One of the most important parts of a plant is the part you seldom see: the roots.  Without that fuzzy network of white shoots and tiny, almost invisible root hairs, nothing would happen above ground.  Below ground, hundreds of processes are taking place.  Air and water alternately occupy pore spaces, bacteria reduce organic matter to manageable dimensions, and roots are constantly trading useless ions for nutrient ions.

A good greenhouse soil will be open and friable (easily crumbled) enough to allow air to penetrate and, at the same time, permit water to drain freely.  It is spongy so a constant supply of moisture will be on hand for plants.  The soil also functions as a nutrient storehouse where plant food is kept available and provides a means of physical support for the plant.

Some gardeners make a lifetime avocation of juggling soil ingredients to come up with the ideal mixture for each plant.  But experts have found that most plants grow well in a single basic mix.  The time-honored formula calls for two parts soil, one part peat moss, and one part perlite, coarse sand, or vermiculite.  Many succulents need a different mix.  Because "damping off" can make a shambles of your seedlings, sterilize any soil brought in from the garden.  Place a pan of soil in an oven set at 200 degrees Fahrenheit, and bake for 45 minutes.  Because sterilization can be a nuisance and thorough mixing somewhat tricky, it may be worth the extra expense to buy premixed (and sterile) soil at a garden supply center.  Several have been developed to provide all your plants will need for initial growth.