44 Home Vegetable Gardening 



why plant food is not of much use unless it is avail- 

 able ; 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 con- 

 sideration. (The non-available plant foods already 

 in the soil may be released or made available to some 

 extent by cultivation. See Chapter VII.) In prac- 

 tically every soil that has been cultivated and 

 cropped, in long-settled districts, the amounts of 

 nitrogen, phosphoric acid and potash which are im- 

 mediately available will be too meager to produce 

 a good crop of vegetables. It becomes absolutely 



