2.6.4.a What are the criteria when choosing between a fertiliser and a soil improver?

Some indicators are necessary to understand the activity in the soil. We have already mentioned the pH, which is crucial and affects the two criteria that will be described here: the percentage of organic matter (OM%) and the carbon/nitrogen ratio (C/N).

Organic matter (OM) is defined as "all living beings and plant and animal debris present in the soil" (Reynier A., Manuel de Viticulture). It is made up of roots, humic compounds, micro-organisms, earthworms etc., and impacts the soil in various ways, both physically (water retention, structure) and biochemically (increases CEC, stimulates biological activity etc.).
Organic matter decomposition of animal origin only leaves simple mineral elements, in solution or fixed to the clay-humus complex.
Organic matter decomposition of plant origin is classified in two main processes: humification and mineralisation.

Humification leads to fairly stable humic complexes (lignin and cellulose). The humus formed will slowly break down into CO2 and mineral substances. Humus increases the soil's biological activity over the long term and releases elements in a regulated manner. It improves soil stability, water retention and soil aeration. It is therefore desirable to have good humification, particularly in certain types of soil, such as sandy soil, which drains very easily.

Mineralisation corresponds to the OM being transformed into mineral compounds (N, S, P, etc.) and carbon dioxide. This is a relatively fast process and the compounds are released and easily accessible to the vine, a portion is fixed to the clay-humus complex. The mineralisation rate is affected by various factors such as pH (optimal around 7), soil temperature (optimal at 30°C), and clay content (the more clay, the slower the mineralisation) and tillage; a well-aerated soil being more efficient.

To estimate the soil's mineralisation capacity or a soil improver, the C/N (carbon/nitrogen) ratio is used, which is a good indicator of the biological activity in the soil and therefore of the mineralisation rate. The higher it is, the slower the mineralisation and vice versa.
Generally, a normal C/N should be between 8 and 12. Understanding this data will help in choosing the type and quantity of fertiliser or soil improver to be applied during fertilisation.
If the plot's C/N ratio is very high, a product with a rapid mineralisation rate (C/N around 10) should be used to stimulate the soil's biological activity. In this case, the product is generally a fertiliser rather than a soil improver.
On the other hand, if the C/N ratio is already low, an input with a high C/N ratio is necessary to balance the mineralisation rate.
The following table summarizes the C/N status according to its value:

In addition to the C/N figure, the organic matter content must also be taken into account. The ideal level depends on the soil's granulometry. Generally, it is between 1.5 and 2.5% of OM and sometimes more in very clayey soils as is the case in Burgundy. Similarly, for each ton of matter added, the amount of stable dry matter should be considered when selecting a product.
To understand the choice to be made according to these two criteria, the following table illustrates a few common cases:

In some cases, soil improvers are not the only option. Grass cover can be used to decompact the soil and is sometimes enough to increase the soil's biological activity, in the case of high OM.
Green manures are also suitable to compensate for certain deficiencies.