Though products to fight against the limitations of an acidic soil are well known (mineral enrichers such as lime or calcium carbonate), farming in calcareous soils requires a sound knowledge of the mineral balances of the soil, if we want to ensure the growth of our plants, both in the garden and on cultivated plots. Often clayey, calcareous soil is known to be difficult to till. But why and how can we overcome its limitations?
How to identify calcareous soils?
Limestone or calcium carbonate is the main constituent of the chalk which gives its whitish colour to calcareous soil. Derived from marine deposits, it is this chalk that causes the calcareous nature of the bedrock of certain regions. Good knowledge of the region’s geology is therefore a first indicator of the presence of limestone rock. Visual observation of traditional housing gives a second clue. As houses use the local rock, they are also a good indicator. A house with white stone walls is often synonymous with a limestone area.
Finally, a more empirical but effective indicator is mixing an acid with a soil sample from the field. Pour white alcohol vinegar on a sample: if it foams, the soil is calcareous.
If these simple preliminary analyses are sufficient for a garden, soil tests are required to ensure yields on hundreds of hectares of different crops by optimising fertiliser applications and by combating deficiencies.
Soil test: total limestone, active limestone, exchangeable calcium What is the difference?
To define the calcium state of soil, soil tests distinguish between two elements: calcium and limestone.
Calcium refers to calcium ions which can be “water-extracted” or exchangeable. “Water-extracted” calcium corresponds to water-soluble free calcium, whereas exchangeable calcium is fixed between the clay and humus layers of the clay-humus complex. This “exchangeable” calcium reserve helps in neutralising the acidity and enriches the soil water through the natural consumption of calcium by plants, leaching, etc.
Limestone is no longer a simple ion, but the combination of carbonate and calcium. A soil test will give two fractions of limestone.
- Active limestone which dissolves very slowly due to the action of bacteria and roots in the soil. It replenishes the soil reservoir with “exchangeable” calcium.
- Total limestone which refers to the active limestone and to the very sparingly soluble fraction of limestone in the bedrock.
Rarely sandy, calcareous soil is often clay-rich, which often gives it a “sticky” structure, fixing mineral elements and even blocking some. In fact, phosphorus and certain trace elements such as iron, boron or manganese are known to be poorly plant-available in calcareous soil.
How can we unblock these elements in calcareous soil?
One of the widely used tricks as part of a curative strategy is using supplements rich in trace elements. The composition of these foliar supplements varies according to the identified deficiencies. As they are sprayed on the surface of the leaves, they should be easily assimilated by the plants. Natural plant-based chelates are therefore to be preferred.
The preventive strategy consists in reproducing and amplifying the biological processes in the soil. We have seen that bacteria and acids excreted by the plant root system dissolve the limestone, thus release mineral elements into the soil water.
By incorporating agricultural crop residues and adding organic matter in the form of manure or compost, we can boost the biological activity, thus increasing the availability of blocked elements. By acting on the structure and the humus content of soil, this strategy also increases organic matter and fertility in the long term, synonymous with better crop yields.