After a fire, the soil sparks interest

The soil has four basic ingredients, and certain ingredients repel water more than others. Soil is composed of minerals, organic matter, water and air. We have already mentioned that some organic compounds can repel water, and the same is true for organic matter. The minerals fraction – from biggest to smallest – is sand, silt and clay. Sand particles, which are coarse and have a low surface area, need very little organic compounds to coat them. Clay particles, on the other hand, are tiny and have over 1,000 times the surface area of sand. They need a lot of organic compounds to coat them in order to become water repellent. As Jorge states, ‘if there is a little bit more clay that may make the difference between if a drop stays on the soil surface or infiltrates.’

In the 21^st century the number of large and uncontrollable fires – fires that can burn off organic matter and change the ability of water to infiltrate the soil – has increased on all vegetated continents. This surge in fire frequency and intensity is due to many factors. In some regions a higher proportion of climatic extremes such as drought has led to drier conditions and an abundance of parched vegetation that is ready to spark. Humans moving into previously uninhabited areas is also an issue as naturally occurring fires, which previously might have been allowed to burn, are extinguished to preserve the new inhabitants’ homes. When a fire does ignite, the glut of vegetative fuel that has accumulated over the years can lead to fires with greater intensity and higher potential to become uncontrollable.

After these destructive fires, it is extremely important to protect the soil. Post-fire management can be in some situations, says Jorge, ‘worse than the fire.’ ‘In many cases the land managers want to remove the dead trees. And if they do this when the soil is very fragile – in the first few months after a fire just when some vegetation is just starting to grow – it can degrade the soil.’

During the International Year of Soils, Jorge has been taking citizens and students from fire-affected areas in Spain to his study sites. They work together to set up plots with different types of management. One plot might have mulch applied while another is compacted as if heavy machinery had been used to remove the trees. Then they use a rainfall simulator to measure which plot has the highest amount of water runoff and erosion. ‘And really, the difference was amazing,’ says Jorge. The mulched plots had little water runoff, and what did runoff was crystal clear. Almost all the water ran off the compacted plot and the water was muddy with eroded soil.

Jorge hopes that experiences like this will encourage citizens to value the soil. He likes the term ‘geoderma’ to describe soil, from the Greek for ‘earth’ (geo) and ‘skin’ (derma). It helps people to understand ‘that the soil is not meters and meters and meters, we have only centimeters of soil. And we must take care of it.’

The views expressed here belong to the speaker and do not necessarily represent FAO’s views, positions, strategies or opinions.

Submitted by Catherine DeLong