Soil fertility: rich or poor, how to tell?

What is soil fertility?

The functioning of soil is complex and the least vague definition of soil fertility could be as follows:

“The capacity of a soil to meet the physical, chemical, and biological needs necessary for plant growth, for their productivity, reproduction, and quality in a manner suited to the type of plant, the type of soil, land use, and climatic conditions.” (Abbot & Murphy: Soil biological fertility – 2003)

In short, soil fertility can be defined by these different factors:

• its humus content: humus, the upper layer of soil created by the decomposition of organic matter, provides structure to the soil and retains water;
• the depth of the soil: or more precisely, the subsoil, the layer beneath the arable land. It must be sufficient for the root development of plants;
• the drainage of the soil: compacted and suffocating soil does not allow for proper transport of gases, water, or nutrients;
• the level of assimilable nutrients: plants need nitrogen, carbon, and trace elements to grow and thrive. If the soil is deficient or if the elements are not assimilable due to soil compaction or excessive acidity, for example, plants will not be able to grow normally;
• its acidity level: most plants grow very well at a pH between 5.5 and 8;
• the life of the soil: insects, earthworms, microorganisms, mycorrhizae… all contribute to creating fertile soil. A dead soil where inhabitants are but a distant memory will not be fertile. Unless it is transformed into a chemical factory with industrial fertilizers…

How to tell if your soil is rich or poor?

Observing Your Plants

To determine if soil is fertile, there’s no need to engage in costly analyses. Initially, you can simply observe what is growing in your garden and how these plants are thriving. Observing your plants provides valuable insights regarding the richness of your soil. If your soil is rich in humus, your plants will grow quickly, with lush foliage, and without excessive diseases or pests.

Observing Adventive and Native Plants

Plants do not grow in a particular location by chance. Nitrophilous native plants will thrive in nitrogen-rich soils: nettles, fat hen, couch grass, dead nettles, annual mercury, hogweed, and white chickweed… If you see these plants in abundance in your garden, you can be assured that your soil is rich enough to support any plant.

Some adventive plants: nettle, white chickweed, and purple dead nettle

Conducting the Hydrogen Peroxide Test

A good way to assess the level of organic matter present in the soil is to perform a simple hydrogen peroxide test (H2O2), available at pharmacies.

There are two types of organic matter: stable humus, which degrades slowly, and fugitive organic matter, which degrades quickly, thus providing minerals to plants rapidly. Each soil contains these two types of organic matter in varying proportions.

To conduct the test:

Place a small amount of soil on a plate. Add a few drops of hydrogen peroxide to this soil. Three reactions are possible:

• The soil does not foam: presence of a lot of stable humus, but very little fugitive organic matter. You will need to stimulate mineralisation, that is, the degradation of organic matter into minerals, by providing oxygen to the soil fauna, for example, through soil scratching.
• The soil foams a lot: organic matter is mainly present in fugitive form. It degrades very quickly, and the C/N (Carbon/Nitrogen) ratio is unbalanced. You need to add carbon: twigs, straw, wood, RCW, untreated cardboard…
• The soil foams little: organic matter is more stable and thus retains more water. This is the best-case scenario. The C/N (Carbon/Nitrogen) ratio is perfectly balanced. The soil has good fertility and is quite stable.

Please note: The Carbon/Nitrogen ratio is an important concept to consider when discussing soil fertility. It indicates the speed of organic matter degradation. The higher the C/N ratio, the slower the organic matter degrades, but the humus or compost created is then very stable. This is why, for example, we alternate between layers of green waste (Nitrogen) and layers of brown waste (Carbon) in a compost heap.

How to make soil fertile?

Understanding Your Soil and Acting Accordingly

It is useless to fertilise soil that is already fertile, but whose structure prevents nutrients from reaching the roots of our plants. Therefore, the first step is to address structural issues, such as lightening the soil, before adding any hypothetical fertiliser.

→ Some guidance is provided in our advice sheet “Gardening in Heavy and Wet Soil“.

Moreover, soil that is too acidic does not allow for proper nutrient transport to the roots of plants: correcting this with a lime amendment can help. Conversely, soil that is too calcareous needs a good humus layer to become fertile again.

→ Read about this in these two articles “Gardening in Acidic Soil” and “Gardening in Calcareous Soil“.

Adding Organic Matter

We all hope to have humus-rich soil like in a forest. However, to create soil rich in humus, thus rich in nutrients and with a good structure that retains water without excess, we must “mimic” what happens in nature: this is precisely what agroforestry practitioners do by combining tree cultivation with more conventional agriculture, thereby protecting soils from degradation. The solution is to add organic matter to the soil: dead leaves, green waste… and well-matured compost, or even well-decomposed manure.

Well-decomposed manure

Green Manures and Other Soil Covers

Green manures are aptly named. These are plants that we sow to prevent soil from being left bare for too long: phacelia, rye, mustard, crimson clover… When we need the area for a crop, we simply cut the green manure and incorporate it into the soil.

Other soil covers: mulching with herbs, RCW, straw, dead leaves… in addition to protecting the soil, they will also be incorporated into the soil, but gradually by soil detritivores (earthworms, insects, fungi… feeding on organic matter).

One can also leave a depleted part of their soil fallow. Soon, plants will colonise the surface and over time contribute to making this area fertile.

Crop Rotation

In the case of a vegetable garden, crop rotation is an essential technique for maintaining soil fertility (among other things!). Leguminous plants (family Fabaceae) such as peas, broad beans, or beans are capable of fixing atmospheric nitrogen. They are, therefore, perfect to be grown just after a “greedy” nitrogen vegetable such as courgettes or other cucurbits.

Leguminous plants make excellent green manures!

Nota bene: Chemical inputs or amendments are not a good solution for addressing soil fertility issues in the medium to long term. The best approach is always to create a natural soil with natural elements: wood, RCW, dead leaves, organic waste…

Does a garden necessarily need to be cultivated in rich soil?

A poor soil is sometimes a blessing

Paradoxically, biodiversity in botany, and thus the resulting faunal biodiversity, is significantly reduced in rich soil. Indeed, in heavily amended or naturally rich soil, a short series of so-called nitrophilous plants (Cow Parsnips, Nettles…) will dominate over other species, reducing botanical diversity.

Some nature lovers work tirelessly or spend fortunes trying to impoverish part of their garden to create a high biodiversity area: this is called a biodiversity “Hotspot”. So if you love nature and have poor soil, well, pop the champagne! It will be an opportunity to establish a lean meadow or simply let nature take its course and wait for native flowers to come to you naturally.

A vegetable garden on poor soil?

Growing vegetables is a practice that requires a lot of nutrients to provide us with quality produce. Imagine what a courgette plant must “eat” to give you a vegetable as long as your arm in just a few days! The quickest solution if you have poor soil is to… not grow on it. Instead, favour alternative growing techniques: on mounds, in lasagne beds, or in a raised bed filled with potting soil. There are plenty of solutions…

Creating a lasagne mound for vegetable cultivation

Plants suited to poor soils

Trees and Bushes

Some trees or bushes thrive in poor soils: Scots pine, oak, juniper, broom, serviceberry, viburnum lantana, sea buckthorn, lavender…

Some trees and bushes suitable for poor soils: Broom, Sea Buckthorn and Oak

Don’t forget the Mediterranean herbs: thyme, savory, rosemary…

Perennials and Annuals

Many flowers can adapt to poor soils: cornflower, wallflower, cineraria, spurge, honeysuckle, mullein, Buenos Aires verbena, garden verbena, oregano… And even some grasses: stipa, fescue…

Some perennials and annuals suitable for poor soils: Cornflower, Mullein and Buenos Aires Verbena

In the Vegetable Garden and Orchard

Some vegetables can still be successfully grown: garlic, onion, shallot, lettuce, asparagus, orache and goosefoot. As for fruits, you might try strawberries, grapevines of course, and perhaps a somewhat forgotten fruit tree from our countryside, the Cornelian cherry.

Some vegetables and fruits suitable for poor soils: Onions, Grapevines and Cornelian Cherry