When Anak Krakatau's eruption ash reaches farmland
The eruption of Anak Krakatau on 5 September showed that the impact of a volcano does not stop around its crater.
A day after the eruption, BMKG detected volcanic ash up to an altitude of 50,000 feet, or about 15 kilometres. In the lower layers of the atmosphere, the ash moved eastwards and reached Banten, DKI Jakarta, and various parts of West Java.
Volcanic ash from Anak Krakatau was even observed as far as the Bandung area, about 260 kilometres from the eruption source. That distance demonstrates the ability of fine volcanic particles to move with atmospheric circulation before eventually returning to the earth’s surface.
The ash’s journey brought Anak Krakatau’s problems into a different domain: agriculture. West Java is one of Indonesia’s important agricultural regions. When ash enters farming areas, the question is no longer only how high Anak Krakatau erupted or how far its ash was carried by the wind.
The question closer to farmers’ lives is: What happens when ash falls on leaves, plants, and soil? The answer is not as simple as assuming that volcanic ash always damages crops or, conversely, immediately fertilises the soil.
Solid material ejected during an eruption varies in size, from large blocks to very fine particles. Material smaller than 2 mm is called volcanic ash. These fine particles are easily carried by the wind and can spread far from the eruption centre, depending on particle size, eruption strength, wind speed and direction, and atmospheric conditions.
Not all volcanic material is formed in the same way. Some consists of crystalline mineral fragments originating from magma or lava that has crystallised and then shattered during the eruption. Others consist of volcanic glass formed when magma melt at around 900–1,200 degrees Celsius cools very rapidly so that its atoms do not have time to arrange themselves into crystals.
This difference is important in soil formation because crystalline minerals and volcanic glass have different resistance to weathering. Volcanic glass is generally more reactive, so it weathers relatively more easily and gradually releases various chemical elements.
Research on material from the 2018 Anak Krakatau eruption provides a picture of the character of very young volcanic material. XRF analysis showed a total CaO content of 15 percent, MgO percent, K₂O percent, P₂O₅ percent, SO₄ percent, and SiO2 or silica of 55 percent. This means the material carries reserves of calcium, magnesium, potassium, phosphorus, and sulphur.
However, total element content is not the same as elements directly available to plants. Some elements are still bound within minerals or volcanic glass and must be released through weathering before they can enter the soil solution and be absorbed by roots.
Therefore, volcanic ash that falls on agricultural land cannot be equated with fertiliser. Ash is new geological material that must still undergo a series of physical, chemical, and biological processes.
Rainwater begins to weather the volcanic glass and minerals. Pioneer organisms such as cyanobacteria or blue-green algae and lichens begin to colonise the material’s surface. Then plants introduce carbon and organic matter through roots and litter. Over time, organic matter interacts with minerals and the geological material slowly develops into soil.
That process can take years to much longer, depending on the nature and size of the material, climate, rainfall, drainage, vegetation, topography, and organism activity.
Ash and plants