Soil is the interface between the biosphere, the lithosphere, the atmosphere and the hydrosphere. It thus forms a continuum of variable thickness that constitutes the building cover of most of the Earth’s surface. Soil’s three phases (solid, liquid and gas), chemical composition, and physical properties determine its type.
Soils are important because they perform essential environmental functions by maintaining biogeochemical and organic matter cycles and providing food and other ecosystem services.
It is crucial to protect the soil because its degradation leads to a loss of productivity, water pollution, reduced air quality, and increased emission of trace gases into the atmosphere. These prejudicial effects on both soil properties and soil mass decline lead to less production in the short term and infertility and desertification in the long run.
THE ROLE OF ORGANIC MATTER IN SOIL
In what follows, we will talk about a fundamental component of soil: organic matter. Why is it so important? Basically, because of its functions in soil systems:
- Soil structuring: water circulation, root penetration, aeration, resistance to incrustation and erosion.
- Ion adsorption and exchange: organic matter takes part in the biogeochemical cycling of nutrients, formation of complexes, and pH control.
- Water retention and supply.
- Stimulation of biological activity: organic matter provides energy and nutrients for soil flora and fauna (edaphic fauna).
For biologists, organic matter is synonymous with substances containing the chemical element carbon (C), which is essential for all organisms, and mindlessly altering them would cause a global disaster.
THE CARBONIATO* EXPERIMENT
The following paragraphs describe the innovative research carried out at El Baix Llobregat Agrarian Park, with the participation of Olivier Chantry, a farmer at El Baix Llobregat Agrarian Park and Joan Romanyà, a soil biologist at the Research Institute of Nutrition and Food Safety (INSA). Much of the information comes from the following video (in Catalan), which we invite you to watch.
Soil that is poor in organic matter affects plant growth. In particular, since farmers apply mineral fertilizers, these plants grow few roots because they don’t need extended root systems to explore the soil in search of nutrients. As a result, plants develop little root volume and depth.
Farmers treat soils poor in organic matter with amounts of fertilisers higher than usual. Where they don’t apply organic residues, the functional part of the soil ends up reduced to a small portion of the total usable soil thickness (only the first centimetres or fifteen in the best case). Moreover, for ecological production, it is important to enrich the soil with the addition of organic matter since the fertility of the soil is seriously compromised because it does not use chemical fertilisers.
* From the Catalan carboni (carbon) and boniato (sweet potato).
What is carboniato?
The application of carbon-rich plant materials is a technique that allows a rapid increase in organic matter content and, at the same time, stimulates the activity of soil microorganisms. In this experiment carried out by the UB, INSA, and the Can Notario team, a large amount of organic matter was applied to the soil with favourable results because they made use of nitrogen-poor plant remains.
And what is the problem with nitrogen?
Applying nitrogen-poor products is a way of considerably increasing organic matter. However, there is a downside: nitrogen-poor products stimulate soil life (edaphic fauna), which also needs nutrients, including nitrogen. Consequently, the plant may experience nitrogen deficiencies for about six months, making harvesting impossible during that period.
So, to avoid yield losses, farmers need to manage these practices that cause transient nitrogen stress in plants. Solutions comprise the application of additional nitrogen or facilitating biological nitrogen fixation.
In the second tactic, nitrogen fixation occurs thanks to a symbiosis between diazotrophic microorganisms (bacteria and archaea that convert atmospheric nitrogen gas into a more biologically usable form, such as ammonium) and the plant, in the zone of the absorbent root hairs (rhizosphere), inside the plants (endophytic nitrogen fixation) or in the soil itself.
The carboniato experience uses sweet potatoes to increase the amount of organic matter in the soil in half a year and, at the same time, solve the problem of nitrogen deficiency because these plants grow very well in nitrogen-poor soils and have a symbiotic capacity with some micro-organisms to obtain nitrogen from the atmosphere.
What is the aim of the experiment?
The idea is to arrive at a farming system that doesn’t need working the soil (by providing only carbon!), with the same production results but without having to wait two or three years to regenerate soil fertility.
In more academic terms, the main objective of the carboniato experiment is to verify whether the following hypothesis is true or not: has sweet potato production been penalised by the lack of nitrogen caused by the addition of 150 tonnes of organic matter (wood and pruning residues) top the soil?
How does the experiment works?
The field research began in May 2021: at each point of the 16 existing experimental plots, participants took control soil samples. Samples were again collected once every two or three weeks. The purpose was to monitor the variations in the amount of nitrogen (plant food) and soluble organic carbon (food for microorganisms). Therefore, with these soil samples, both substances are measured in the laboratory and analysed afterwards.
This monitoring to assess improvements in such a short period is ongoing. So far, the measurements obtained are encouraging because sweet potatoes have grown well, and the soil is behaving well under irrigation. It holds moisture better, retains more water, and seems to have stimulated the proliferation of microorganisms, as indicated by mushrooms growing during cultivation. In addition, as of the time of writing, there has been no loss of production, which means that sweet potatoes are adapting well to this nitrogen-poor situation.
In addition to collecting soil, participants took samples from sweet potato leaves and surrounding weeds for isotope analysis. Nitrogen from the soil absorbed by the plant through the roots is rich in nitrogen 15. On the other hand, atmospheric nitrogen (captured by symbiotic microorganisms) is poor in nitrogen 15.
Discovering differences in nitrogen between the sweet potatoes grown without soil nitrogen and those grown under more normal conditions (with soil nitrogen) would indicate that some treatments allowed the integration of atmospheric nitrogen inputs into the system. The good news is the use of services provided by microorganisms to improve nitrogen-poor vegetable crops, such as sweet potatoes.
Why is it important?
On the one hand, there is the idea in organic farming to look for fertility in closed cycles and not in the fertiliser industry: to recover soil fertility by recycling organic compounds.
On the other hand, it is essential to improve soil conditions, i.e. to facilitate rooting and to make the soil larger in volume so that plants are more resilient in drought spells.
And finally, climate change must be addressed, as extreme weather events are and will be more recurrent, such as extended droughts or heavy rains. Soil and plants have a greater capacity to respond to such phenomena in the abundance of organic matter; more specifically: there is more and better infiltration, better water retention in times of drought, less damage caused by drowning of roots, more edaphic fauna, more healthy crops and, therefore, fewer disease and pest problems.
As well as improving the land and helping prepare for climate change, we must emphasise the role of organic soils as carbon reservoirs. Rich soils help remove carbon from the atmosphere and retain it for a long time.
We must consider that the agricultural sector is already suffering the first effects of climate change, for example, on harvests. In the words of Olivier Chantry: the forecasts are disastrous, and we need to start building alliances, which is why we need to work together with farmers, researchers and the consumer community.