Soils are more than dirt ... in fact they are essential to our survival. After all, about 95% of our food comes from it, either directly or indirectly. The plants that feed us, are themselves nurtured by the soil and its unique ecosystem. Yet, we know surprisingly little about what's beneath our feet. In fact, around 59% of all species on Earth live in soil, but still most of them remain undiscovered and unnamed. Tragic! Not only because these organisms play a decisive role for growing our food, but also because soils can play a major role in reversing climate change.

Soils are home to the most diverse and complex microbiome on this planet and it is this microbiome that interacts in a multitude of ways with plants. Microbes and plants mainly meet at the plant’s roots. Here, at the so called “rhizosphere”, plants are “recruiting microbes”, a process that works via fluids they excrete. These tasty root exudates attract and feed soil microbes, which pay back to the plants by helping them to get easier access to water, protecting them against diseases and by delivering essential nutrients like nitrogen or phosphorus. And this cooperation is worth it! Microbial contribution to feeding plants makes up to 80% of nitrogen and 90% of phosphorus taken up by plants.

But microbes not only feed and protect the plants that feed us, they also offer a very potent pathway of mitigating climate change. This is because bacteria and fungi play an important role in storing carbon in soil. Their carbon-containing dead bodies and glue-like substances, help to form soil micro- and macro aggregates. These bio-organic complexes are very stable and can protect the entrapped carbon up to 300 years. And all carbon that is contained in the microbes’ bodies was once carbon dioxide. This carbon dioxide was taken up by plants via photosynthesis, transferred by the plant tissue or by root exudates to the soil, and then digested by microbes, whose dead bodies now help to store carbon dioxide in soil.

What only few people know is that soils are the largest terrestrial organic carbon pool. They store about 3,012 Gt of carbon up to a soil depth of 2m, which is 80 times the amount of annual fuel emissions (37.5 Gt in 2022). This is why increasing the amount of carbon in soil, even by a few percent, can be a potent way of removing carbon dioxide from the atmosphere. And the role of microorganisms in this process is significant, because they contribute to more than half of the carbon stored in soil. 

One of the most important factors in maintaining healthy soils is organic matter. Organic matter helps to build humus, a substance that supports a wide range of soil life, including microbes, fungi, algae, and earthworms. When soils are consistently replenished with organic matter, their ecosystems remain healthy, and their ability to store carbon is enhanced. The most effective way to increase humus in the soil is by applying organic fertilizers, such as compost, manure, or nutrient-enriched biochar; amendments, which are both rich in nutrients and carbon. 

Manures and composts, which can derive from plants, from animal manure and even from human excreta, are particularly effective in improving soil health. Mixing biochar into these materials is a further method of increasing their efficiency. This is because these organic fertilizer not only increase humus and soil biodiversity but also boost nutrient levels and improve the soil’s capacity to retain water. With the last benefit being especially important in the face of climate change, which brings more unpredictable weather patterns, including both heavy rainfall and droughts. Humus acts like a sponge, absorbing water during heavy rains and storing up to five times its weight in water. This stored water can be a valuable resource during periods of drought. Applying only 48 tons of compost to a 1-hectare field every three years can raise soil humus levels by 0.1%, which increases the soil's water storage capacity by 30,000 litres. This reservoir of water is replenished each year during rainy seasons and can be used during dry spells. 

This is why the 4 per 1000 initiative, a global effort launched at the 2015 Paris Climate Summit, seeks to enhance soil fertility and combat climate change by increasing the humus content of soil. The initiative suggests that even a modest increase of 4 per 1000 (0.4%) in soil carbon could play a significant role in offsetting the rising carbon dioxide levels in the atmosphere. 

This is all the more important since in summer 2024 the UNESCO highlighted the troubling fact that 75% of the world’s soils are already degraded and in urgent need of restoration. And they pointed out that, without action, the percentage of soils unable to sustain productive agriculture could rise to 90% over the next 25 years. 

What is most tragic about this fact, is that it is precisely in areas where soil fertility is particularly low, such as sub-Saharan Africa, that the potential for producing organic fertilizer is greatest. This is because here up to 80% of the waste created in urban households, is organic and dumped on open landfills. Human excreta undergo here a similar fate. Mostly they are discarded on fields or open water bodies without any further treatment. However, instead of being dumped they can easily be converted into a safe and nutrient-rich fertilizer by hot composting. This is a low-tech method, which is also called thermophilic composting and widely known for its sanitizing properties due to process temperatures of up to 60 and even 70°C.

Transforming these neglected resources into valuable and urgently needed fertilizer, is thus an approach that is being propagated by more and more scientists and practitioners, not only for developing countries but as a worldwide applicable recycling and fertilizing strategy.

Want to learn more about soil, microbes and the contribution of (s)crap to climate change mitigation?

Lewis, W., & Lowenfels, J. (2010). Teaming with Microbes: The Organic Gardener's Guide to the Soil Food Web, Revised Edition (p. 220). Timber Press.

Jenkins, JC (2019): The humanure handbook. A guide to composting human manure. 4th edition. Grove City, PA: Jenkins Pub.