperse soil aggregates, limit water and nutrient absorption by plant roots and cause tissue damage in plants.

Soil salinity is often expressed as exchangeable sodium percentage (ESP) -- i.e., the percentage of sodium (Na+) exchangeable between the soil clay surface and soil solution relative to the amount of exchangeable potassium, calcium, magnesium, aluminum and hydrogen.

Rice, carrots, taro roots and lettuce, for example, can tolerate an ESP of 20 percent, but will not survive an ESP of over 40 percent. Peanuts, soybeans, sweet potatoes, tea and mungbeans are affected by an ESP of 5 to 15 percent, with peanuts being the most sensitive. These crops cannot survive when the ESP exceeds 25 percent.

The desurfacing of topsoil is caused by serious erosion from the waves' passage and by the clearing of debris by heavy machinery. Using a bulldozer to clear debris would likely desurface the topsoil and cause compaction of the soil. While bulldozing is a quick and economical clearing method for residential areas, it can cause serious damage to agricultural land and should be avoided as much as possible. Clearing manually or by the use of animal power, such as elephants, can minimize desurfacing and compaction.

The extent of these three major forms of damage must be assessed prior to implementing rehabilitation efforts. The use of satellite imagery and geographic information system (GIS) would be very helpful in accelerating the initial assessment.

Meanwhile, ground transect analyses -- which extend from the least affected point on the upper slopes down to the coast -- of soil properties will show a gradient in damage and thus soil fertility along the transects; the closest area to the beach will be the most seriously affected. This kind of analysis is essential for evaluating soil fertility and land suitability in determining agricultural or conservation plants as well as in developing land rehabilitation techniques.

Included among rehabilitation programs must be the addition of organic matter to increase nutrient and water retention of the soil, and split or stepwise fertilization.

A high ESP could be rehabilitated by fertilization with potassium (K+), magnesium (Mg++) and calcium (Ca++). Nitrogen and phosphorous fertilization are also indispensable. The addition of organic matter is important to increase the nutrient retention capacity of the soil.

Washing away the salt with fresh water is theoretically sensible, but technically complicated and costly. Rainfall will gradually do the job.

Rehabilitation techniques for desurfaced soils include heavy fertilization and organic matter addition. At times, planting nitrogen-fixing leguminous cover crops, such as velvet beans, is necessary. Other organic matter sources include barnyard manure and plant residues.

Supplies of seeds and seedlings of suitable crops should follow rehabilitation, as these are among the vanished resources in Aceh and North Sumatra.

Owing to the gigantic scale and intensity of the damage, those areas with the highest probability of success using the transect analysis approach should be prioritized. Damage assessment, rehabilitation program development and implementation should be an integrated and coordinated endeavor among both national and international institutions -- as opposed to the conventional, "working together", uncoordinated approach.

Dr. Fahmuddin Agus is a researcher and head of the Soil Research Institute, Bogor.