December 5^th  2024 marks the 10^th World Soil Day, the theme of which is “Caring for soils: measure, monitor, manage’. This year’s theme emphasises the importance of gathering accurate soil data and information to promote well-informed decisions on soil management. Understanding the quality, availability and health of soils is crucial to ensuring this resource is managed properly.

To mark World Soil Day this year, we’re looking at some of the research on soils in MDPI’s journals, exploring the varied ways it is and can be better managed, as well as the role soil will play in tackling problems such as food scarcity and climate change.

Underlining the importance of soil

Soil serves a number of useful purposes and is arguably overlooked as a vital resource. It is essential for growing food, carbon sequestration and storing groundwater. At a time when food shortages, climate change and water scarcity are major concerns for researchers and the world at large, better managing soil is crucial.

Soil management and measurement is not a recent concept; ancient civilisations are acknowledged for their soil engineering ingenuity. For example, the ancient Amazonians engineered their own soil. Known as terra preta, the soil was created by pre-Hispanic indigenous civilisations intentionally to improve crop yields, which has inspired modern-day approaches to soil management in the area.

Today, over 95% of our food comes from soil, making it an essential resource to bolster food security in the coming years. To meet global demand, agricultural production will have to increase by around 60% by 2050. The UN notes that up to 58% more food could be produced through sustainable soil management, which would help with increasing food supplies significantly.

Soils are also key to tackling climate change. Earth’s soils act as a massive carbon sink, with terrestrial ecosystems removing about 30% of the carbon dioxide emitted by human activity each year. Soil captures carbon dioxide from the atmosphere which is then turned into biomass through photosynthesis. Developing a deeper understanding of soil composition can help to better manage soil resources for plant growth and carbon capture as well as measure the way that human activity impacts soil health.

Human impact on soil

Soils are made up of billions of microorganisms; human activity has a considerable impact on their composition. One of the main elements deposited by human activity is nitrogen, which in excess has been found to lower the quality of soils and groundwater alongside affecting plant respiration. The primary cause of excess nitrogen deposition is agricultural fertilizer. To put into perspective the importance of considering the health of soil in agriculture, 37% of Earth’s surface or 13 billion hectares is currently used as agricultural land. Soil management is key in these areas to increase land-use efficiency and to ensure soils remain fertile.

Increased nitrogen deposition leads to an imbalance in the nitrogen to phosphorous ratio in soils which is required to maintain soil health. The process by which soils and water see increases in certain nutrients is known as eutrophication, which for soil is primarily caused by agricultural fertilizer. All the various elements found in soil contribute to the overall soil ‘solution’; imbalances can lead to changes in the pH balance and related changes that affect soil health.

Nitrogen deposition and eutrophication

The authors of a paper published in Microorganisms use 16s rRNA sequencing technology to analyse the effects of changing nitrogen–phosphorous ratios on soil microbes. Specifically, they look at the effects of the nitrogen–phosphorous ratio on both bulk soil and rhizosphere soil bacterial communities in coastal wetlands, using the Yellow River Delta, China, as their study area. Soils can be found in different textures and form dependent on the location; river deltas typically hold soils that consist of finer sediments that are usually clayey in nature.

“The Yellow River Delta, with its fragile habitats, is experiencing greater rates of N deposition, rapidly changing the N:P input–input ratio.” – Ma et al.

The rhizosphere is the area from which plants obtain their nutrients and into which they secrete waste products like carbon. Understanding how this type of soil might be affected is therefore key to managing and measuring plant health.

The authors note that changes to the nitrogen phosphorous ratio have been found to have both a positive and negative effect in various studies. Increased nitrogen has been found to increase carbon release from soils during microbial growth but has also been found to increase soil acidity which decreases the availability of essential nutrients and reduces bacterial community diversity.

In cases where nitrogen has a negative impact, phosphorous has been found to mitigate its negative effects.

Evaluating soil health

Overall, the authors found that increases in the N:P ratio had a positive effect in the Yellow River Delta.

They note that increased N uptake in plants may have limited the harmful effects of increased nitrogen input to the soil. The addition of phosphorous, which often only occurs through natural processes unlike with nitrogen, which is deposited in excess by human activity, may have counteracted the decrease in soil biodiversity caused by additional nitrogen.

They found that there were variations in the effects on bulk soil versus rhizosphere soil, demonstrating that nutrient addition has different effects on their respective bacterial communities.

Certain nutrients were found more abundantly in the rhizosphere soil, with Halomonas being amongst the main rhizobacteria detected, to provide an example. Halomonas increase soil permeability, improve soil structure and provide root and shoot-growth-promoting nutrients as well as increasing nutrient use efficiency.

Understanding soil composition and the bacterial communities that help plants to grow is crucial to supporting increased agricultural activity in future. The authors note that further field experiments are necessary to verify the effects of the nitrogen to phosphorous ratio on soil.

To learn more, you can access the study in MDPI’s journal Microorganisms.

Developing cost-effective management strategies

Soil is arguably poorly represented in the UN Sustainable Development Goals, as noted in a paper published in the journal Sustainability. Here, the authors highlight that soil is overlooked in policy on the international and statutory level, arguing the need for ‘specific ambitions on soil health’.

Much like water, soil is a valuable resource on which we rely on for the sustainment of life, including that of plants, other animals and humans. With 33% of soils today being degraded, and 24 billion tons of fertile soil being lost annually from agricultural systems worldwide, addressing soil in terms of the sustainable development goals and developing better ways to manage it are crucial.

Soil-related legislature

There have been steps in the right direction, with SDG 15 mentioning land degradation and 15.3 including a specific soil indicator. The authors also note that SDG 2 (zero hunger), SDG 3 (Good health and well-being), SDG 6 (Clean water and sanitation) and SDG 11 (Sustainable cities and communities), among others, relate to soil health.

Soil monitoring frameworks and guidelines do exist today (the authors specifically name the IPCC Guidelines for Greenhouse Gas Inventories) that include methodologies for tracking changes in soil composition. However, the authors make the case for more legislation specifically related to soil health.

There is a gap in the monitoring of soil on a statutory level too, unlike with air and water, despite its growing importance. The authors note the following:

“There is therefore a clear and important need for specific ambitions and accompanying metrics to fill existing knowledge gaps around soil if soil health is to be improved.”

Citizen science as an effective strategy

The authors suggest that citizen science soil health monitoring could be a useful avenue for monitoring and managing soil health, especially given the labour demands of such a project. Thus, in their paper, they provide a review of existing citizen science methods and platforms for soil health monitoring.

Briefly, citizen science is a practice which entails public participation in scientific research to increase scientific knowledge. They begin by conducting a review of citizen science soil monitoring methods reported in the literature.

Criteria for inclusion include the ability to be carried out by a non-expert with no or minimal training, that the method provides reliable data and the existence of interdependencies that interlink with other metrics to provide a comprehensive outlook of soil health.

They assess soil monitoring methods based on a few parameters, classifying each method as either red, amber or green in terms of a) time and repetition requirements, b) cost and required materials and c) strengths and limitations in terms of reliability/accuracy.

Looking to future management strategies

The review classifies 32 citizen soil monitoring methods that crossed the five indicators of soil as amber or green. The authors note that there ‘are a whole host of citizen science methods and toolkits that are appropriate for monitoring different aspects of soil health’.

While citizen science monitoring methods could be a useful supplement that fills knowledge gaps, it cannot replace efforts on the international and statutory level to better manage and monitor soil health.

Rather, citizen science methods and robust policy frameworks can act in tandem to support the protection and improvement of soil health globally.

Soil, the largest terrestrial storehouse of carbon

Climate change is one of the most pressing issues of the current era; greenhouse gas emissions are the main driver of this problem. Soil, in this instance, is indispensable in tackling such an enormous challenge.

As noted in an article published in the journal Soil Systems, soil plays an important role in carbon sequestration. To illustrate the point, the authors note that 8.7 gigatons (Gt) of carbon is emitted every year globally by anthropogenic sources, yet the atmospheric increase is approximately 3.8 Gt. Soil is responsible for a huge reduction in the increase in carbon emissions each year.

Understanding soil as a carbon sink

The review makes the case that with the need to mitigate the amount of greenhouse gases released into the atmosphere becoming more urgent, understanding the role that soil plays in removing greenhouse gases is crucial. Optimizing the carbon capture abilities of soils with different land-use purposes is key to developing resilience in the face of climate change.

The authors, in their study, ‘recognize the diverse roles of soil, while reasserting its well-documented significance in carbon sequestration.’ Although atmospheric CO2 can be dissolved in soil moisture, photosynthesis is the main method of transferring carbon to the soil. Briefly, the review looks at the impact of five key factors affecting the amount of organic carbon in soil, including the following:

• Climate conditions
• Topography
• Parent material
• Organisms
• Soil qualities

The authors note that small increases in soil organic carbon as a result of the removal of atmospheric CO2 by plants could mitigate the greenhouse effect. It would also lead to better soil health and greater fertility—both of which are essential for increasing agricultural production by 2050, thus helping to meet the global increase in demand for food.

Future research directions

Understanding the impact human activity has on soil and how to mitigate the negative impacts that arise from human activity is key to developing ways to better manage soil.

MDPI welcomes research on all topics related to soil and has dedicated a Special Issue in the journal Land to mark World Soil Day this year. To contribute to the global effort to better manage, measure and maintain our soil, we encourage researchers to submit to this Special Issue by 30^th April 2025.

Soil engineering and monitoring has taken place for centuries; supporting the publishing of research in this area will contribute to this year’s aim of caring for soils by proposing ways to measure, monitor and manage this essential resource. You can read more about the management of soil in agriculture, the role of citizen science in maintaining soil health and carbon sequestration in soil and much more in MDPI’s various journals, including Soil Systems.