How you prepare the soil for planting seeds or cuttings, or for repotting, depends on the type of plant involved. There are considerations that are generally true for almost all houseplants, and specific items and measurements that are unique to a species.
Water, nitrogen and phosphorus, and many other chemicals are needed to sustain plant growth for any plant. Unlike mammals, obviously, plants receive the majority of their needed nutrients through the soil.
Less obvious is that not 100% comes from the soil, nor do all species get nearly all of their substance from the earth. Some needed elements are delivered through the air and, for some unusual species, nearly all of it that way.
There are less obvious nutrients that are required in certain cases. Magnesium and sulfur are needed in small amounts, as is iron and other elements. Research is required for your specific plant and don’t rely solely on a ‘general’ fertilizer to do the job in all circumstances.
Test the soil for proper levels of nitrogen and phosphorus and add according to what’s needed for your species. On average, a complete fertilizer (10-10-10 of Nitrogen, Phosphorus and Potassium, NPK as it’s known from the chemical symbols) goes in at just over a teaspoon per square foot. Change the amount proportional to the size of your pot and your plant’s needs.
If potassium isn’t needed, blood meal can be used as a substitute. About a teaspoon per square foot is plenty for most plants. For plants that are more slowly growing, reduce the amount of fertilizer accordingly. For special plants, like herbs, very little or no fertilizer is needed.
Remember that adding fertilizer is timing-dependent. Most need a little more in early Spring, less in Summer. But variations among species are common. Check what your specific plant requires.
If your soil is very clay-like, you’ll have difficulty maintaining the proper moisture level. Clay retains water. But a certain amount of clay is desirable for that very reason.
Testing the soil can be carried out in a number of ways. Touch the surface with a finger. If your finger feels moist but not sopping that’s a good first step. Probe deeper with a tongue depressor or small round piece of wood. If the soil is moist below, the wood will absorb some of the water and appear wet and darker. Special instruments (hygrometers) can be purchased for more accurate measurements and they range in price from super cheap to outrageous.
You can adjust the amount of water and repeat the tests, probing different areas. But if you’re finding it difficult to get consistency or good results there’s a simple way to solve the problem. Add a little gypsum to areas that are too moist. That will help break up the clay and provide a little better draining soil.
One aspect of soil preparation and maintenance involves considering the sheer volume of earth.
For smaller plants, providing too large a pot – this is too much soil to grow in – can lead to excess water retention. That encourages disease, especially root rot. For larger plants placed in too small a pot, becoming root bound is a problem just around the corner. Match the amount of soil to the type of plant you have, especially when repotting.