In the last years, on 5 December, World Soil Day invited us to reflect on an often undervalued yet vital asset , fundamental to our survival. The theme chosen for the 2024 edition is no exception in highlighting urgent challenges. FAO - which as part of the Global Soil Partnership (GSP) has the official mandate to facilitate the implementation of World Soil Day - chose ”Taking Care of Soil: Measure, Monitor, Manage” as the key message, reminding that sustainable soil management is essential to ensure food security.
Today, more than three quarters of Earth's surface is already degraded, and without concrete action more than 90% could follow the same fate by 2050. However, soil is not only a resource to protect for food and biodiversity: its “indissoluble” link with water, which nourishes and sustains it, must be at the core of any environmental management strategy. Soil health is water health. Therefore on this day, as the COP 16 Desertification negotiations are underway in Riyadh (Saudi Arabia), we propose an in-depth look at this link for our thematic channel The Water Observer. We will travel from chemistry to regeneration, passing through reforestation projects, such as the Great Green Wall in Africa.
The link between soil and water, towards the concept of “health”
We step on soil every day, yet the recognition of the importance of this ecosystem has changed in recent decades. In the 90s, agronomic quality was the main driver for classifying soil conditions: a good quality soil was the one capable of guaranteeing higher agricultural yields. Thus the focus was on the single function of substrate to produce food, fodder, fibre and biomass. Conversely, the emergence of the concept of ecosystem services and “soil health” allowed the gradual recognition of soil multifunctionality. In addition to supporting agricultural activities, soil provide fundamental (free) services: it filters water, regulates the hydrological cycle, stores carbon - contributing to the regulation of greenhouse gases - it is home to extraordinary biodiversity, often greater than what is visible on the surface.
“Soil and water are closely interconnected, as soil is an extremely complex ecosystem composed of three phases: the solid phase (mineral and organic), the gaseous phase (the telluric air) and the liquid phase, that is the soil water, more appropriately defined as soil solution. This liquid component is crucial because plants mainly absorb water from the soil. Consequently, the liquid solution plays a key role in plant growth and production,” explains to Renewable Matter Claudio Ciavatta, Professor of Agricultural Chemistry at the University of Bologna and board member of the Re Soil Foundation.
“Soil differs in his ability to retain water, which depends on its specific physical and chemical characteristics. For example, soil with a low water retention capacity is typically characterised by a looser, sand-rich texture. In contrast, clay soil has a much higher retention capacity. As an example, sandy soil can retain around 60-80 litres of water per cubic metre, while clay soil can exceed 200 litres per cubic metre. These significant differences illustrate the great variability between different soil types'.
The quality of a moving resource also depends on human intervention
Water in the soil is anything but stationary: it moves towards the deep layers under the influence of gravity or escapes through evaporation, directly or through plants. Soil solution is therefore not pure but contains soluble salts and ions. These elements are essential for plant nutrition through microorganisms, such as bacteria and fungi, that are widespread in the soil. It is said that in just one teaspoon of healthy soil there are more living organisms than people on Earth. The quality of water moving through the soil, including that which penetrates the deepest layers, is closely related to the quality characteristics of the soil itself. There is indeed a direct relationship between the liquid phase, the water in the soil, and the solid phase that makes it up.
”Water flowing through the soil may encounter different geological layers, such as limestone or carbonate formations, which can influence its composition. For example, calcium carbonate may dissolve to generate carbon dioxide, or the latter may dissolve due to gaseous mechanisms. Other types of water may contain sulphur, iron or even undesirable metals such as arsenic. In certain areas, water may naturally have a high concentration of arsenic (in Bangladesh, for example) showing poor quality for natural, rather than strictly anthropogenic, reasons.
“However, human activities can both improve and worsen water quality through agricultural practices and soil management,” Ciavatta continues. “Soil can retain water according to its characteristics and the agricultural practices adopted. Among these, practices like ploughing or spading aim to increase the so-called useful water reserve, i.e. the amount of water accessible to plants. This reserve lies between two limits: gravitational water, which moves downwards under the effect of gravity and is only temporarily available, and microscopic water, which lies beyond the permanent wilting point and cannot be absorbed by plants. Soil tillage can increase this useful water reserve, especially when combined with organic matter additions.”
A good state of aggregation and a well-defined soil structure improve the useful water reserve by increasing the organic matter content. This aggregates mineral particles, stabilise the structure and optimise the water-to-air ratio in the voids. Soil rich in organic matter retains more water for plants, while crop roots further contribute to soil improvement. However, in cultivated systems, it is up to man to act responsibly.
The importance of reforestation
Soil loss also concerns the deterioration of soil functions, in particular fertility. When soil loses its ability to retain water, the risks of erosion, flooding and stable vegetation cover loss increase. The scarcity of vegetation amplifies erosion, creating a vicious circle. Covering 41% of the Earth’s surface, drylands are home to some of the most degraded areas on the planet, according to IPBES and FAO. It is therefore important to rehabilitate the land, an ambitious but achievable challenge, to improve livelihoods and water resilience, protect biodiversity and capture carbon.
In Africa, for example, large-scale and ambitious initiatives are already in place, such as the Great Green Wall (GGW) that spans from Senegal to Djibouti in the Sahel region. Key targets of the GGW include restoring 100 million hectares of land, while another 130 million hectares are pledged by 34 countries within the AFR100. Both are to be completed by 2030. Additionally, the Pan-African Agenda on Ecosystem Restoration (that includes initiatives such as AFR100) pledges to restore 200 million hectares. These initiatives are part of the Bonn Challenge, which aims to restore a total of 350 million hectares globally by 2030.
10 million hectares per year would need to be restored along the GGW to reduce land degradation to zero by 2030. However, according to figures released by FAO, which kickstarted the Action Against Desertification programme to support the GGW, the rate of recovery currently stands at 1.9 million hectares per year.
“Through large scale land restoration for small scale farming, improved livelihood, climate change mitigation and reduced loss of biodiversity, with rural communities at the heart of the interventions, afforestation contributes to combating desertification in arid and semi-arid regions. The example of the Great Green Wall, an Africa-led initiative, has become a symbol of large-scale restoration in drylands. So far, with extensive interventions, it has restored around 4 million hectares of degraded agro-pastoral lands, created over 350 000 jobs, and significantly improved food security and climate resilience across the Sahel. This initiative showcases how landscape and ecosystem restoration can drive socio-economic transformation.”
A truly radical change to allow life to flourish will have to come from better preparing soil for rainwater harvesting through the implementation of mechanised deep ploughing of the soil at scale. This would give plants a better chance to survive and grow even in the harsh conditions of the arid areas of the Sahel.
This article is also available in Italian / Questo articolo è disponibile anche in italiano
Image: Alicia Christin Gerald, Unsplash