The best known and most widely used substrate for sowing is soil.
But soil has some disadvantages :
- there is a risk of rot
- it is sometimes difficult to assess whether the soil is too wet or too dry
- it is not always easy to maintain the proper humidity level
- it is difficult to see if the buried seeds germinated
However, there are other soil less substrates that maintain a good moisture and are inert.
Soil is the most commonly used substrate for sowing. It is easy to find and inexpensive.
The big advantage of using soil is that the seedling roots are not disturbed during transplanting.
However, it has several inconveniences:
- soil is organic, it contains microorganisms that may cause rot. This can be remedied by sterilizing soil before using it in a microwave oven.
- it is not always easy to monitor the germination
- it is not always easy to maintain the correct level of humidity
Sand has the advantage of being mineral which reduces the risk of rot.
It’s quite easy to see with the naked eye if it is sufficiently moist.
Sand is recommended for tiny seeds or cactus seeds.
Expanded clay pellets:
Expanded clay pellets reduce the risk of rot.
Placed in a sealed container with water at the bottom and the seeds above water level: the water will be supplied to the seeds by capillary action.
Expanded clay granules are particularly suitable for large seeds such as avocado, coconut, mango…
Perlite has the advantage of being inert, so less risk of rot.
However, it is not easy to have a correct degree of humidity in the immediate vicinity of the seed.
Perlite sticks to fingers, which is extremely annoying.
This is a possible substrate but not a very practical one.
Vermiculite is mineral, the risk of rot is very low.
It absorbs and retains water in the granules and the seeds are kept in a humid atmosphere.
It is suitable for all sizes of seeds, tiny to very large ones.
Personally, this is my favorite substrate: it maintains perfect humidity, is inert and suitable for most seeds. I sow in soil only seeds of plants which I will grow in soil afterwards and require swampy conditions or plants that suffer if the roots are damaged when transplanting.
Coir is made from fibers that surround the coconut.
Even if the substrate is organic, it decomposes very slowly and rather behaves as an inorganic substrate, with little risk of rot.
It keeps moisture well enough.
Coir is found loose or compressed into small pots ready to be planted.
Rock wool is the most widely used substrate for sowing in horticulture.
It is inert and helps maintain a good level of humidity.