Soils provide us with valuable services that are still too often invisible: it is thanks to them that many species—including our own—can feed themselves and even clothe themselves, thanks to textile crops. They are the physical basis on which a large part of terrestrial ecosystems—as well as human infrastructure—are built.

They provide incomparable ecosystem services. Not only do soils provide plants with the water and nutrients they need to grow, but they also help regulate the water cycle, between rain runoff and infiltration. As a result, they play a key role in mitigating the extent of flooding. Their function as carbon sinks also makes them valuable allies in decarbonization.

Soil provides an essential habitat for the survival of certain organisms such as microorganisms (bacteria, fungi, algae, protozoa, etc.) as well as animals of varying sizes (earthworms, arthropods, nematodes, etc.), all linked by a complex food chain.

These organisms are very numerous: a teaspoon of soil contains more living beings than there are humans on Earth! It is home to several hundred million bacteria, tens of thousands of fungi, hundreds of protozoa, and dozens of arthropods such as mites and springtails.

Under these conditions, healthy soil is soil that functions well, i.e., that provides a suitable habitat for all these living organisms. It offers them shelter and sustenance: a safe and constant roof over their heads and enough food to eat. Good soil health is an undeniable asset for agriculture in terms of fertility, production, and disease control. But how can it be measured simply?

From the blood sausage test to the holey underpants test

As researchers specializing in agriculture, we have seen it all: from the blood sausage test to the glass of water test, from the color to the smell of the soil, from the spade test to counting earthworms, there is plenty to choose from.

Let's mention one of these tests that has the merit of being simple, effective, and not without humor: the famous underwear test. To determine the health of soil, agricultural soil for example, you can bury a pair of untreated white cotton underwear, then dig it up a few months later to see what condition it is in.

The slip test is easy to interpret: rapid degradation is actually good news. Gabriela Braendle / Agroscope / University of Zurich

The interpretation is quick and easy: underpants found in relatively good condition are bad news, while underpants with holes in them are a sign of cellulose degradation in the cotton. The explanation is a little more complex: healthy soil is home to a wide variety of organisms, which enable it to function optimally, particularly in terms of the decomposition and mineralization of organic matter.
It is these soil functions that enable the recycling of nutrients necessary for plant growth. The cellulose in the underwear, for example, is an organic material. So if the underwear is in good condition when it is dug up, this indicates that it has not been degraded and that the soil is not performing its decomposition functions properly.

Tea bags, a benchmark test

To refine the analysis, there is another less publicized test: the tea bag test.

Simply bury tea bags of different compositions (green tea or rooibos), which are more or less “digestible” for soil organisms—and therefore more or less difficult to break down—and study the weight loss of the bags after a given period of time. If they have become lighter, it means they have lost material – and therefore that this material has been decomposed by soil organisms.

After all, a tea bag simply contains dead plant material inside a nylon mesh. This is good bait for decomposing organisms.

But not all teas are created equal: green tea is easier to break down, while rooibos is more woody and takes longer to decompose. Comparing the two therefore allows us to assess the extent to which the soil can break down different types of organic matter.

It may raise a smile, but it is actually a standardized method developed by researchers. Its protocol is available to anyone wishing to assess the degradation efficiency of a soil.

Not only does this provide valuable information on the soil's ability to decompose organic matter, but such a protocol makes the results comparable between different sites around the world, regardless of how the soils are managed.

Using this method, researchers from UniLaSalle and farmers in the Hauts-de-France region were able to demonstrate a higher level of degradation of organic matter in soils under conservation agriculture, a less intensive farming method than the conventional method.

Healthy soil is essential for farmers. In cases of poor decomposition of matter (i.e., intact underwear and tea bags that appear ready to be brewed despite several weeks in the soil), further laboratory analysis is useful to understand where the problem lies.

Soil analyses: invaluable for farmers

Soil analyses provide an overview of soil health and are therefore invaluable for farmers. These analyses can focus on several parameters and are carried out in the laboratory after sampling the soil from the plots.

They can measure:

• Soil texture, which provides information on the proportion of sand, silt, and clay. This is important because certain textures are favorable for some types of crops but not others. Regardless of their practices, farmers have no control over this parameter, but must take it into account when adapting their production and crops.
• The organic matter content, which corresponds to the proportion of substances resulting from the decomposition of plants, animals, and microorganisms present in the soil. A high content improves the soil's ability to retain nutrients and strengthens its structure. Farmers can increase it by regularly adding different types of organic matter, such as livestock manure, anaerobic digestion digestates, or by promoting the return of plant matter to the soil.
• The concentration of available nutrients in the soil (phosphorus, potassium, magnesium, etc.) provides information on whether or not there is enough to meet the needs of the crops being grown. A low concentration of a nutrient can be compensated for by adding mineral fertilizers or various organic materials which, when decomposed by soil organisms, will release that nutrient.
• Finally, the pH, which indicates the acidity of the soil, impacts plant development. For example, it can influence the availability of nutrients, but also the presence and activity of organisms that are beneficial to plants. Farmers can influence this parameter through various types of inputs, such as lime, which helps prevent the pH from becoming too acidic.

All this information helps guide farmers in their soil management. Soil analyses can be repeated over the years, in particular to monitor the condition of a plot according to the agricultural practices implemented.

New bioindicators currently being developed

It is clear that soil organisms are largely responsible for the proper functioning and health of the soil. However, they are sensitive to their environment and soil management practices. Therefore, in order to take their presence into account, new indicators based on soil life have emerged in recent years: these species are referred to as bioindicators, as their presence provides information about the ecological characteristics of the environment.

Of course, different bioindicators provide different information: the abundance and diversity of mesofauna (springtails and mites) provide information on the soil's ability to break down matter, while fungi provide information on the efficiency of nutrient recycling. The sensitivity of these organisms to agricultural practices makes them good bioindicators.

Other biological indicators are currently being tested, for example to assess the proper functioning of the carbon cycle and the nitrogen cycle.

These tools are not necessarily accessible to the majority of farmers, as they are currently too expensive. One area of research would therefore be to develop tests based on bioindicators that are easier to implement and interpret.

Coline Deveautour, Lecturer-Researcher in Soil Microbial Ecology, UniLaSalle; Anne-Maïmiti Dulaurent, Lecturer-Researcher in Animal Ecology and Agroecology, UniLaSalle and Marie-Pierre Bruyant, Plant Sciences, UniLaSalle

This article is republished from The Conversation under a Creative Commons license. Read the original article.