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