The Deux-Sèvriens soils are mostly acidic and quite poor in carbonates. The causes are multiple:
- soil leaching by rain (loss of 200 to 500 jg of CaO per hectare per year)
- export by cultivation
- effects of some fertilizers

This natural acidity is maintained by the fact that roots release H+ ions and that nitrate leaching also mobilizes H+ ions. The pH measures the acidity of the soil, i.e. the concentration of H+ ions in the soil solution. It conditions a large number of chemical and organic phenomena.

PHYSICAL EFFECTS: "bad aeration", difficult water circulation, deflocculation of clays, icing of silty soils.

CHEMICAL EFFECTS: blocking of fertilizing elements, reduced microbial activity (poor degradation of organic matter), saturation of the clay-humus complex by ions that are toxic to the plant.

The adjustment of a pH is reasoned in base requirement (BEB) which is expressed in neutralizing values. The neutralizing value (VN) corresponds to the capacity of the amendment to neutralize the acidity of the soil: 1 kg of CaO is equivalent to 1 VN unit and 1 kg of MgO (magnesium) to 1.4 VN unit.
To raise the pH by one point, an average of 3,500 units of neutralizing values/ha are required. Maintenance requirements vary according to the system. In a standard situation, 300 units of neutralizing values/ha will be used for maintenance and 150 units if manure or poultry droppings (rich in carbonates) are regularly added.
Basic amendments are distinguished according to their composition in CaO, MgO, which gives their neutralizing value. We also distinguish the potential speed of action of the product which depends on the fineness and the type of rock. It can be fast, medium fast or slow.

Fired products such as quicklime and magnesian lime have neutralizing values of 90 to 100 units per 100 kg of product. Their action is very fast.
In the raw products, a distinction is made between products according to their fineness. Fine products such as carbonate of lime have a neutralizing value of 46 to 54 units per 100 kg. The action is moderately fast for ground products and fast for soft and pulverized products.
Raw products (wet lime carbonate, lime sand) have a neutralizing value between 32 (sands) and 52 (carbonates). Their action is slow.

To choose a basic amendment, one must take into account the neutralizing value and the speed of action. Depending on the urgency of the liming, one will choose fast or slow acting products.
In maintenance liming, the speed of action is of little importance. For spreading, each type of product requires specific equipment (powder, raw product, wet product). It is strongly recommended to avoid, as much as possible, the close application of dejecta rich in ammonia (liquid manure) and of fast-acting products, which would increase the risks of nitrogen loss. Ideally, fall liming should be preferred to intercropping.