Soil testing: how to analyze the quality or safety of your soil
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Time to read 6 min
Summary
Introduction
Testing your soil through soil analysis offers numerous benefits, particularly if you want to verify that your soil is free of pollutants. It also allows you to assess its fertility level and maximize your chances of success in your vegetable garden.
Why test the soil?
Testing the soil in the garden, vegetable patch or before building
Soil testing is useful in the following main cases:
- If you're planning to design a garden with trees, grass, or shrubs, the type of soil will help you determine which plants will thrive best on your land. For example, for an orchard, loamy and/or sandy soil is ideal because it promotes good air and water circulation.
- For a vegetable garden, the type of soil (clay, silt, sand, or humus-rich) also plays a role. Consider the soil's acidity level (pH) and its nitrogen, phosphorus, and potassium (NPK) content as well. Depending on the results of the analysis, adding amendments may be beneficial.
- Before building a house on a plot of land, detecting pollutants may be necessary. This is especially important if the land has an industrial past (industrial record) or if it is a former agricultural area that has been rezoned for construction.
Identify the risks: pollution, acidity, soil unsuitable for cultivation
The first risk to identify? The potential presence of pollutants in your soil. Suspicions of pollution can be visible. This is the case if you observe traces of hydrocarbons, a pile of inert waste in a section of the land, or an area where nothing grows and the soil is particularly compacted. The major elements that can contaminate your soil are heavy metals (lead, arsenic, cadmium, zinc, nickel, etc.), hydrocarbons, particularly polycyclic aromatic hydrocarbons (PAHs), VOCs, pesticides, and PFAS, which are also pollutants found in water.
You can assess the soil's acidity level (pH). Ideally, it should be between 5.5 and 7 for optimal plant growth. Soil that is too acidic can be completely unsuitable for growing plants. The extensive use of synthetic nitrogen fertilizers such as ammonium nitrate, urea, or ammonium sulfate contributes to soil acidification.
Testing the soil in your garden or vegetable garden
Why test the soil?
Preparing a vegetable garden is a good reason to test your soil. Before you begin, you'll need to know your soil type and fertility level to determine which plants to choose. These tests can be done using simple methods like the soil sausage test or pH strips. However, nothing replaces laboratory analysis. To minimize costs, you can collect soil samples yourself before sending them in. And if you suspect contamination, it's best to get it checked to avoid consuming fruits or vegetables containing PFAS or heavy metals .
Before planting a tree or hedge, or to check soil fertility, testing the soil is a good idea. For example, roots will struggle to grow in overly compacted soil, and you will likely only be able to plant shrubs. Soil fertility is indicated by the presence of nitrogen, phosphorus, and potassium. These are the main elements that govern plant growth, flowering, and fruiting.
What can we measure?
The table below lists the main measurable elements in a soil test, as well as ideal values and related advantages and disadvantages, such as those associated with soil type.
| Element analyzed |
Ideal value |
Advantages and disadvantages |
pH |
Between 5.5 and 7 |
A pH that is too acidic (often below 5) is harmful, but so is one that is too high. In this case, it limits the absorption of iron by plants, causing yellowing leaves and reduced yield. |
Soil type |
Clayey, silty, sandy or humus-rich |
Clay soil absorbs water rather poorly, while sandy soil drains better. Depending on the nature of your soil, some plants will thrive better than others. For example, root vegetables grow well in sandy soil, while shrubs with decorative foliage, lettuce, tomatoes, eggplants, or peppers grow best in clay soil. |
Fertility level (NPK) |
The nitrogen, phosphorus, and potassium content should be in a balanced ratio, such as 10-10-10 or 15-15-15. If you have flowers or fruit trees, a higher nitrogen level than the other two elements is beneficial for flowering and fruiting. |
Elements not concerned |
| Organic matter (OM) content |
This rate is measured by the C/N ratio, which is the ratio between total organic carbon content and total nitrogen level in the soil. The optimal ratio is between 10 and 12. It primarily assesses soil mineralization. |
Elements not concerned |
Chemical equilibrium |
Specifically, the evaluation of CEC, or cation exchange capacity, is an indicator of soil fertility. A relatively high CEC is at least 12 milliequivalents per 100 grams of soil (meg/100g). Below 9, it is considered low. |
Item not concerned |
We list here the main elements that can be looked for when testing soil. One can also assess the level of total organic carbon or the presence of trace elements such as iron, manganese, copper, zinc, or boron.
Potential pollution: how to protect yourself?
To avoid potential pollution , refrain from using nitrogen fertilizers or pesticides in your garden. They deplete the soil, increasing its acidity and harming ecosystems. Opt instead for natural NPK fertilizers and sustainable techniques such as planting cover crops or nitrogen-fixing plants like alfalfa, lupin, nettle, or horsetail. Also, use compost to enrich the soil.
To identify potentially polluted land, several reference systems exist in France, such as the Soil Information Sectors (SIS). This map, produced by the State, includes land affected by pollution management measures implemented to protect safety, health, public hygiene, and the environment. The CASIAS (Areas of Industrial and Service Activities) map shows former industrial sites and service activities that may be the source of soil pollution. These maps allow for tracing the industrial history of a territory since the late 19th century.
How to take a soil sample?
Simple 4-step method
To take a soil sample, use this simple 4-step method:
- Collect soil from different locations on your land: garden, vegetable garden, near trees, in a fallow field… A minimum of three distinct locations provides an interesting overview of the state of your soil.
- Dig with a spade to a depth of approximately 20 to 30 centimeters. Avoid doing this during periods of drought, heavy rain, or frost. The ideal time is early autumn or spring.
- Mix the soil in a bucket to obtain a single sample. Remove branches, leaves, and large stones to obtain a homogeneous sample that is easier to analyze.
- Send your soil sample to the laboratory in a sealed bag. If you use a sampling kit, such as the one from Pouryère, you can send it by mail in a specially designed package. This method is less expensive than hiring a professional sampler to take core samples.
What tools can be used to test soil?
Commercial kits: pH, NPK test, etc.
In stores, online, or at garden centers, you can find kits for testing soil pH using test strips. You can also find electronic testers that you insert into the soil to measure acidity, moisture, or light levels. These tools provide a general idea but are not as precise as a laboratory test, mainly because the soil analysis depth is shallow.
It's also possible to test the chemical balance of your soil and determine its nitrogen, phosphorus, and potassium content. Here too, there are soil analyzer kits and more sophisticated versions with probes and a smartphone app for data interpretation.
Laboratory analysis (complex or contaminated cases)
Comprehensive laboratory analyses, with samples taken by an expert at your location, can be quite expensive. Costs range from a few hundred to nearly a thousand euros, depending on the number of samples taken and the quantity of elements analyzed. These analyses are generally reserved for those who wish to market their produce, such as farmers, winegrowers, market gardeners, or horticulturalists… or in cases of strong suspicion of pesticide, heavy metal, or PFAS pollution, to verify that the consumption of agricultural products is safe.
Professional services like those of Pouryère
Finally, there's a middle ground between at-home kits and full laboratory analyses for testing your soil. This is what Pouryère offers with its soil analysis kits, starting at under €100. You collect the samples yourself, but the analysis is carried out in a laboratory using the sample you send in.
