The pH reflects the concentration of protons (positive elementary particles) in the soil. This property 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. In these conditions, proper control of soil acidity is essential. Several questions then arise: How to measure it? How often? Which indicators should be considered? How should they be taken into account to properly adjust basic amendment practices? Which products to choose according to situations? How much to apply?

These are all questions that this article will answer. Here we will consider a rather acidic soil. Alkaline soils have a specific functioning for which the practices are different: we will discuss this subject in a future article.

I. SOIL ACIDIFICATION: A NATURAL PHENOMENONL

A. Multiple causes

Most of the causes are natural. Acidification can also be caused by an imbalance in the choice of fertilizers used. Here is a list of the main causes:

Leaching of nitrates,
The functioning of soil life and the mineralisation of organic matter,
The excretion of H+ in the rhizosphere as part of the root function of plants (especially legumes),
Nitrification of soil nitrogen,
Volatilisation of ammoniacal nitrogen,
Or the predominance of mineral fertilisation rather acidifying.

Soil PH - Control its acidity - Acidification - Causes

B. Negative consequences on the soil

Soil acidification causes problems on several levels. All of the following consequences contribute to reducing its fertility and limiting the growth of the plant, or even making it susceptible to the development of diseases and the attack of pests:

Decalcification of the soil by leaching of calcium,
Destructuring of the clay-humic complex,
Decrease in the structural stability of the soill,
The risk of aluminium toxicity (detrimental to plant growth) increases below a pH of 5.5,
The risk of molybdenum deficiency.

Soil PH - Acidification - Main negative consequences on the soil

C. Amplification factors of the phenomenone

Several factors can accentuate the phenomenon of soil acidification, such as:

Drainage -> it promotes leaching of mineral elements,
Excessive nitrogen fertilisation -> it brings a large quantity of H+ ions into the soil,
High yields -> the quantities of elements exported are then higher,
Straw export -> mineral elements are also exported from the soil,
A predominance of legumes in the rotation -> the symbiotic functioning of the roots contributes to increase the concentration of H+ ions in the soil,
Heavy rainfall -> like drainage, it increases the leaching of chemical elements.

D. Mitigation factors

Fortunately, several practices can slow down or even reverse the acidification process:

Well-managed nitrogen fertilisation -> excess H+ ions are then limited,
The establishment of intermediate crops -> their root action limits the leaching of mineral elements,
The regular addition of organic amendments -> they have mostly an alkaline pH. They thus contribute to bring OH- ions that will neutralize H+ ions and increase pH,
The regular supply of basic amendments -> like organic amendments, they provide powerful bases that will fix the H+ ions and raise the pH.

II. SOIL ACIDITY, INDICATORS, MEASUREMENT AND VALUES

A. Soil analysis, the indispensable tool

Soil acidity is a dynamic parameter in time and space. To know its condition, the soil analysis is the necessary adequate tool. For its realization, it is recommended to follow several rules:

Take soil samples preferably during intercropping, before any cultivation operation (especially fertilisation),
Take into account spatial heterogeneity (as in the example opposite): for heterogeneous plots, it will be advisable to carry out a soil analysis by identified sector,
To constitute the soil sample, take about fifteen samples with an auger, either within a circle of 20 m radius or along a straight line. The sample should be taken to the depth of the cultivated soil area,
Repeat the soil analysis every 3 to 5 years to adjust practices accordingly.

Soil PH - Indicators - pH - Mapping - Spatial heterogeneity — Illustration of the spatial heterogeneity of the pH: under these conditions, it is advisable to carry out a soil analysis for each zone delimited in orange

B. pH and Cation Exchange Capacity (CEC) as main indicators

1. Soil pH

For this indicator, the soil test provides two types of complementary information:

a. The pH KCl

It is lower than the water pH. It reflects the maximum acidification limit of the soil: the lower it is, the more the soil will tend to acidify, the more it will be necessary to be vigilant and to carry out regular basic amendments.

b. The pH H2O

t reflects the pH of the soil at the time of sampling. As shown in the graph opposite, it varies significantly over the course of the year (from 0.5 to 1 point): in these circumstances, its value will depend on when the soil sample is taken.

Soil PH - Control its acidity - Water pH and CEC Metson - Temporal evolution — The pH water and CEC change over the course of the season. The greater stability of the CEC makes it a more reliable indicator for adapting basic amendment practices

For the vast majority of crops, the water pH value is ideal between 6.2 and 6.8. It may be a little higher for legumes or crops like beetroot.

The significant variation in water pH over the year makes it an unreliable indicator for deciding on the precise application of basic soil conditioners. Fortunately, soil analysis provides a second interesting indicator: the cation exchange capacity (CEC).

2. The CEC of the soil

It is calculated according to the Metson method. It corresponds, as its name indicates, to the capacity of the soil to fix cations and to allow exchanges. It is mainly filled by calcium, magnesium, potassium, sodium and hydrogen ions. It is measured in meq/100 g or cmol/kg

Soil analysis provides 3 types of additional information for this indicator:

a. The size of the CEC: its value reflects the quantity of negative charges, mainly carried by the clay-humic complex, capable of fixing cations. Consequently, it will be all the greater as the soil contains clay and organic matter.

b. Its saturation rate: this is the cumulative share of calcium, magnesium, potassium and sodium ions in the total volume of the CEC.

c. The respective distribution of each major ion on the CEC, expressed in %.

The CEC has several advantages:

a. It is statistically correlated with pH.

b. It is more stable over time (see graph above).

Consequently, it is more reliable than pH for deciding on the quantities of basic amendments to be applied. With the exception of situations with a CEC < 5 meq/100 g (for which the correction of acidity will be based on the pH), it is the best indicator for adapting amendment practices.

n field crops, the objective of fertilisation is to lead to a saturation rate of 85-90% of the CEC. Nature being well made, the pH will, as a general rule, have an ideal value between 6.2 and 6.8 during the season!

III. HOW TO PROPERLY MANAGE SOIL ACIDITY

A. The different situations to be managed

There are three different situations:

1. Maintenance situation: The CEC filling rate is between 80 and 85%. In practice, at the beginning of the season, an application of 250 to 350 neutralizing values/ha of long-acting product will be sufficient in most cases.

2. Light correction situation: This corresponds to a CEC filling rate of 70-80%. It is then appropriate to use products with a medium speed of action over time (e.g. dolomite, calcium carbonate, etc.).

3. Emergency situation: The saturation rate of the CEC is less than 65%. It is then preferable to use fast acting products (quicklime or magnesium).

This type of situation is normally exceptional if the soil acidity management is good.

The use of the formulas provided in chapter IV. E will allow the calculation of the quantities to be applied to obtain an optimal saturation of the CEC.

B. Good practices to follow

To control soil acidity over time, 4 rules are important:

Repeat a soil test regularly (every 3 to 5 years) for a given plot,
Adapt the choice of the basic amendment to the situation encountered. As such, the flowchart in Part III. D can be used to help identify the most appropriate products,
Determine the basic amendment needs from the results of the soil analysis and the formulas in part IV. E,
Modulate the frequency of inputs according to the buffer capacity of the soil: for example, in sandy soils, an annual application will be systematic.

C. The importance of balancing the composition of the CEC, especially the calcium and magnesium slider

Beyond the search for an ideal CEC saturation rate (between 85 and 90%), the addition of basic amendments must also allow the calcium-magnesium slider to be adjusted for optimal soil functioning. 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.

Soil PH - Control its acidity - Properties of the main elements of CEC — Calcium and magnesium have the power to flocculate clays and organic matter. Calcium gives the soil supple properties, while magnesium gives it a better water retention capacity.

The work of William ALBRECHT has made it possible to identify an ideal distribution of CEC cations.

Soil PH - Control its acidity - Ideal composition of CEC — Calcium and magnesium are the two main elements of CEC. They play a major role in the behaviour of clays and of the soil.

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 share of these two ions is 80%.

Soil PH - Control its acidity - Calcium-magnesium balance on CEC — In all cases, the cumulative sum of the share of calcium and magnesium is 80%

IV. HOW TO DECIDE WHICH BASIC SOIL AMENDMENTS TO USE AND HOW MUCH TO APPLY

Basic amendments are made up of a basic anion and a cation (Ca ++ or Mg ++). E.g.: CaO, CaCO3, MgCO3, Ca(OH)2,... They have favourable effects on both the soil and the plant.

A. The use of basic amendment, a positive practice for the soil and the plant

1. Soil fertility stimulated...

The effects occur on several levels:

Adjustment of the pH,
Restructuring of the clay-humic complex by flocculation of clay and organic matter,
Improvement of the saturation of the CEC and the exchanges between the soil and the plant,
Suppression of aluminium toxicity,
Better concentration of the soil solution,
Improvement of soil structure,
Limitation of soil erosion,
More sustained soil life.

Soil PH - Addition of a basic amendment - Beneficial effects on the soil

2. ... And sustained plant growth

The improvement of the soil environment leads, in a second step, to positive effects on the plant:

Better crop emergence,
Better root and aerial development,
Efficient plant nutrition process,
Improved symbiotic nitrogen fixation for legumes.

Soil PH - Basic amendment - Positive effects plant

B. Several criteria to be taken into account for the choice of basic amendments

Several criteria must be taken into account to guide the choice of amendments:

Calcium and magnesium content,
The neutralising value: this is linked to the calcium and magnesium content. It expresses the capacity of a product to neutralise the acidity of the soil. For example, it is around 54 for dolomite and 90 for quicklime,
The fineness of the product: this criterion reflects the fineness of the particles of the product used. The finer a product is, the more it can dissolve in water and neutralise acidity quickly,
The agronomic positioning index: this indicator, created by UNIFA, reflects the capacity of a product to act quickly. It is of course dependent on the nature and fineness of the product.

C. Basic amendments from different sources

The table opposite shows the different types of products:

Soil PH - Control its acidity - The different categories of basic amendments

Products such as powdered lime or limestone are to be reserved for emergency situations when the time is short before planting the next crop. In other cases, it is preferable to bring coarser products whose action is more prolonged in time.

D. How to choose your amendments properly

The following flow chart illustrates the approach for choosing a basic amendment. In view of the results of the soil analysis, three questions arise in succession:

Does my soil need more calcium?
Does it need more magnesium?
Does it also need potassium?

For the flowchart opposite, the term "target rate" refers to the calcium or magnesium content mentioned in the table in Part III. C, indicating the ideal balance between the two elements for optimal soil functioning.

Soil PH - Control its acidity - Determining basic amendment - Calcium rate < target

Soil PH - Control its acidity - Determining basic amendment - Calcium rate = objective

Soil PH - Control its acidity - Determining the basic soil improver - Calcium rate > target

E. How to calculate the quantities to be applied

To determine the base requirements per hectare, the calculation formula is different depending on the size of the soil CEC: the pivotal value is 5 meq/100.

For situations where the CEC is higher than this value, the calculation formula also makes it possible to know the precise calcium and magnesium requirements necessary to correspond to the ideal value identified by the work of W. ALBRECHT (see table in chapter III. C).