Agricultural soil analysis: a strategic lever to improve your yields

Soil analysis is crucial for optimal yields while preserving the fertility and long-term health of your land. This requires detailed analysis of soil structure, especially if you practice organic farming where chemical amendments are not permitted.

Adjust inputs (NPK, amendments, organic matter)

The primary benefit of a soil analysis? To detect potential deficiencies in your soil's composition, such as insufficient minerals for plant growth, a lack of organic matter, or poor drainage. This will help you determine whether you need to apply NPK fertilizers (nitrogen, phosphorus, and potassium), enrich your soil with organic matter, or make amendments, particularly to regulate its pH and acidity levels.

Optimizing farming practices

A better understanding of your soil's strengths and weaknesses will also help optimize your farming practices. These practices come into play at every stage of the plants' lives: soil preparation and tillage, planting, maintenance, irrigation and pest control, harvesting, and practices such as three-year crop rotation.

Preventing imbalances and yield losses

An agricultural soil analysis also identifies soil imbalances that can lead to yield losses. For example, excessively acidic soil (pH below 5.5 or 5) prevents plants from developing optimally. Some crops are particularly sensitive to this, such as barley, vegetables, and alfalfa.

Meeting regulatory requirements or certifications (organic, HVE…)

Finally, analyzing your soil helps you meet certain regulatory requirements, especially if you practice organic farming. Indeed, you are only permitted to use organic amendments such as compost or manure, as well as fertilizers, a list of which is available on the European Union website. Examples include:

• Peat whose use is limited to market gardening, floriculture, arboriculture and nurseries.
• Worm compost and insect droppings.
• Seaweed and seaweed products.
• Sawdust and wood shavings.
• Mollusc remains.
• Rock powders and clays…

Similarly, a farm certified as High Environmental Value (HVE), or aiming for this certification (which rewards operations that preserve ecosystems), must comply with certain standards. A soil analysis is therefore essential for a relevant assessment.

pH, CEC, nutrient content (NPK, trace elements)

pH is the fundamental measurement for all agricultural land. It should be between 5.5 and 7 for maximum yield. Nutrient content is also important, particularly phosphorus, nitrogen, and potassium, which are essential for plant growth. Other trace elements also play a role in this development. Examples include zinc and iron for photosynthesis, and copper and iron for respiration.

The analysis also serves to assess the CEC, or cation exchange capacity, of the soil. This indicator reflects the level of soil fertility and indicates whether certain enrichments are necessary. A sufficient CEC is considered to be between 12 and 15 milliequivalents (meg) per 100 grams of soil.

Organic matter content, humus

The organic matter (OM) content is the ratio of your soil's organic carbon content to its nitrogen content. This matter is composed of undecomposed plant and animal remains, as well as bacteria, fungi, and small living organisms such as nematodes. These tiny worms are generally invisible to the naked eye.

Humus is more readily found in humus-rich soil but can be present in all soil types. It offers good water and nutrient absorption, which is beneficial for plant growth. It is formed from the decomposition of plants and soil animals, such as earthworms.

Soil structure, compaction, salinity

Soil structure is also important, if only to adapt the types of crops to the soil type. There are four main soil structures and numerous variations that can combine several of them. These are clay, silt, sand, and humus-rich soils. Clay soil retains water easily, while sandy soil needs irrigation because it is very absorbent. Root vegetables like carrots or asparagus are better suited to sandy soil, while lettuce, leeks, tomatoes, and eggplants thrive particularly well in clay soil.

The level of compaction must also be analyzed. Very compacted soil has difficulty draining water, roots struggle to develop, and nutrient exchange is reduced. This compaction is often linked to soil that has been extensively tilled with agricultural machinery. Plough pans have formed, rendering part of the soil sterile.

Finally, salinity levels are also important in assessing agricultural soil. This can be visible to the naked eye: the soil appears white due to excessive levels of sodium, potassium, magnesium, calcium, or chlorine. This phenomenon is exacerbated by irrigation, drought, or human activities such as wastewater treatment, desalination, winter road salting, or potash mining.

Presence of chemical residues or pollutants

The presence of chemical residues or pollutants must also be detected, especially if you decide to convert some of your plots to organic or sustainable agriculture, or if you have just acquired a farm or new land to cultivate. In an agricultural setting, the primary focus is on detecting the presence of pesticides or heavy metals. However, if the site is located on a former industrial brownfield, the presence of PFAS, hydrocarbons such as PAHs, or VOCs may also be detected.

Step 1: Sampling

In agricultural soil, the sampling depth should be at least 20 to 25 cm, which corresponds to the maximum depth of tillage. Perform an analysis on 5 to 10 hectares. The coring method is preferred and therefore, in most cases, requires the intervention of a professional technician. To minimize costs, you can also carry out the operation yourself using a tool such as a spade. This is possible if the number of samples is small and if you have the time to do so. It is advisable to remove large stones, larger than 2 cm, as well as pieces of wood and leaves.

Step 2: Sending to the specialized laboratory

Next, the samples are sent to an accredited laboratory for analysis. Choose an organization with accreditation, such as Cofrac, which guarantees its technical expertise and impartiality. This ensures you have an independent, neutral, and reliable analysis to guide your crops and improve your soil.

Step 3: Analysis and interpretation of the results

The laboratory analyzes various criteria and establishes a complete assessment of the condition of your agricultural soil. The following elements are specifically examined:

• Soil texture and particle size.
• Acid-base status and pH.
• Organic and biological state with the measurement of total organic carbon.
• Chemical equilibrium with CEC measurement.
• The trace elements are iron, manganese, copper, zinc and boron.
• The content of major elements such as phosphorus.

You can also request testing for heavy metals or specific pollutants, such as PFAS, in your samples. This is particularly relevant if you observe visible signs, traces, or areas of barren soil, if you are converting to organic farming, or if your farm is located near industrial sites.

If you have your agricultural soil analyzed by a laboratory, you will receive an explanation of the results. And, sometimes, advice on how to improve your soil.

What do the results mean?

Results are generally provided based on target values. For example, for pH, a value below 5 to 5.5 is considered acidic. For some metrics, there is no ideal value. This is the case for soil texture, each having its own advantages and disadvantages. Clay soil retains water more easily for plant growth but is susceptible to ground movement. Sandy soil drains well, is less prone to flooding, but retains fewer nutrients beneficial to plant growth.

Agronomic reading: thresholds, deficiencies, toxicities

The table below gives some normal values ​​for the most important indicators:

This list is not exhaustive and there may be more values ​​if you are looking for other components.

What should be done depending on the type of crop?

Depending on the type of crop, the work involved may vary. This can include crop rotation and leaving certain areas fallow to allow the soil to rest. Adding lime or NPK amendments can be beneficial, as can planting cover crops, which will enrich the organic matter and nourish the growing plants. It can also be advantageous to minimize soil disturbance beforehand and to use direct seeding, a technique that eliminates the need for compaction, aeration, or soil preparation.

Cost of a simple vs. complete analysis

A simple analysis can cost as little as a few hundred euros, especially if you collect the soil samples yourself. You will only pay for the laboratory analysis and interpretation of the results.

A complete agricultural soil analysis can cost up to several thousand euros, especially if the area is large, you use core sampling and you want to analyze many indicators, both related to fertility and to possible pollution.

Prices for large agricultural areas

For a large agricultural area, the price will depend on the number of samples to be analyzed and the quantity of indicators measured. It must include the technician's travel expenses, the time taken for core sampling, and the analysis itself. Unlike an analysis for a private individual, an analysis for agricultural land is not performed on just a single sample. Indeed, if you have several cultivated plots with different plants and trees, it is beneficial to have the results for each section and to tailor corrective measures accordingly.

Are there any grants or subsidies available for agricultural soil analysis?

There is no specific financial aid for soil analysis. Some regions provide subsidies to support conversion to organic farming or the preservation of ecosystems: permanent soil cover, reduction of mechanical work or extension of crop rotation periods.