This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
The conditions of air and moisture are attained by an efficient system of drainage, on which, indeed, the success of pot cultivation mainly depends. Water must be often applied, and yet, unless it passes through freely, the soil will become stagnant and heavy, and unfit for healthy vegetable life. The subject of drainage is so well treated in Dr. Lindley'a Theory of Horticulture (1855, p. 438), that we cannot improve upon it, and shall therefore quote a passage:
"The ordinary way of putting at the bottom of the pot a large quantity of crocks is but a clumsy proceeding, and one which, if it affords an opportunity for roots to spread themselves freely, affords also a harbor for worms, slugs, wood-lice, and other vermin. To remedy this, I put at the bottom a piece of perforated zinc, an inch and a-quarter, or more, square, according to the size of the pot, so as completely to cover the hole; this may be had, for a trifle, of any brazier or tin-plate worker, and may, by the help of a strong pair of scissors or small shears, be readily cut to the requisite size. Upon this I place a small potsherd, with its convex side upwards, taking care that, by resting partly upon the zinc, it renders it immovable. I then put in a quantity of good moss, so as to form a layer of a third of an inch or more thick, when pressed together by the mould, and then proceed to finish, as usual, the operation of potting the plant I have found this method to succeed perfectly. Constant drainage is effected; the moss, particularly with the addition of the potsherd, prevents the earth from choking the sides of the zinc, and by partial decomposition, where it is in contact with the soil, affords an agreeable receptacle for the roots of the plants in which they appear to delight.
All sorts of vermin are excluded; the operation of shifting is facilitated, as the earth comes out of the pot unbroken; and it is, moreover, a much more cleanly process than the one commonly used".
Always use moss at the top of the crocks, to prevent the light soil being carried through to the shelves of the green-house - an inconvenience much felt by the ordinary method.