Many of the things around us come from the ground soils. But how to understand soils in their diversity?

Attention, we are dealing here with the ground soil itself. The soil is a medium and serves as a support, an intermediary for other things. Among other things, we cannot neglect the living without which the soil would not be structured in this way. Although this part does not speak of the living, it is essential.

Resource website on the subject of life in the ground soil
https://compost.seedtohumus.org
Understanding compost is already a first approach to understanding living organisms, interactions, degradation, chemical nutrients (from a naturally occurring chemistry point of view).

Soil analysis

It is necessary to understand ground soils in many cases. For example, to build, cultivate, make a pond or retention basin (without adding material), make bricks (raw or terracotta), make earth plaster, make pottery, separate elements from the ground soil to make natural filters or have another use of their biochemical properties...

Soil types

There are several types of soil, each with different properties and are able to support different species of plants.

The ground textures

Soil texture corresponds to the distribution in this soil of minerals and organic matter by size category, independently of the nature and composition of these minerals. [1]

The grain size itself relates to fine earth. [2]

• sands: > 50 µm
• silts: from 50 µm to 2 µm
• clays: < 2 µm

Sandy texture: well aerated soil, easy to work, poor in water reserves, poor in nutrients, low anion and cation exchange capacity.
Silty Texture: Excess silt and insufficient clay can cause a massive structure to form, along with poor physical properties. This tendency is corrected by a sufficient content of humus and calcium.
Clay texture: soil chemically rich, but with poor physical properties; impermeable and poorly ventilated environment, forming an obstacle to root penetration; difficult tillage, due to high plasticity (wet state), or compactness (dry soil). A good structure favored by humification partly corrects these unfavorable properties.
Balanced texture: it corresponds to the optimum, insofar as it presents most of the qualities of the three preceding types, without having the defects. [3]

Sand is a granular solid material made up of small particles resulting from the disintegration of materials of mineral origin (essentially rocks) or organic (shells, coral skeletons, etc.) [4]

Silt is the ultimate product of fluvial erosion of rocks in the watershed of rivers. Silty particles are thus made up of very fine debris of quartz, mica and feldspar, or even clay minerals. [5]

Historically, in geology and soil science, the term clay corresponds to all minerals with a size of less than 2 µm. This grain size, invisible to the eye is inherited from studies carried out by optical microscopy at the end of the 19th century. The crystals then having a size of less than 2 µm were not recognizable and classified under the name clay. [6]

In order to know the texture of your soil, you can perform the jar test.
For clay, the sausage test can be carried out.

The tests

We do not detail here tests, such as tasting the ground (which requires a lot of iteration before mastering with relative precision), nor the test of the buried underwear (in cotton, which shows the degradation after a certain time). Plenty more tests exist, we let you discover this for yourself, if you want to know more! The ones showed here are sufficient to have a rather good knowledge about your soil, outside professional fields requiring precise data.
A test in a lab, will show other things such as pollutants.

Touch

We take a little soil at 5-10 cm deep.
The soil must be dry (if not, set it aside and wait for it to dry).
We crumble the earth between our fingers. The grainier it is, the more sand it contains. The smoother it is (like wet flour), the more silt or clay it contains.
With experience, we ended up having a certain precision in the touch test.

Jar test

• We take soil (about the equivalent of a large handful) at a depth of about 10-15 centimeters.
• We put it in a jar after breaking the large pieces.
• Water is added to the jar approximately 3/4.
• We shake energetically.
• After 30 minutes, stir with a spoon. If there are pieces left, we shake so that everything is in suspension.
• We repeat the previous step as long as there are pieces left.
• Leave to settle for 24 to 48 hours.
• 3 phases are formed, sand at the bottom, silt in the middle and clay above.
• We measure the height of what has decanted and the height of each phase.
• We apply a cross-multiplication to know the percentages.

cross-multiplication principle.
The total height is 100%.
To know the percentages of each element, we do:
Phase height (sand, silt, or clay) / total height = percent of the phase

• The percentages are reported in the triangle below.
• Several measurements can be made to define an area in which the ground soil is located. If you do several tests, pay attention to the bio-indicators! If on 5 square meters, the ground is different with lighter or darker grass, with plants that are found only on a restricted area, then it can be very different. The reasons are multiple, a buried stump, an underground deposit of rubble, greater leaching, a different relief, pollution... Whatever the reason, this is not representative of the average ground soil.

pdf file to print

Source file

For people who master vector (svg made on Inkscape).

Sausage test

For the sausage test, a handful of soil is taken from about 10-15 centimeters deep.
It is compacted by moistening it with a little water. Then, we do the following steps in order.

Step 1
We form a ball of 3 cm in diameter that we roll in the hand.
Place it on a flat, hard surface. If the ball does not hold, it is a sand. (Less than 10% clay)

2nd step
We form a sausage of 6 to 7 cm that we roll with the hand on a flat and hard surface.
If the sausage doesn’t hold, it’s loamy sand. (10% to 15% clay)

Step 3
We form a sausage of 15 cm that we roll with the hand on a flat and hard surface.
If the sausage does not hold, it is a sandy loam. (0% to 20% clay).

Step 4
Form a semi-circle with the sausage.
If the sausage does not hold, it is a silt. (8% to 28% clay).

Step 5
We form a complete circle with the sausage.
If the sausage does not hold, it is silty clay loam or silty loam clay. (10% to 40% clay).
If the sausage holds, but cracks slightly, it is a light clay (30% to 45% clay).
If the sausage holds without cracking, it is a heavy clay. (45% to 100% clay)

Leaching test

Leaching is the loss of nutrients from the soil when it rains.
This can be done below (towards the water table, vertically or obliquely) or on the surface (lateral) in the case of very compact soil, such as clay. In the latter case, we speak of runoff.

• We cut a plastic bottle 10 centimeters from the top, to form a funnel.
• We make holes in the cap.
• Water is poured into the funnel.
• The cloudier the escaping water, the more leaching there is.
• If the water remains on the surface, runoff may occur.

Source file

For people who master vector (svg made on Inkscape).

Limestone and pH

Limestone forms aggregates (clay and humus form the clay-humic complex thanks to calcium and the work of earthworms) which retain nutrients and help raise the pH.

It’s all about quantity, you need enough, but not too much... See the “perfect soil” section.

• We gather soil taken from 10-15 centimeters deep.
• We put soil in a first glass (about a third).
• White vinegar or alcohol vinegar is added to wet the earth.
• The more foamy the result, the more basic the soil pH. There are several levels of reaction: You can hear effervescence at a minimum or have abundant foam at a maximum.
• We put earth in a second glass (about a third).
• We add water to make a mud while mixing.
• Add a teaspoon of sodium bicarbonate (or baking soda).
• The greater the foam, the more acidic the soil pH.

Bio-indicators

The concept of bio-indicators was discussed in the part of the jar test.
If the ground soil has similar visual characteristics (Plants present and plants distribution...) then this defines two main things.

pH
Every plant grows with a certain pH. We can identify endemic plants (not the one we planted, but those naturally present) via applications such as plantnet (https://plantnet.org/), via determination books, online forums from a photo, through more expert friends...
The ideal pH for plants is easily found on the internet.
For example, if we have a plant growing with a pH of 5.5 to 6.5 and another with a pH of 6.0 to 7.5 on the same area, then the pH is between 6.0 and 6.5, therefore slightly acidic.

Soil type
Likewise, every plant grows on a certain type of soil. We can identify them as mentionned previously and know if the soil is clayey, sandy, loamy, calcareous or humus.
We can also see if the soil is compact or airy, poor or rich in nutrients (and what kind)…
Databases on the type of plant matching the type of soil are easily found on the internet.

Organic material

• We take a little soil from about 5 centimeters deep (without roots or grass).
• We put hydrogen peroxide on the sample
  • If a foam appears immediately and leaves quickly, the soil is rich in nitrogenous matter and poor in carbon,
  • If a foam appears late, the soil is rich in carbonaceous matter and poor in nitrogen,
  • If we observe a continuous, long and intense effervescence, the soil is balanced in carbon and nitrogen.

Humic clay complex

As said before, clay and humus form the clay-humus complex thanks to calcium and the work of earthworms.
This is the creation of aggregate in the soil to fix the positive ions that are not carried away by leaching.

Carbon and Nitrogen

The elements carbon and nitrogen are very important in the soil. The C/N ratio (carbon to nitrogen) largely determines the life of the soil.

Find more information here, with this exhibition (part “The chemistry of compost”)

The perfect soil

Land whose agronomic balance of the different elements that compose it would ensure regular growth of vegetation. Its theoretical composition is 65% sand, 15% clay, 10% humus and 10% limestone. [7]

Available water capacity

The available water capacity of a soil (AWC) is the quantity of water that the soil can absorb (soil water retention) and return to the plant. [8]
It is strongly related to soil type as we can see below in another texture triangle typology. (Source: Benitronne, CC BY SA license)
The higher the number, the higher the available water capacity.
It is also known as available water content (AWC), profile available water (PAW)[2] or total available water (TAW). [9]

pdf file to print

Source file

For people who master vector (svg made on Inkscape).