FEEDING THE PLANT 97 



gen, other things being equal, will generally be greater 

 than from other forms. 



These considerations apply to all of the constitu- 

 ents, though more particularly to nitrogen and phos- 

 phoric acid. The insoluble form of phosphoric acid, 

 for example, must change to the "reverted," or soluble 

 form, before the plant can use it; great care should, 

 therefore, be exercised in the use of the insoluble. That 

 form which will be most likely to change quickly 

 should be selected, though there are conditions where 

 insoluble forms are quite as useful as those more 

 quickly available, owing to the differences that occur in 

 soils and crops and in their periods of growth. 



WHAT SHALL BE USED 



Naturally, the first question the thoughtful man 

 asks is, "What shall I apply, nitrogen, phosphoric acid 

 or potash, or all, and in what form ?" The question can- 

 not be answered in a categorical way. The answer will 

 depend upon the conditions that exist. In many in- 

 stances one element only may be deficient in the soil ; 

 in others two, and in still others all three may be re- 

 quired. Positive and helpful information on these 

 points cannot be obtained except by actual experiment. 

 Still, much very useful information may be given the 

 practical man who is unable to make these experiments 

 for himself, and. it is based upon a knowledge of the 

 original character of the soil, the kind of cropping? that 

 has been practiced, the rotation, and the methods of 

 management adopted. 



As already indicated, soils differ widely in their 

 composition. The poorest are those made up chiefly of 

 sand, which can, from the nature of the substances 

 composing it, contain but small quantities of plant 

 food; the best, those which are admixtures of sand, 



