The soil is calcareous or alkaline when the pH is above 7. Generally it is made up of 12% to 30% of calcium carbonate. You can tell a soil is calcareous by the presence of stones and pale clear earth clods on which plants such as mustard, poppy and thistle grow without problems. These plants are also a sign of the land’s basicity. Arguably a limestone soil is depleted although some plants can tolerate it.

A calcareous soil is easy to work provided that it does not rain otherwise it tends to become muddy and slimy, due to its high clay content. Despite the dry nature of limestone soil, some plant varieties are able to grow on it without any problem. In general these plants come from the Mediterranean region. However, these plants to resume growth quickly after being planted. It is best to grow them in the fall in order for the roots to cling better to the earth. On the other hand, roses generally do not like alkaline soils and the goes for some flowering shrubs such as lilacs or Kolkwitzia. As for rhododendrons and camellias, they are plants that only grow in acidic environments.

Amending calcareous soil

When limestone binds to other nutrients from the soil, they form a concentration of various chemical compounds that are necessary for the plant’s growth. The roots that are in constant contact with these substances absorb whatever the plant needs. However, excess limestone could lead to harmful consequences for the plant. Among these, we can include the massive preservation of nutrients and minerals that will no longer be available to plants at a given time. Iron is involved, for example, in the metabolism of organic substances in plants. Iron deficiency may cause foliage chlorosis that manifests itself by a yellowish discolouration of the leaves. This means that the plant is no longer able to grow more, even if the amount of limestone increases in winter and decreases in the fall to increase again during the following frost. Therefore it is advisable to grow calcifugous plants because an abundant supply of limestone in no way impairs their normal development. Yet there are many plants that need iron to grow well. Therefore, it is preferable to proceed to an amendment of acid fertilizer to balance the soil. Heath soil, cow manure and peat are all ideal. This type of fertilizer can make up for the iron shortage and also provides the elements essential to the plants’ vital requirements. The other alternative consists of growing white lupins or clovers on extremely calcareous soil. It is also recommended to turn the soil from time to time without digging too deep to avoid the appearance of the clay layer.

Plant varieties for a garden with calcareous soil

Because of the many reasons that denigrate the culture on a limestone soil, some gardeners tend to fertilize the soil to make it more acid. Yet, there are countless plants that are not at all calcifugous varieties. We can cite, among others, laurestine and laurel, boxwood, acacia, rosemary, maple, olive, sorbus, beech, juniper and almond. However, these plants, which appreciate droughts and cannot bear the frost, should be planted them in areas where temperatures are rather mild. If your garden soil is rather calcareous and you want to plant clumps of perennials, use Achillea, lavender, rockrose or crown vetch. Rockery plants like asters and carnations also appreciate basic soil. If you want to integrate some shrubs in your garden, then plant daphnes or mock orange. Fruit trees that are about perfect for a calcareous soil are mandarin, lemon and hazelnut. Feel free to grow clematis or trumpet creeper to train on your garden gate, even if the soil is alkaline.

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Published in Specific by Alexander on 06 Jul 2011