Manure of some sort is essential to the growing of plants in pots or boxes, both because of the plant-food it adds to the soil, and because it improves its mechanical condition and sponginess or water-holding quality. Thoroughly rotted horse manure or horse and cow manure mixed is by far the best. Cow manure alone, or pig manure, is lumpy and cold, and hen, sheep, pigeon or other special manures are not safe in the hands of the beginner, as they are one-sided, being especially rich in nitrogen and likely either to burn the plants or to cause too soft and watery growth.
This brings us to the point where it is necessary to say a few words about the theory of manures, for they are not all alike and what would be wise to give a plant under some circumstances under others would be quite wrong, just as you would not think of feeding beefsteak to a baby just recovering from the colic, while it might be a very good thing for a hungry man who was going to saw up your wood-pile.
Plants of all sorts—in pots, in the garden or in a ten-acre lot—require three kinds of food elements: nitrogen, phosphoric acid and potash. These elements may be fed to the plants in various forms; for instance, the nitrogen in hen manure, or in cottonseed meal, or in salts from the nitrate fields of Chile, known as nitrate of soda; the phosphoric acid from bone, or from acid phosphate (a ground rock treated with acid); the potash from wood ashes or from German potash salts (muriate or sulphate of potash). Plants, to do their best, require that all three elements shall be present in sufficient amounts to supply their wants.
It is not necessary, however, to go very deeply into the science of plant foods in order to grow plants successfully. Fortunately, manure rotted as described above, furnishes all three elements in about the right proportions. Cow, sheep, hen and pigeon manure are best used as described later, under "Liquid Manuring."
FERTILIZERS
There are many brands of mixed fertilizers prepared specially for use in the greenhouse or on plants in pots. There is a temptation to use these on account of their convenient compact form, and because they are more agreeable to handle. As a general rule, however, much better results will be obtained by relying on rotted manure.
If you want to use fertilizers at all—and for certain purposes they will be very valuable—I would advise restricting the list to the following pure materials which are not mixed, and which are always uniform; nitrate of soda, cottonseed meal, pure fine ground bone, and wood ashes. (Several of the other chemicals are good, but not so commonly used.)
Ground bone is the most valuable of these. It should be what is known as "fine ground," or bone dust. It induces a strong but firm growth, and can be used safely in the potting soil, supplementing the manure as a source of plant food. From two to three quarts to a bushel of soil is the right amount to use. It should be thoroughly mixed through the soil. It may also be frequently used to advantage as a top dressing on plants that have exhausted the food in their pots, or while developing buds or blooming. Work two or three spoonfuls into the top of the soil.
Nitrate of soda is the next in importance. It is very strong and must be carefully used, the safest way being to use it as a liquid manure, one or two teaspoonsful dissolved in three gallons of water. If first dissolved in a pint of hot water, and then added to the other, it will be more quickly done. Use a pint or so of this solution in watering. The results will often be wonderful.
Cottonseed meal may be safely mixed with the soil, like ground bone, but requires some time in which to rot, before the plant can make use of it.