Liming : why and how to correct soil acidity?

• Mineralisation of organic matter acidifies the soil. It is the transformation of humus and organic matter, complex elements into simple molecules which the plant can then absorb.
• Biological activity of plants.
• It is also the inherent characteristic of soils which are easily washed out, such as silt, sand, sandstone and some granites.

When the soil is too acidic, aluminium (Al) is freed into the soil, and becomes toxic to plants, thus restricting root development.

Nutritive elements such as potassium (K), phosphorous (P) and nitrogen (N) are not utilised correctly and become unavailable to the plant.

Certain plants will reveal the characteristics of a plot

• Acidic soil : Moss, dock, sheep’s sorrel
• Low CEC (Cation exchange capacity), low fertility: Red fescue, sweet vernal grass, agrostis, trefoil, bird’s foot trefoil.

Cation Exchange Capacity (CEC)

Stems from a soil analysis, it corresponds to the capacity of clay and humus in the soil to retain all types of cations (Ca, Mg, K, Na, H, Al, Fe, Mn). CEC represents the reservoir of stored fertilising elements.

Saturation rate (S/CEC)

Stems from a soil analysis, it shows the occupancy rate of the CEC by nutritive cations ( Ca, Mg, K, Na). It is linked to the pH. Liming is necessary when saturation point is below 80 %.

• Example 1 : S/CEC = 80%, pH = 6,2 |clay| liming is unnecessary
• Example 2 : S/CEC = 20%, pH < 5 | sandy soil | liming is necessary.

Enriching the soil with a calcic supplement:

• Reduces the aluminium toxicity
• Enhances availability for the plant of the mineral elements NH₃, P, Mg and trace elements Mo, Mn, B, Zn, Cu.
• Enhances root system development, and increases calcium content of the grass.
• Improves the structural state of the soil, for soils with a low organic matter content, and a low CEC.

Two types of liming practices are used:

• Rectifier liming, which consists in using large quantities of lime over several years to improve a low pH. The lime spread in several small quantities so as to enhance the pH slowly, otherwise the mineral elements could be blocked.
• Maintenance liming which consists in supplementing the soil every 3 or 4 years to maintain a high enough pH. The quantities to be used are estimated taking into account the exports on the plot (e.g mowing) and the acidifying action of mineral fertilisers when used regularly.

On average, 350kgs/ ha of CaO can be spread every 3 years on grazing land.

Calculating the quantity of CaO to rectify the pH of the soil

What products should be used?

• Burnt lime has been calcinated at a temperature of 1000°C.
• Crude non-heated  products are less expensive, but their action is slower.
• The finer the grain of the product used, the more expensive it is, and the faster it will act. Quicklime is the most soluble.

When to lime fields?

Liming can be carried out at any time of the year, when the soil is firm enough (firm enough to carry the machinery without deterioration) :

• In summer or autumn after hay-making
• In winter, but not on snow covered ground, which would wash it out.

If using quicklime, spreading should be carried out in the absence of animals.