Soil Testing

Soil analysis allows understanding the structure and fertility of your soil through 4 parameters: texture, acidity, the organic profile and the mineral state of the earth. Laboratory results help to manage fertilization, adapt agricultural practices, identify the type of soil tillage to adopt, and optimize your cropping strategy. Thus, soil analysis helps to improve yields and save fuel, fertilizers, phytosanitary products, etc.

Assessment of the homogeneous study zone

To approach the diagnosis of your soil, it is important to identify the variations in the terrain (present vegetation, moisture level, leveling, etc.) on your land to segment it into several homogeneous zones and make soil analyses relevant.

A homogeneous study zone is characterized by a uniform spontaneous flora present on the plot, as well as a comparable soil appearance. Simply pushing the spade in allows feeling soil variations in space.

General data of the land

Once your land has been segmented into homogeneous zones, it is necessary to gather a set of data to support the understanding/analysis of each of these zones and their agronomic adaptation :

Observation date
Location
Parent rock (if known)

The parent rock is the surface layer of the earth's crust whose weathering contributes to soil formation. It also determines the water infiltration capacity of the soil (depending on whether it is permeable or impermeable). It can be of different types: limestone, chalk, marl, flint, sandstone, clay, schist, granite, or gypsum. The BRGM website provides information on the subsoil. Sometimes explicit (clay), sometimes difficult to understand due to specialized vocabulary.

Screenshot of the Infoterre site showing the subsoil description "Plateau silts"

To access the service, click on the right on “add a layer”, then select the “printed geological map 1/50 000 BRGM”. Then click on the “i” icon at the top to get the description. By clicking on the legend, you access the different types of local subsoils available.

Extract from the geological map legends of the Infoterre site

More information is available by downloading the notice in pdf format.

Height of spontaneous biomass growth

The growth height of the biomass present on the plot is an excellent indicator of the amount of OM present in the soil. It is not the height of the tallest blade on the plot but the maximum height where the grass is dense. Referring to the following table, you can estimate this amount without laboratory analysis : if the total biomass present exceeds 10 tons

of DM/ha at maximum growth (June or even autumn), the soil is clearly very fertile : it grows by itself!

Visual estimation of cover biomass (in tons of DM/ha)

Be careful, this measurement cannot be done if the soil has been recently worked or if the grassland has been mowed. You can then assess the potential of the site by observing field edges or by letting the grass reinvade the plot (which will take more time...).

If observing outside the maximum growth periods, you must adjust your growth state evaluation according to the period. For example, a few centimeters of growth at the end of April indicates poor fertility potential. Very bright green grass, broad leaves, and a height of about ten centimeters in March is a good sign.

Land history

Land history refers to the set of agricultural practices prior to the soil diagnosis. Each agricultural practice or land management can have positive or negative consequences on the state of the land. The most recent practices will have the greatest impact on the soil state at the time of diagnosis.

Worked soil : For example, soil worked in the last three years is considered to have an OM content of about 2 %. The management history of the grassland is also important. Only plots with very high organic matter inputs (large quantities of manure) or no-till under cover crops for several years have OM levels above 3 %.

Grassland : In France, grassland has an average OM content of 3 %. This rate varies greatly depending on the age of the grassland or its grazing method. Thus, overgrazed grassland (almost constant presence of animals) or undergrazed (mowing every 2 years) will have a lower OM content : 2 to 3 %. The rainy regions with good grazing methods have higher OM levels between 4 and 8 % OM. In mountain areas, grasslands can have OM levels between 10 and 15 % due to cold causing low biological activity and thus low mineralization of organic matter. The passage of mechanized equipment, or frequent grazing are determining factors in compaction of the surface layer of the plot as well as in the risk of erosion.

Getting information on your land's history

There are different ways to learn about your land's history. The first is to talk with the previous owner if possible, and also with neighbors, so they can tell you about recent agricultural practices. The site Remonter le temps allows you to observe the evolution of your plot by aerial and satellite views. Besides being easy to use, it is very informative.

Soil profile

Spade test

In addition to the history and initial visual evaluation of the plot, it is necessary to assess your soil profile to get familiar with it. In a profile, one seeks to know, for example by a spade test, the depth of the soil layer, the nature of the parent rock, the soil texture (clay/silt/sand) and its structure.

Detailed article : Réaliser un test bêche - VESS

Thickness

We call soil thickness the height of soil where one can push a spade without too much difficulty. This is to check the viability of the soil for establishing a vegetable crop. Although root systems of each vegetable variety have varied characteristics (architecture and rooting depth : fibrous, taproot, spreading, etc.), the minimum suitable soil thickness for vegetable cultivation is about 15 cm. Therefore, it is not necessary in a first observation to dig beyond 40 cm depth (depth allowing detection of potential plow pans).

Texture

Soil texture indicates the distribution of mineral particles in the soil classified according to their size. Soil texture is the proportion of constituents according to 3 components: sand (greater than 50 mm), silt (between 2 and 50 mm), and clay (less than 2 mm) which allows assigning a texture to your soil placed in a texture triangle on the next page.

Depending on the proportion of each class, we distinguish several soil types, some specific to a region:

Sandy soil
Silty soil (0-10% clay)
Clay soil (25-40% clay)
Humiferous soil (10% or more humus)

Soil texture is relatively stable over time. It is decisive for the overall functioning of the soil since it directly conditions its structure, porosity, water regime. A large part of its fertility through clays, during the formation of the clay-humus complex also depends on it.

Analyzing your texture

The settling test (or jar test) gives a fairly precise idea of the soil texture. Take a one-liter jar and fill it two-thirds with water. Take a soil sample and sieve it. Add the soil to the jar so that the water reaches the top of the jar. Close, shake well and let it settle. After an hour or several days, you will see the different layers appear.

Characteristic textures

Soil texture is rarely unique; it is a mixture of clay, sand, and silt. To determine it, the texture triangle is used. For example, a soil composed of 70% silt, 60% sand, and 50% clay is a sandy-silty-clay soil.

Sandy soils : mainly composed of quartz and minerals from rock weathering, they are granular and abrasive. These soils retain little water and nutrients but are less prone to compaction and allow good rooting of crops. –› To detect them : they tend to crunch under the finger or crack under the teeth.
Silty soils : composed of elements from rock weathering, they contain rich elements but are particularly sensitive to compaction and prone to forming a crust of surface sealing. –› To detect them : the finger leaves a clear fingerprint.
Clay soils : composed of granulometric and mineralogical clays. In large quantities, they can cause difficulties such as the formation of an impermeable soil horizon, heavy and sticky. –› To detect them : the soil forms a roll at more than 25% clay and does not break beyond 40% clay.

Impact on crops

Depending on soil texture, some crops develop better or worse. For example, the carrot from sandy soils is adapted to a light, well-drained sandy soil which allows it to keep a straight shape and avoid diseases. Some textures are also exposed to risks. A silty soil is less stable and thus more exposed to water erosion and surface sealing crust formation. This is why ACS recommends establishing a cover crop which improves soil structure and protects it from mechanical erosion by rain.

Structure

Soil structure is the spatial arrangement of its elementary constituents, organized or not into aggregates. Soil structure must allow:

water and gas exchanges
promote quality of plant rooting
optimize biological activity

If these functions are not fulfilled, the soil risks erosion. Improving physical fertility is central in conservation agriculture.

There are several tests you can perform yourself to evaluate your soil structure, a crucial indicator of soil condition:

The VESS test

It is a visual evaluation of soil structure, developed by Pascal Boivin, it is a spade test that consists of extracting a block of at least 20 cm thick whose horizons are studied. The test characterizes soil structure with a score from 1 to 5. The evaluation protocol is detailed in the video Visual evaluation of soil structure – VESS method. The evaluation can be done with the naked eye by comparing to the reference sheets provided with the method, but also using the VESS app available on Google Play.

The structure of a recently worked soil is good, so it is not very informative. But it is a mechanical structure, which tends not to last over time. It is therefore interesting to evaluate the structure below the tillage horizon as well as the evolution of the structure 6 to 9 months after the last soil work. An untilled soil rated 1 or 2 indicates a crumbly and fertile structure. If the score is lower, care must be taken to prevent further degradation. Structure can be improved by regular carbon inputs (20 T DM/ha/year) and even better by roots of vigorous plants. While it helps understand your soil structure, the VESS test also gives an idea of its porosity (an important point reflecting soil health, as a soil without aggregates is inhabited by a larger root system). It also allows observing soil color, smell, and taste as well as giving an overview of biological life, notably the presence of earthworms (a superficial shovel of soil should contain 2 to 4 but the more there are, the better).

You can see soils rich in OM with poor structure (4 or 5) but excellent biological porosity. Be careful though

of waterlogging of these soils which can be avoided by removing organic mulches (plastic sheeting rather has a positive warming effect)

but especially by using cover crops with powerful root systems.

The slake test

Detailed article : Slake test

Organic matter (OM) content

Organic matter (OM) in soils is a pillar of fertility. OM refers to what is found in the surface layer of the soil (0-30 cm) and represents about 1 to 10% of soil mass. It includes all dead or living organic constituents of animal, plant, or microbial origin present in the soil.

The organic matter content is a basic element allowing management of organic fertilization. To conserve and enrich your soil in OM, it is advised to limit soil tillage, cover the soil, and regularly return plant biomass. For example, you can use Reduced Tillage or No-till.

By compiling: history, structure, and spontaneous biomass, you can approximately estimate the OM content using the following table :

Observations	Estimated OM content
Worked soil, poor spontaneous grass growth, poor structure	1 %
Worked soil, poor spontaneous grass growth, good structure a few months after soil work	1.5 - 2.5 %
Average grass growth, good structure, reduced soil work, poorly managed grassland	2.5 - 3.5 %
Very good grass growth, good structure, well-managed grassland	4 - 6 %
Very good grass growth, good structure, well-managed grassland, abundant water or mountain	6 - 9 %

A soil rich in organic matter is more resilient

Note : it is possible to cultivate with an OM content below 1% on small surfaces and with a lot of OM available to start the system : these are deserts, fill zones, extremely stony soils, concrete slabs, etc. They require real revival.

Detailed article : Fertility management strategy

The electrical model of soil according to Olivier Husson

A field can be seen as a electrical system where the size of the battery would be the amount of organic matter and its charge level would be the biological activity of the soil. The bigger the battery, the more biological activity can develop and easily supply the plant's needs. Below 3.5% OM, your battery may not start every morning...

Organic matter content does not inform about the quality of soil OM!

Soil acidity analysis

The soil pH expresses the degree of acidity or alkalinity of a soil on a scale from 0 (very acidic) to 14 (very alkaline or basic).

Soil pH is used to detect a deficiency or a toxic element. It also characterizes the Cation Exchange Capacity (CEC), i.e., the amount of exchangeable cations plants can use to grow.

Soil acidity impacts crop development and yield. For example, some soluble forms of aluminum can become toxic when pH is below 5.5. This phenomenon is called aluminum toxicity and is the main cause of yield loss in acidic soils.

Liming helps bring the pH back to a range favorable for plant development and soil life. This improves organic matter decomposition and modifies the soil organic profile.

pH management is central because it conditions the blocking of certain elements and the biological activity that transforms organic matter.

On a tilled soil, the acidity of fertilizers is “diluted” in the top 30 cm. But in RT, all the acidity accumulates on the surface. To prevent this, small lime applications are needed rather than massive ones to restore a neutral pH.

Mineral Soil Analysis

The elements that characterize the mineral state of the soil are:

phosphorus (P)
potassium (K)
magnesium (Mg)
calcium (Ca)
iron (Fe)
copper (Cu)
manganese (Mn)
boron (B)
silica (Si)
sulfur (S)

‍ Each element occupies a specific place in the development of a crop and is essential for the good health of the plant. A deficiency in one element can prevent other elements, even if present in the soil, from being available to the plant and make it ill. It is important to characterize all the elements present to avoid deficiencies.

Mineral element	Very sensitive crop
Iron	Vine, pea, soybean
Manganese	wheat, barley, oat sorghum, beet, pea, soybean
Zinc	corn, flax, bean
Copper	Wheat, barley, oat
Boron	Beet, alfalfa, sunflower
Molybdenum	Alfalfa, pea, clover

Sulfur is necessary for protein synthesis, magnesium is an essential component of chlorophyll allowing the plant to perform photosynthesis. A sulfur deficiency causes chlorosis.

Studying these 4 parameters helps to understand how your soil works and how the crops you plant react. Besides providing an overview of the health of your soil, a soil analysis is essential to take action. It is then a matter of characterizing the fertility of your soil and adapting your practices to optimize your yield.

How to perform a soil analysis?

The sample must best reflect the state of the plot to provide all necessary information. Sampling errors are often the biggest source of error in soil analyses. You can geolocate the point to resample the same area in a few years. There are free apps for that.

When to sample?

Soil biological activity depends on temperature, the disturbance rate linked to tillage, organic inputs or the sampling period during the year.

It is preferable to sample in autumn because the soil is still warm and the biological activity is strong under the vegetation cover. This also allows to anticipate winter inputs and plan fertilization strategies.
Ideally, soil should be sampled at least 2 months after fertilizer application, 6 months after liming amendments and one month after the last tillage. Otherwise, the analysis may be biased!

How to sample?

Two sampling methods can be used:

in a circle for heterogeneous plots.
diagonally for homogeneous plots.

5 steps to perform sampling for soil analysis:

Target the most representative part of the plot, or the most problematic, depending on your objective.
Mark a point in the plot in the middle of the area you want to sample.
From this point, take 10 to 15 samples within a radius of 6 to 10 meters.
Avoid edges of plots or sprayer tracks.
Each sample is taken from 0 to 15 cm depth, which is important to ensure analysis quality.

The Agroleague method

This method uses biological and chemical fertility parameters of the soil, taking into account organic matter, CEC, biological activity, etc. It allows a good understanding of nitrogen in the soil, thus establishing adapted fertilization plans.

The protocol is detailed in a "Soil Analysis" kit that farmers can order from AgroLeague.

Once samples are taken, mix them in a bucket, fill the analysis kit bag halfway, drop the kit at a Colissimo relay point or post office. The samples are sent to the laboratory which will analyze the soil of your plot.

Considering that no French laboratory protocol effectively reported the overall soil health, taking into account the complexity and diversity of situations, AgroLeague decided, for its analysis, to partner with a laboratory in the United States, which uses the Haney test, a globally recognized analysis technique as the most relevant to know the health status of a soil.

The Haney test allows to:

Measure the biological activity of the plot.
Identify limiting factors to unlock biological activity.
Measure the nitrogen released by the plot to adapt nitrogen doses.
Establish a complete profile of macro and trace elements available for the crop.

Laboratory analysis

In France, accredited soil analysis laboratories are defined each year by decree published in the Official Journal, available on Légifrance. Several laboratories offer different soil analysis methods. Some focus on chemical fertility (study of soil chemical balances and crop nutrition) like the Albrecht-Kinsey method. But in this case, biological fertility is left aside. Soil analyses that take biological fertility into account are very expensive and few data are available to interpret the results.

In addition to field analyses, a laboratory soil analysis will allow you to make a definitive diagnosis. An analysis costs on average between 40 and 80 euros, it estimates the OM content, the soil texture and the charge level of various mineral and trace elements. The soil carbon/nitrogen ratio (C/N) is normally 10. If higher, it indicates ongoing degradation of the organic matter. In this case, the OM content is systematically overestimated : you must wait at least 6 months for the materials to degrade and to correctly read the OM content. Regarding pH, when the soil has an OM content around 3.5-4%, the pH is buffered around 6-7.5.

A soil analysis only measures leachable available elements, since the soil is “washed” in the laboratory and only elements in the “rinse” solution are measured. However, leachable elements do not correspond to all available elements, for example biological activity also provides elements (fixing symbioses, etc.). Therefore, soil analysis does not allow to know the quantities of available elements (nitrogen, phosphorus, etc.). Care must also be taken to thoroughly mix the different topsoil horizons when sampling soil for laboratory analysis. Knowing that a well-proportioned organic matter input provides all minerals necessary for plant needs, the OM content is therefore the main focus of your soil analysis.

Reading a laboratory analysis

Note that in the majority of the GIEE MSV Normandy group, indicators are all in the green : soils rich in organic matter guarantee good results! Life will restore small imbalances. The important thing remains to check that soil life is active : temperature, air, water and toxic substances (heavy metals - hydrocarbons - etc.) are the parameters of the fertility equation. The variable is the quantity and quality of organic matter inputs.

Physical and soil constitution analysis

Cation Exchange Capacity (CEC) : It represents the size of the chemical element storage space in the soil. It depends on the clay content but especially on the organic matter content.
Organic matter & carbon content : measurement of carbon present in the soil by oxidation. Then the rate is calculated as follows : OM = 1.72 x C.
Texture : the clay/silt/sand ratio gives the temperament of your soil. Soil life differs according to texture, hence the need to adapt fertilization strategy. Sandy soil contains on average fewer earthworms than silty soil and stores less organic matter.
Kjeldahl nitrogen : organic nitrogen contained in our soil, very different from rapidly mobilizable nitrogen. Nevertheless, this figure gives an idea of the stock.
C/N : a too high C/N indicates that the decomposition chain does not go to completion. This ratio is usually between 8 and 15. Already digested compost averages 10.

Chemical analyses/chemical soil fertility

PH: measured after soil solution preparation. pH varies throughout the seasons, soil moisture...

and distance to the root. The different species present determine the pH.

Cation distribution on the CEC

The important factor is not the absolute concentration value of nutrients but the distribution on the CEC. "The fridge is full, but of what?" A balance is sought in the distribution of K/ Mg/Ca.
Here are the critical thresholds for each base depending on soil type :

Ideal saturation rates of cations adsorbed on the clay-humus complex. Source: Doucet, 2006. p.329^[1]
Soil categories	Ideal % saturation
Soil categories	Ca²⁺	Mg²⁺	K⁺	Na⁺	H⁺
Heavy soils CEC > 23 meq/100 g soil	80 – 95 %	10 – 15 %	2.5 %	< 1 %	5 – 10 %
Loams CEC = 10 to 23 meq/100 g soil	60 – 70 %	10 – 12 %	2.5 – 3.5 %	< 1 %	20 %
Light soils CEC < 10 meq/100 g soil	20 – 30 %	6 – 10 %	4 – 5 %	< 1 %	60 – 75 %
All soils where pH is not too acidic	65 – 85 %	6 – 12 %	2 – 5 %	< 1 %	10 – 26 %

Note, the % saturation for each base is calculated as follows : (meq /100 g soil ÷ CEC) x 100

To convert analysis values in "kg/ha" to "meq/100 g soil", the same units as CEC, you must first perform the following operations :

K : kg K/ha ÷ 876 = meq K /100 g soil
Mg : kg Mg/ha ÷ 272 = meq Mg /100 g soil
Ca : kg Ca/ha ÷ 448 = meq Ca /100 g soil

Balance between mineral elements:

Source: Doucet, 2006. p.329^[1]
Ratios	Balance between elements (meq/100 g soil)
Ratios	Insufficient	Acceptable	Optimal	Strong	Too strong
Ca/Mg Acid-loving plants	< 1	1 – 2	2 – 9 0.8 - 5	10 – 30	> 30
K/Mg	< 0.05	0.05 – 0.10	0.10 – 0.50	0.5 – 1	> 1
(Ca + Mg) / K Acid-loving plants	< 12	12 – 15	15 – 30 6 – 8	30 – 40	> 40

Note, ratios are calculated in "meq/100 g soil". You must convert "Kg/ha" to "meq/100 g soil".

"It is in soils with low CEC that the balance between exchangeable cations must be more carefully considered." (Doucet, 2006. p.329)^[1]

Going further with bioindicator plants

To conclude this chapter, we propose a new approach based on reading the flora inhabiting your plots. Bioindicator plants, in addition to offering a more integrative understanding of the ecosystem in which you will produce, provide an additional key to understanding the soil diagnosis of your territory. According to the work of Gérard Ducerf, plants spontaneously present on a plot are a key indicator of soil quality. Indeed, a plant starts its growth thanks to dormancy breaking, a phenomenon during which the seed germinates. However, each plant species has its own dormancy breaking conditions.

Plants never colonize soil randomly. Knowing the criteria that favor their establishment helps understand

on what type of soil they grow. Here are some examples :

The presence of certain species can indicate imbalances :

Clay deficiency in soil for sheep sorrel (Rumex acetosella)

Poor clay and OM content in soil for corn spurry (Spergulas arvensis)

Soil compaction for broadleaf plantain (Plantago major),...

Other species can on the contrary assure us of good soil health :

Good balance of the clay-humus complex for common chickweed (Stellaria media) or bird's-foot trefoil (Lotus corniculatus).

WARNING

However, reading your soil through bioindicator plants requires precise identification of them, referring to specialized books. Indeed, although among the 60 species of rumex, 59 indicate soil problems, common sorrel (Rumex Acetosa) testifies to excellent meadow quality and can be difficult to distinguish from its relatives.

How to perform a soil diagnosis using bioindicator plants?

According to Gérard Ducerf, during a soil evaluation by bioindicator plants, a complete diagnosis can be obtained : estimation of pH, base quantity in the soil, OM content, biological activity...

The diagnosis is done in three steps :

Inventory of species present on one m² (relatively homogeneous and representative area of the plot to study)
Association of each species with a coverage coefficient to determine the significance of its presence. This coefficient varies between 1 and 5 according to the percentage of soil surface covered by the plant (100% for coefficient 5, 75 for 4, 50 for 3, 25 for 2 and less than 25 for 1). If the plant is present only by a few scattered individuals, it is assigned only the sign +.
Analysis of the plot characteristics.

Points of caution

At least 6-8 species are needed to perform a good diagnosis. The best period for diagnosis is when the maximum number of plants are at flowering stages, i.e. late spring. This method provides only a qualitative soil result.

Sources

Performing a soil analysis to evaluate the structure and fertility of agricultural land, AgroLeague

MSV

↑ ^1.0 ^1.1 ^1.2 Doucet, R. 2006. Le climat et les sols agricoles. ed. Berger, Eastman, Québec. xv, 443 pp

[:0-1] 1.0 ^1.1 ^1.2 Doucet, R. 2006. Le climat et les sols agricoles. ed. Berger, Eastman, Québec. xv, 443 pp

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