Plants take all their nourishment in the form of soups, and very weak ones at that. Plant food to be available must be soluble to the action of the feeding root tubes; and unless it is available it might, as far as the present benefiting of your garden is concerned, just as well not be there at all. Plants take up their food through innumerable and microscopic feeding rootlets, which possess the power of absorbing moisture, and furnishing it, distributed by the plant juices, or sap, to stem, branch, leaf, flower and fruit. There is one startling fact which may help to fix these things in your memory: it takes from 300 to 500 pounds of water to furnish food for the building of one pound of dry plant matter. You can see why plant food is not of much use unless it is available; and it is not available unless it is soluble.
THE THEORY OF MANURING
The food of plants consists of chemical elements, or rather, of numerous substances which contain these elements in greater or less degrees. There is not room here to go into the interesting science of this matter. It is evident, however, as we have already seen that the plants must get their food from the soil, that there are but two sources for such food: it must either be in the soil already, or we must put it there. The practice of adding plant food to the soil is what is called manuring.
The only three of the chemical elements mentioned which we need consider are: nitrogen, phosphoric acid, and potash. The average soil contains large amounts of all three, but they are for the most part in forms which are not available and, therefore, to that extent, may be at once dismissed from our consideration. (The non-available plant foods already in the soil may be released or made available to some extent by cultivation. See Chapter VII.) In practically every soil that has been cultivated and cropped, in long-settled districts, the amounts of nitrogen, phosphoric acid and potash which are immediately available will be too meager to produce a good crop of vegetables. It becomes absolutely necessary then, if one would have a really successful garden, no matter how small it is, to add plant foods to the soil abundantly. When you realize, (1) that the number of plant foods containing the three essential elements is almost unlimited, (2) that each contains them in different proportions and in differing degrees of availability, (3) that the amount of the available elements already in the soil varies greatly and is practically undeterminable, and (4) that different plants, and even different varieties of the same plant, use these elements in widely differing proportions; then you begin to understand what a complex matter this question of manuring is and why it is so much discussed and so little understood. What a labyrinth it offers for any writer—to say nothing of the reader—to go astray in!
I have tried to present this matter clearly. If I have succeeded it may have been only to make the reader hopelessly discouraged of ever getting at anything definite in the question of enriching the soil. In that case my advice would be that, for the time being, he forget all about it. Fortunately, in the question of manuring, a little knowledge is not often a dangerous thing. Fortunately, too, your plants do not insist that you solve the food problem for them. Set a full table and they will help themselves and take the right dishes. The only thing to worry about is that of the three important foods mentioned (nitrogen, phosphoric acid and potash) there will not be enough: for it has been proved that when any one of these is exhausted the plant practically stops growth; it will not continue to "fill up" on the other two. Of course there is such a thing as going to extremes and wasting plant foods, even if it does not, as a rule, hurt the plants. If, however, the fertilizers and manures described in the following sections are applied as directed, and as mentioned in Chapter VII., good results will be certain, provided the seed, cultivation and season are right.
VARIOUS MANURES
The terms "manure" and "fertilizer" are used somewhat ambiguously and interchangeably. Using the former term in a broad sense—as meaning any substance containing available plant food applied to the soil, we may say that manure is of two kinds: organic, such as stable manure, or decayed vegetable matter; and inorganic, such as potash salts, phosphatic rock and commercial mixed fertilizers. In a general way the term "fertilizer" applies to these inorganic manures, and I shall use it in this sense through the following text.
Between the organic manures, or "natural" manures as they are often called, and fertilizers there is a very important difference which should never be lost sight of. In theory, and as a chemical fact too, a bag of fertilizer may contain twice the available plant food of a ton of well rotted manure; but out of a hundred practical gardeners ninety- nine—and probably one more—would prefer the manure. There is a reason why—two reasons, even if not one of the hundred gardeners could give them to you. First, natural manures have a decided physical effect upon most soils (altogether aside from the plant food they contain); and second, plants seem to have a preference as to the form in which their food elements are served to them. Fertilizers, on the other hand, are valuable only for the plant food they contain, and sometimes have a bad effect upon the physical condition of the soil.
When it comes right down to the practical question of what to put on your garden patch to grow big crops, nothing has yet been discovered that is better than the old reliable stand-by—well rotted, thoroughly fined stable or barnyard manure. Heed those adjectives! We have already seen that plant food which is not available might as well be, for our immediate purposes, at the North Pole. The plant food in "green" or fresh manure is not available, and does not become so until it is released by the decay of the organic matters therein. Now the time possible for growing a crop of garden vegetables is limited; in many instances it is only sixty to ninety days. The plants want their food ready at once; there is no time to be lost waiting for manure to rot in the soil. That is a slow process—especially so in clayey or heavy soils. So on your garden use only manure that is well rotted and broken up. On the other hand, see that it has not "fire-fanged" or burned out, as horse manure, if piled by itself and left, is very sure to do. If you keep any animals of your own, see that the various sorts of manure —excepting poultry manure, which is so rich that it is a good plan to keep it for special purposes—are mixed together and kept in a compact, built-up square heap, not a loose pyramidal pile. Keep it under cover and where it cannot wash out. If you have a pig or so, your manure will be greatly improved by the rooting, treading and mixing they will give it. If not, the pile should be turned from bottom to top and outside in and rebuilt, treading down firmly in the process, every month or two— applying water, but not soaking, if it has dried out in the meantime. Such manure will be worth two or three times as much, for garden purposes, as that left to burn or remain in frozen lumps. If you have to buy all your manure, get that which has been properly kept; and if you are not familiar with the condition in which it should be, get a disinterested gardener or farmer to select it for you. When possible, it will pay you to procure manure several months before you want to use it and work it over as suggested above. In buying manure keep in mind not what animals made it, but what food was fed—that is the important thing. For instance, the manure from highly-fed livery horses may be, weight for weight, worth three to five times that from cattle wintered over on poor hay, straw and a few roots.
There are other organic manures which it is sometimes possible for one to procure, such as refuse brewery hops, fish scraps and sewage, but they are as a rule out of the reach of, or objectionable for, the purposes of the home gardener.