The soil is a constantly changing environment: exchanges and transformations are permanent. By way of illustration, here are some aspects:
During its cycle, the plant takes different elements from the soil.
In return, it gives back some of the products of photosynthesis to the roots.
This exchange stimulates soil life and leads to a change of parameters in the rhizosphere periphery. In return, soil-plant exchanges are modified.
This whole process also fluctuates according to climatic conditions: for example, the photosynthesis process is greatly stimulated in sunny periods.
In addition, excessive rainfall can lead to the leaching of molecules such as nitrates.
The balance between export (harvest) and restitution of course influences the soil and its structure in return.
These different transformations modify the soil conditions and the loop starts again.
Of course, many other phenomena can be added to this list. It is therefore essential to monitor the way in which the soil evolves in order to adapt one's practices over time. In this sense, soil analysis is an important monitoring tool. Several questions arise: How to prepare a sample properly to obtain a representative soil analysis? What information is commonly available in this document? What are the different indicators? What is their meaning? How to interpret their value in order to adapt one's practices accordingly? Is the analyse sufficient in itself?
This article addresses all of these questions to provide an answer. We will place ourselves here in the configuration of a conventional soil analysis.
I. PROPERLY PREPARE YOUR SOIL SAMPLE FOR A REPRESENTATIVE SOIL ANALYSIS
As a general rule, a volume of soil 25 cm deep on a hectare weighs between 2700 and 3200 T. The weight of a sample sent for analysis is between 500 g and 1 kg: in these conditions, it is easy to understand the importance of taking a sample that is as representative as possible of the plot. In this respect, several points should be kept in mind.
Firstly, the sampling technique: there are three main techniques (see figure opposite).
The following elements are then added:
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From the time of dispatch, the analysis time is generally 45 days. Once the analysis has been received, it is time to interpret the results.
II. SOIL ANALYSIS, A TOOL RICH IN INFORMATION
A. Soil analysis, several main categories of indicators
A conventional soil analysis includes five main indicator categories: "granulometry and texture", "organic matter", "cationic exchange capacity", "pH and acid-base balance", "macro and trace elements".
Each category has several indicators, each providing information. Let's look at them in turn.
B. Soil analysis, granulometry and texture
The textural classification is based on the respective proportion of clays, silts and sands. They are classified according to their size, independently of their mineralogical constitution.
Secondly, the distribution between the different particle size categories makes it possible to identify the dominant soil type. In this respect, there are several texture triangles, the one opposite is the most commonly used.
C. Soil analysis and organic matter
Soil organic matter comes back to the centre of considerations, thanks in particular to the emergence of currents such as soil conservation or agroecology. It is the keystone of physical, chemical and biological soil fertility. As such, it is strongly recommended that a soil carbon management policy be put in place, in parallel with the classic management of fertilising elements.
The amount of organic matter in a soil sample is determined by measuring the mass of carbon present. This figure is then multiplied by 1.72 to estimate the organic matter content.
1. What is the right level of organic matter for a soil?
This is a recurring question in the agricultural landscape. The interpretation of the OM content depends on the type of soil considered : the more clayey the soil, the higher the minimum rate should be. The calculation of a threshold rate will be all the more accurate as the clay rate is reported in the soil analysis.
a. If the clay content is not indicated
In this situation, the right value zones are determined by the dominant texture.
If the thresholds indicated for the sands or silts are close enough to the reality to be obtained, that for the clays will certainly be too low as soon as the clay level is well above 20%.
b. A more precise answer thanks to the MO/clay ratio
The work of Pascal BOIVIN, a Swiss agronomist, and his team have established a fairly good relationship between the structural state of the soil (assessed using the spade test) and the balance between clay and organic matter.
In this sense, the soil structure becomes interesting when the OM%/Argil% ratio is higher than 0.17. It is ideal above a value of 0.24.
2. The C/N ratio
This indicator is obtained from the comparison of carbon and nitrogen contents. It is useful for assessing the level of functioning of the soil life.
Ideally, this ratio should be between 9 and 11 in a pH range of 6.2 to 6.8.
3. The coefficient of mineralization of organic matter
The soil contains a more or less important stock of organic matter. Each year, this stock gradually decreases, given the functioning of the soil life, which feeds mainly on carbon. The calculation of the mineralization coefficient K2 makes it possible to determine the level of consumption.
At the level of the French territory, two formulas exist: the choice between one or the other depends on the geographical location of the parcel. The factors vary from one to the other.
The infographic opposite shows the optimal area for this indicator.
If its calculation is not included in the soil analysis, it can be done directly by clicking here.
4. The microbial activity index
This index measures the enzymatic degradation potential of organic substrates in the soil. It varies according to the quantity and quality of the organic restitutions, the type of soil, the fertilisation practices, the quality and quantity of the microbial flora.
It is generally evaluated between 0 and 5, or 0 and 20 depending on the laboratory: the highest score being the best.
5. The humic balance
Every year, the soil loses part of its humus stock. The humic balance is an important step to check if the carbon inputs to the soil in different forms (crop residues, plant cover or organic effluents) are sufficient to compensate for the losses. If the humic balance is negative, the organic matter level decreases little by little, the soil structure is more compact, the soil life is negatively affected, the plant nutrition is less good. In the opposite case, a virtuous cycle is set up.
The result of the humic balance is expressed in T or kg of carbon/ha.
If this indicator is missing from the soil analysis, it can be calculated by clicking here.
D. Soil analysis and cation exchange capacity (CEC)
Soil contains nutrients in varying amounts. They are contained in 3 types of compartments: the soil crystals (the largest reserve but also the least accessible), the soil solution (very small quantity of ions) or the clay-humic complex.
The quantity fixed on the latter represents the cation reserve of the soil, otherwise known as the "cation exchange capacity" or CEC. It corresponds, as its name indicates, to the capacity of the soil to fix cations and to allow exchanges.
It is often determined using the Metson method. It is measured in meq/100 g or cmol/kg (some laboratories give it in meq/kg, in which case the value is 10 times higher).
About the CEC, soil analysis provides 3 types of complementary information.
1. The size of the CEC
The size of the CEC depends above all on the dominant texture of the soil: it will be low for sands, high for clays. It also increases with the level of organic matter.
In the illustration opposite, the CEC is compared to a glass of water, the volume of which will be greater the more clayey and rich in OM the soil is.
2. Le taux de saturation de la CEC
The CEC is filled mostly with calcium, magnesium, potassium, sodium and hydrogen ions. The saturation level measures the proportion of the first four compared to the overall volume of the CEC.
There is a fairly good correlation between the saturation level and the soil pH. In practice, for the majority of crops, basic amendment applications should achieve a saturation level of 85-90% after operation.
For more information on the choice of products and quantities to be applied, all the necessary information is available by clicking here.
3. Composition and balance of the CEC
Beyond the value of the ideal CEC saturation rate (between 85 and 90%), the soil analysis is an opportunity to check the balance of the calcium-magnesium cursor. Indeed, as shown in the infographic opposite, the properties of the main cations of the CEC are different. Its composition has an influence on the behaviour of the soil, particularly that of the clays: depending on the distribution of the cations, the clays will be more or less loose or cohesive, and the dynamics of air flow and water management will be modified.
The work of William ALBRECHT has made it possible to identify an ideal distribution of CEC cations.
The cursor between calcium and magnesium depends on the size of the CEC (see table opposite): the greater the proportion of calcium, the greater the amount of clay in the soil, and vice versa for magnesium. In all cases, the added proportion of these two ions is 80%.
The practices of adding basic amendments should allow this balance to be adjusted afterwards..
E. Soil analysis, pH and acid-base balance
The pH reflects the concentration of protons (positive elementary particles) in the soil. This parameter plays a fundamental role in its functioning and in the growth of the plant: when the pH is badly adjusted, the soil degrades, its fertility is reduced, mineral elements are less available, toxicities appear, the development of the plant is more difficult, and it becomes more sensitive to diseases.
Soil analysis provides a variety of information for this category of indicators.
1. The pH KCl and the reserve acidity of the soil
The pH KCl represents the reserve acidity of the soil. It reports its maximum acidification limit: the lower it is, the more acidic the soil will tend to become, and the greater the need to be vigilant and make regular inputs of basic amendments.
It is measured by immersing the soil sample in a potassium chloride solution: the potassium then takes the place of the aluminium and hydrogen ions on the clay-humic complex. In a second step, the hydrogen ion concentration is evaluated to obtain the pH KCl.
Below a value of 5.3, aluminium, copper or manganese toxicity may occur.
2. The pH H2O and the current acidity of the soil
The pH H2O is the one that best reflects the pH of the soil solution. It is determined by placing the soil sample in a deionised solution. As shown in the graph opposite, it varies significantly over the year (from 0.5 to 1 point): in these circumstances, its value depends on when the soil sample is taken.
For the vast majority of crops, the water pH value is ideally between 6.2 and 6.8. It may be a little higher for legumes or for crops such as beet.
The significant variation in water pH over the year makes it an unreliable indicator for deciding on the precise application of basic amendments. With the exception of sandy soils, it is preferable to use the CEC indicator. For more information on this subject, click here.
3. Calcium carbonate, active limestone and chlorotic power index
Initially, the calcium carbonate content can be measured to determine its calcareous nature: a soil is considered to be "calcareous" above 10%.
This first calculation is then used as a basis for measuring the amount of active limestone in the soil. Active limestone corresponds to the proportion of calcium carbonate in the soil that is soluble in water: its content is considered high above 10%.
The determination of active limestone allows, in a last step, the calculation of the chlorotic power index of the soil with regard to its concentration of easily assimilable iron.
The chlorotic power index is used to measure the risk of iron deficiency: it is considered high above a value of 30.
This calculation is particularly useful for guiding the choice of rootstocks in perennial crops such as viticulture or arboriculture.
F. Soil analysis, macro and trace elements
1. The main macroelements
a. Soil nitrogen
The vast majority of soil nitrogen is in organic form, only a small proportion is soluble in the soil and readily available to the plant.
The measurement of total nitrogen serves as a basis for calculating the C/N ratio.
The soluble part, in the form of ammonium and nitrate, corresponds to the nitrogen residue. This information is useful for adjusting fertilisation practices. The nitrogenous residue varies during the year according to several factors: the amount of rainfall, the type of soil, the organic and mineral amendment practices, or the presence of intermediate cover (which have the capacity to retain elements in the crop horizon). Consequently, its value will vary depending on when the soil sample is taken.
b. Phosphorus and potassium
Before considering how to interpret the test results for phosphorus and potassium, it is important to discuss the concept of requirement.
1. Notion of need and requirement
The requirement is defined by the importance of a given element for the physiological development of a crop. In this sense, the more demanding a crop is, the more the lack of this element in the soil has negative consequences on the yield.
For its part, the need corresponds to the total amount of an element needed to enable the crop to complete its cycle.
The two infographics opposite show the level of requirement of the main crops for phosphorus and potassium.
2. The phosphorus
The soil contains several tonnes of phosphorus per hectare, but only a small fraction is soluble in the soil solution and readily available to the plant.
Another fraction, fixed on the clay-humus complex, can be exchanged with the soil solution and assimilated by the plant.
The last part, which is in the majority, is very difficult for the crop to access. Only a well-developed soil life can "dip" into this reservoir.
Three main analytical techniques are used to assess phosphorus content. They differ in their ability to extract more or less phosphorus from the soil.
The Olsen method: It can be applied to all soil types. It is the softest extraction technique. It measures the amount of phosphorus available to the plant fairly quickly.
The Dyer method: This is used only for acid soils. It is the most aggressive method. It assesses the phosphorus in the soil solution, the phosphorus fixed on the clay-humus complex and part of the bound form in the soil.
The Joret-Herbert method: It is applicable to all soils. Its extraction capacity is intermediate compared to the other two methods.
As a result, the content measured with the Olsen method is always the lowest, while that with the Dyer method is the highest.
The infographics below indicate, for each extraction method and for each level of requirement, the threshold values to be retained in order to decide whether it is necessary to reinforce one's amendment practices or, on the contrary, to adjust them downwards.
For the reinforcement situation: the inputs must be increased to take into account the need to correct the soil content,
For the skip situation: to adjust fertilisation practices precisely, it is then necessary to take into account phosphorus inputs during previous seasons and the fate of crop residues (exported or returned).
These reference values vary according to the regions and the type of soil.
3. The potassium
For potassium, the approach is identical to that of phosphorus. Here again, the threshold values must allow for the adjustment of fertilisation practices according to the requirements of the crop, past potassium inputs and the fate of the crop.
Here too, these reference values vary according to the regions and the type of soil.
2. The trace elements
Trace elements play an important role both in the soil and in the physiological development of the plant:
In the soil, they intervene to ensure the proper functioning of enzymes and to support the activity of the soil biology.
For the plant, their action is essential to support its good growth during the cycle.
Consequently, in the event of a deficiency identified in the soil, it will be essential to first make the appropriate corrections in the soil before considering foliar applications.
The infographic opposite shows the importance of micronutrients according to the crop.
Let us now turn to the subject of optimal content ranges for the main trace elements in the soil.
a. Copper
For copper, the thresholds vary according to the pH and texture of the soil.
b. Zinc
For zinc, the minimum threshold varies according to the type of crop and the pH.
c. Manganese
For manganese, the minimum threshold varies slightly with the pH value.
d. Iron
Desirable levels of iron are between 40 and 100 ppm.
e. Boron
For boron, the thresholds vary according to the type of crop: in field crops, they depend on the pH, the type of soil, the crop and the climatic conditions; in viticulture and arboriculture, they are mainly related to the pH.
f. Molybdenum
Molybdenum availability increases with pH: for alkaline soils (pH>7), the risk of deficiency is almost zero.
The evaluation of molybdenum is only interesting for acidic soils and for sensitive crops (see table above).
3. Balances between elements
In an ideal world, it would be very convenient to have a good content of all the elements in the soil to be sure to have enough of everything. Unfortunately, the reality is quite different: ions interact with each other sometimes positively (synergy), quite often negatively (antagonism).
a. Between antagonism and synergy
The infographic opposite, called Mulder's chart, summarises the interactions between the main mineral elements. How to read it properly?
Let's take potassium as an example. It positively stimulates the concentration of manganese and iron. On the other hand, too much potassium blocks magnesium and boron. In the other direction, too high a concentration of magnesium, calcium, nitrogen or phosphorus limits its absorption.
b. True deficiency and induced deficiency
If the notion of excess is defined by the excessive concentration of an element leading to negative consequences, that of deficiency can have two faces.
Intuitively, we would be tempted to think that it is the low content of a given element........yes but not only! The example opposite is a perfect illustration.
Beyond the importance of having sufficient elements, it is essential to respect certain balances between them. In this sense, the calculation of different ratios provides valuable information.
c. The main ratios of the soil analysis
In general, four ratios are calculated in a classical soil analysis: K/Mg, Ca/Mg, P/Zn and Cu/MO. The infographic opposite shows the optimal ranges for each of these.
There are many more ratios, which we will try to cover in a future article.
In practice, if an imbalance is found, two questions must be asked in succession:
Is one of the elements present in very large quantities?
Or, is the content of one of them very low?
In order, it will always be important to solve the excesses first (by drastically reducing the intake of the element in question) before solving the deficiencies.
III. SOIL ANALYSIS, A TOOL TO BE COMPLETED
A. Soil analysis and spatial heterogeneity
The soil can experience great spatial variability within a single plot. In this case, it is important, if possible, to identify several homogeneous areas and to carry out a soil analysis for each of them. In the example opposite, the large differences in pH certainly reflect heterogeneity of soil type: three zones can be delineated. If the first two appear relatively homogeneous, the third is much less so, and the analysis will certainly be less representative.
B. Soil analysis, a partial view of soil fertility
Although soil analysis provides interesting information for a number of indicators, as a whole it gives only a partial picture of the overall fertility of the soil. The infographic opposite provides a summary.
In terms of physical fertility, although the information is sufficient to identify the dominant soil type based on granulometry, additional observations (spade test, soil profile, etc.) are essential to assess the quality of the clays, the structure of the soil in the different crop horizons and the relationship between them.
At the chemical level, classical analysis provides an interesting overview of the soil's ionic state. However, elements such as sulphur, molybdenum or boron are often missing. Also, the calculated ratios are limited in number, although there are about thirty of them. It is therefore necessary to subscribe to a more complete analysis formula.
Finally, the soil analysis does not make it possible to know whether the elements are in a form available to the plant or not.
In terms of soil life, although the biological activity coefficient gives an overall idea of its level of functioning, there is no information on its diversity. Additional analyses, carried out in specialised laboratories, are essential to measure the genetic richness of the soil. Also, additional counts are useful to estimate the quantity of underground biomass.
More generally, soil analysis does not allow us to know how all the soil parameters interact with each other. Observations such as soil profiles are therefore recommended to assess this interaction.
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Raphaël from TERREOM
Useful links :
AUREA - WikiAurea - https://wiki.aurea.eu
P. BOIVIN - Le rôle du carbone en agriculture - https://www.youtube.com/playlist?list=PL-rKfI2fHaALyyq0aEwET1D9Kg69qw5sm
WikiAurea - Calculette pour estimer le coefficient de minéralisation de la matière organique K2 - https://wiki.aurea.eu/index.php?title=L%27Agro-calculette_K2
SIMEOS AMG- Outil de calcul du bilan humique - http://www.simeos-amg.org/index.php?action=delog
UNIFA - Outil de calcul des besoins en bases - https://ipa-chaulage.info/index.php/le-raisonnement-du-chaulage-et-calcul-des-besoins#logiciel
BUCAILLE Francis - Revitaliser le sol https://www.cnra-france.org/francis-bucaille-revitaliser-les-sols/