Mixing your own potting soil is a great way to control the health of your indoor garden. Knowing exactly what has gone into the soil for your indoor plants is the best way to know that you have started growing them in good conditions. Good potting soil must be absorbent enough to retain moisture but loose enough to allow for good drainage and air circulation; its physical composition is actually far more important than its chemical content.
POTTING SOIL SHOULD CONTAIN:
- loam, the equivalent of purified topsoil
- peat moss or leaf mold, for roughage and water retention
- sharp sand or perlite (a substance made from volcanic rock), for drainage. Sand from the seashore is no good; not only is it likely to be salty, which is harmful to plants
A GOOD FORMULA IS:
- one-third packaged soil + one-third peat + one-third perlite. The formula, sometimes referred to as "houseplant thirds," can be adjusted for special situations.
Certain plants demand an entirely different mix.
- Orchids, for example, require a mix that contains tree bark or some similar porous substance
- Cacti thrive on very sandy soil.
- Houseplant thirds will do for most other indoor plants, from ficuses to geraniums
STEP 1: CHECK INGREDIENTS
- After purchasing packaged soil, peat moss, and perlite or sand, make sure the soil and peat moss are slightly moist; if they are not, they will probably not absorb moisture effectively after mixing.
- If the peat is dry, empty it into a pail of warm water and squeeze handfuls under the water until no dry flakes float on top.
- Then remove the saturated peat, squeeze out the water until the peat is merely moist, and return it to the bag. If the bag is kept sealed thereafter, the peat will stay moist.
STEP 2: MIX SMALL BATCHES
- Pour small helpings of each ingredient into a pail or basin in roughly equal amounts (absolute exactness is not essential) and mix thoroughly with a trowel.
- Add more of the ingredients in small, equal batches, increasing the total amount until you have what you need for the job at hand.
STEP 3: ADJUST FOR SPECIAL REQUIREMENTS
- Add extra amounts of peat moss or perlite to suit plants with particular needs.
- If you are mixing soil for African violets, for example, you should add extra peat plus more perlite;
- Succulents double the perlite or sand.
- Some experts advise adding ground limestone if a more alkaline mix is needed, or extra peat moss for a very acid one.
- Most houseplants are happy enough with a mix that is just slightly acid, as the houseplant thirds is.
- To give plants an extra boost, some experts recommend adding a pinch or two of bone meal. This is not necessary, however, as the packaged soil will almost certainly include some kind of nutritive substance. In any case, do not add any fertilizer to the mix—it is probably rich enough as it is.
STEP 4: LABEL ANY LEFTOVER MIX
- If you end up with soil mix left over, put it in a plastic bag, seal the bag tightly with a twist-tie, and label it explicitly so that you will know at a later date precisely what ingredients have gone into it.
- Larger quantities can be stored in small plastic garbage cans.