Rehabilitation Aceh's soil

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.