To a very small extent garden vegetables get their food from the air. The amount obtained in this way however, is so infinitesimal that from the practical standpoint it need not be considered at all. Practically speaking, your vegetables must get all their food from the garden soil.
This important garden fact may seem self-evident, but, if one may judge by their practice, amateur gardeners very frequently fail to realize it. The professional gardener must come to realize it for the simple reason that if he does not he will go out of business. Without an abundant supply of suitable food it is just as impossible to grow good vegetables as it would be to train a winning football team on a diet of soda pop and angel cake. Without plenty of plant food, all the care, coddling, coaxing, cultivating, spraying and worrying you may give will avail little. The soil must be rich or the garden will be poor.
Plant food is of as many kinds, or, more accurately speaking, in as many forms, as is food for human beings. But the first distinction to make in plant foods is that between available and non-available foods--that is, between foods which it is possible for the plant to use, and those which must undergo a change of some sort before the plant can take them up, assimilate them, and turn them into a healthy growth of foliage, fruit or root. It is just as readily possible for a plant to starve in a soil abounding in plant food, if that food is not available, as it would be for you to go unnourished in the midst of soups and tender meats if the latter were frozen solid.
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 potassium. 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 potassium 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 potassium) 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.