Soils are essential for life on earth and critical in many of the environmental challenges facing the earth. Most plants depend on soils as a suitable medium for growth by providing at least six factors for plant growth: physical support for anchoring the root system, aeration and ventilation for the roots, pores for absorbing rainwater and holding it for use by the roots, moderation of temperature fluctuations, protection from phytotoxic substances, and supplying inorganic minerals in the form of dissolved nutrients.
The four major components of soil are air, water, mineral matter, and organic matter. An ideal soil for plant growth has a composition by volume of 50% solid (45% mineral, 5% organic) and 50% pore space (25% filled with air and 25% filled with water).
The most important consideration in making a potting soil for succulents is that it will drain well. The soil must be porous so that water penetrates easily and drains away quickly. At the roots a rapid exchange between water and air is essential but cannot take place when water is excessive. A soil that remains wet for long periods of time can quickly kill most succulent plants.
There is no one way to prepare a succulent soil mix and any that drain well should support healthy succulent growth. By experimenting you can find the one that works the best for you. An example of a good succulent soil mix is 2 parts by volume of a potting soil, 1 part perlite, and 1 part small size gravel, e.g., pumice, turface, or crushed granite. Even simpler is a 1:1 mixture of potting soil and perlite. If sand is added to a mix, it should be coarse grained such as builders sand. Addition of organic material is not necessary since in their native habitats most succulents live in weathered soils which are very low in humus.
A soil mixture for succulent plants should have a good crumbly structure. To test your soil mix, moisten and then squeeze with your hands: the mixture should not form a lump but crumble loosely.
Often it is recommended to repot your succulents every couple years. The use of an appropriate soil mix will promote their good health and keep them looking their best.
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