Just when you thought you had picked the perfect potted plant, you realize that it doesn't like the soil it is in. But could it be because the soil you used technically isn't soil or it may not be the right kind for the plant?

The stuff you can purchase in plastic bags is not really considered proper soil by experts, according to an expert gardener. And if your succulents keep dying, not from the cold or overwatering, it may be the time to look at the root of the problem.

"What we get in the shops are usually substrates – mixtures of different ingredients," Marcus Scheel, the master gardener at the Royal Garden Academy in Berlin said.

If you want to keep happy plants, it pays to know what substrates are often made of, which of them are actually useful and what's important when fertilizing.

What are substrates made of and why are there so many?

Industrial substrates are based on high and lowland peat or peat substitutes such as coconut fiber. They are then mixed with wood fibers, sand, clay, compost or other components.

Manufacturers offer a different substrate for every purpose, like geranium or herb soil, "to make shopping easier – and to make customers believe that they have to buy several different substrates," Scheel says.

Which substrate is best for sowing?

"For seeds and cuttings, there is special sowing substrate. It is a very fine substrate without recognizable pieces but still has a coarse, airy structure," Scheel explained.

One defining feature of sowing substrate is that there is no fertilizer added. "This stimulates root growth. Because the seed is stimulated to form roots after germination to absorb water and nutrients through them," Scheel said. The young plant that has emerged after germination must therefore use its roots to "search" for nutrients.

But what about special pricking substrate?

If you sow fine seeds densely, you should separate the plants once the seeds have germinated, a process called pricking.

"Pricking is done when you can see the beginnings of foliage leaves appear after the seed leaves," Scheel said. This provides the small plants with more light and nutrients. (Check this article for inspiration about plants that can strive on little light.)

Special pricking substrate has a small additional share of fertilizer, Scheel explained. However, sowing substrate, which is often sold as herb soil, usually works just as well. This substrate is frequently mixed with sand and is lower in nutrients, which makes it ideal for many herbs.

What’s best for repotting plants?

It depends on the plant, Scheel said. Commercial potting soil is usually suitable for vegetables as well as balcony plants.

"It's usually fertilized for four to six weeks. After that, you can feed the plant with a liquid fertilizer." How much depends primarily on the fertilizer concentration as well as the type of plant and the time of year, he said.

As a rule of thumb, "garden plants should only be fertilized a little or not at all" a few months before the cold season begins. This allows them to prepare for hibernation.