How to deal with calcareous soil?

A limestone soil can be a problem in the garden… if you don’t do it right…

We will therefore see in this article how best to understand basic soil or alkaline soil (other terms to indicate calcareous soil).

But first, let’s start with a better understanding of our purpose.

What is calcareous soil?

To begin with, let’s reiterate that knowing the soil in your garden is fundamental to a natural vegetable garden.

Calcareous soil consists of 10 to 30% chalk (lime carbonate).

We can measure the alkalinity of an earth on a scale (based on the presence of H+ ions) from 0 to 14, called PH (Potential Hydrogen):

  • At a pH less than 7, the soil is said to be acidic;
  • It is neutral if the pH is equal to 7;
  • And if the pH is above 7, the soil is basic (or calcareous).

But these figures are only indicative. In concrete terms, we assume that a soil is only really acidic if its PH is lower than 6.5 (even 6) and that it is calcareous with a PH higher than 7.5.

So a PH between 6 (low acid soil) and 7.5 (slightly calcareous soil) will allow most crops.

However, some plants will appreciate soil with a certain alkalinity…

The presence of rocks or limestone (chalk), white, is a first indication, yet to be confirmed…

Certain plants, when they predominate in the spontaneous state, tell us that a soil is calcareous.

In the calcareous soilswe will find in particular wild chicory, hellebore, field mustard, chanterelles, meadow sage, viburnum, cherry trees, elms, elderberries…

I also present, in the soil knowledge article, a simple test with vinegar. Because vinegar is acidic, an effervescent reaction occurs, more or less strongly depending on the degree of alkalinity, in the presence of limestone.

Finally, if you want to determine the PH of your soil more precisely, you can ask for a chemical analysis (Many garden centers offer it… And you will know other data about your soil… But count on a hundred euros), or use a PH meter for the soil, or strips (unreliable).

What are the disadvantages of calcareous soil for crops?

In a very alkaline soil, certain mineral elements remain blocked and therefore inaccessible to plants.

This has the possible consequences, depending on the deficiencies, of poor plant growth, as well as poor flowering or fruiting.

These plants, which are not vital, will also be more easily susceptible to disease (especially chlorosis) or animal attacks.

In addition, calcareous soils generally retain poor water and nutrients (which are leached).

Which plants like limestone?

Weak calcareous soil

A soil is fairly calcareous if the pH is between 7 and 8.5.

In such soil, the following plants normally grow without any problems:

  • Vegetables: garlic, artichokes, beets, celery, chicory, broccoli, cauliflower, cabbage, cucumber, pumpkin, watercress, spinach, beans, lettuce, corn, turnip, onions, leek, peas, radish, rhubarb, tomato
  • Herbs: chives, lavender, mint, thyme
  • Flowers: yarrow, anemone, knapweed, clematis, poppy, carnations, iris, lavender, daisy, poppy, peony, hollyhock, speedwell…

Strong calcareous soil

At a pH higher than 8.5, the cultivation options are more limited. Besides the flowers mentioned above, chicory and mint will be, so to speak, the only crops that appreciate such alkalinity…

Among fruit trees, hazelnuts, walnut trees, cherry trees, plum trees, peach trees (PH not too high) or even vines appreciate limestone. Nevertheless, rootstocks adapted to this soil property are recommended.

In general, the following crops will often be more difficult to grow in limestone soil: asparagus, basil, carrot, zucchini, shallot, endive, fennel, strawberry, raspberry, currant, sorrel, parsley, pepper, potato.

But this is not an absolute truth…

Increase cultivation possibilities in calcareous soil

It is more difficult to lower the pH of calcareous soil than to raise it of acidic soil.

Nevertheless, with the right actions, you will be able to grow most of the plants there…

Distinguish clay soils from sandy soils

Limestone (calcium carbonate CO3 6), as it decomposes, releases calcium into the soil.

In the presence of calcium, the clays will fuse into stable aggregates (we speak of flocculation). It is an ideal structure for crops.

But if the calcium is missing, the clays will spread. This will result in soil compaction and therefore poor circulation of water and air.

Thereby, in clay soil, for a correct structure of clay soil the pH must be relatively high (between 7 and 7.5 for very clay soil).

While for sandy soil, a PH of 6.5 (slightly acidic soil) is optimal.

We therefore understand that it is essential to know the texture of the soil in your garden before you want to do anything about the PH.

So if your soil is clayey and slightly calcareous, just enrich it with the right additives, as we’ll see below.

But when it’s sandy and calcareous, things get a little more complicated…

Apply acidic organics

Adding more or less acidic organic materials (semi-ripe compost, cattle manure, softwood RCW, pine bark and thorns, heath soil, clippings, dead leaves, etc.), preferably in the autumn, will improve the structure of the floor.

These changes will also allow the formation, on the surface, of a stable humus, almost neutral (slightly acidic) and beneficial for crops.

Note that they will also help to lower the PH a little… But this decrease will be temporary, not very long lasting (the limestone will remain)… So they will have to renew these contributions every year.

Sandy and calcareous soils benefit from additions of clay (e.g. clay balls) for a better soil structure.

Growing green manures

A mixture of mustard, alfalfa, clover and phacelia is perfect for calcareous soils.

When it breaks down, this mix of green manure helps balance the PH.

Change growing medium

If the soil in your garden is very calcareous, the previous methods probably won’t suffice…

In this case, the only solution, to grow all kinds of vegetables, will be to artificially create a new culture medium. This can be viewed in 2 ways:

  • Creating living mounds
  • A thick supply (20 or 30 cm) of balanced topsoil (rather hefty)

The sulfur issue

You will read here and there that sulfur lowers the PH… It’s true, but the acidifying effect is long (more than the time of a grow), and not very durable… So it has to be added constantly.

And above all, it is not without consequences for life: sulfur is also a fungicide (thus harmful to mycorrhizal fungi, necessary for soil life) and an insecticide (which will lead to imbalances in animal populations)…

I therefore do not recommend using it in a natural vegetable garden.

And you yourself, how do you perceive a calcareous soil? What difficulties do you encounter there?