With an abundance of soil all around you it is tempting to use this if you want to plant things in containers, or re-pot existing ones. After all, it is free, so what’s the point of buying something for doing the job?
Unfortunately, it is not as easy as that. For a start, most soils are far from ideal and plants grown under stress – container growing is a sort of stress, because roots are restricted, watering is often neglected (or done to excess), and the growing medium tends to get over-warm in summer and over-cold in winter, especially if the pots are outside, or under glass — need the best conditions you can provide. Furthermore, even a really good loam tends to lose texture and quality if packed indefinitely into a confined receptacle, and you cannot improve it as you can garden soil by digging it over and adding lots of compost, or other soil conditioners. Another disadvantage of using soil straight from the garden for potting is that it contains many pests and diseases which are fairly insignificant when given an unrestricted area in which to operate, but can become extremely harmful to plants when imprisoned in a comparatively small space.
It is therefore essential, if you want to keep your containerized and potted plants as healthy as you can, to give them something to grow in of good texture and as free as possible from bugs and blights.
Originally, all seed and potting composts were based on sterilized loam. In the 1930s the John Innes Institute devised standard formulae for seed, cuttings and potting compost comprised of loam, peat, limestone or chalk and coarse sand, with a fairly quick-acting fertilizer mixture added according to what the compost was to be used for. Even today, soil-based composts are still the most desirable for long-term container growing as they encourage the formation of the best type of root system for this sort of cultivation, keep their texture, and do not dry out so readily.
Formulae for John Innes Composts (all measures are by bulk):
Seed mixture: 2 parts sterilized medium loam; 1 part peat, 1 part coarse sand. To each bushel (a bushel is equivalent to 4 2-gallon buckets) add 1Η oz (40 g) superphosphate of lime and ¾ oz (20 g) finely ground chalk or powdered limestone.
Cutting mixture: 1 part medium loam; 2 parts peat; 1 part coarse sand.