[Macky]

Cheap potting soil is cheap for a reason. Drainage is usually poor, which means the soil gets compacted and the roots can't get the air they need, which can cause them to rot. It usually doesn't retain water well either. Occasionally, unscrupulous manufacturers will use contaminated ingredients as well. So yeah, not a good situation for the plants.

Some potting soil comes with slow-release fertilizer already in it (granules gradually dissolve over time, releasing multiple layers of fertilizer). Most doesn't, especially the cheap stuff. You need to fertilize them because there are essentially no nutrients in potting soil. In fact, there isn't usually any "soil" at all in potting soil. It's usually made up of: mostly peat moss (varies widely in quality), perlite (drainage) compost (varies widely in quality and can be from contaminated sources) and sometimes some vermiculite (water and nutrient retention).

Good potting mixes will use MUCH better quality peat (that won't compact on the roots), a wetting agent (surfactant), lime (to counter the acidity of the peat) and various micronutrients and other things you'd find in natural soils. Consumer potting soils often have gel beads that help retain water as well, releasing it to the plant slowly.

Back to fertilizer, though. You need to "feed" them regularly when they're in pots because otherwise they won't get the nutrients they need to grow well. Fertilizers you'll find in the stores have three numbers on the front: the first represents nitrogen (N), the second is phosphorous (P) and the third is potassium (K). Plants need other things, too, but you're just starting out, so let's stick with the basics. You tend to use more nitrogen on plants that you're growing for the leaves (herbs are in this category, lettuce, kale, etc.) Phosphorous is said to encourage root growth (thus often used for young transplants). Potash promotes health and fruiting.

We're sticking with the basics, though, right. Go get yourself a fertilizer where all three number are the same; it doesn't matter what the numbers are, just that they're all the same. If the numbers are higher, you just use less; if they're lower, you use more. When all the numbers are the same, it's called a "balanced" or "all-purpose" fertilizer. Then just follow the label. If they give you a range of frequency, err on the side of too little rather than too much. Over-fertilizing can cause "salts" to build up in the soil of potted plants and you don't want that.