FEEDING THE TREES 95 



most general of general principles can be given. Some 

 of these, however, are worth studying. 



It may be assumed that the reader has a fair under- 

 standing of the ordinary fertilizers, and the general 

 principles governing their use. As a matter of review 

 it may be proper to recall that plant foods supplied to 

 the soil in the form of fertilizers are of three kinds ; 

 i.e., furnishing three different chemical elements. First 

 and most expensive of these is nitrogen, second is pot- 

 ash, and third is phosphoric acid. Plants require many 

 other chemical elements in the course of their growth, 

 but these other elements are almost always present in 

 the soil in sufficient quantity. Lime is sometimes used 

 as a fertilizer, but usually it is employed only for its 

 secondary effects and not because the plants really 

 need more lime. 



It may be said roughly that nitrogen is useful to the 

 plant in making a large growth of fresh green wood ; 

 potash is valuable for the assistance it gives in the 

 physiological process of plant growth, 'enabling the 

 leaves to take up the carbon in the atmosphere ; while 

 phosphoric acid helps in building up the fruit. This 

 statement of the various offices of the different ele- 

 ments is extremely rough and not really very useful. 



In practice, nitrogen is applied when the plants do 

 not make enough growth. When an orchard looks 

 yellow, instead of a dark, bluish green, the indications 

 are that more nitrogen is needed. It may be true that 

 the soil is in such a condition that the plants cannot 

 use the nitrogen already present; but at any rate, for 

 one reason or another, the plant is not getting what 

 nitrogen it needs. In such cases nitrogen fertilization 

 may be adopted or the methods of cultivation may be 

 revised. 



