108 The Sweet Potato 



ical analysis of a specific soil would be of little practical 

 value in determining the " best " fertilizer. Every 

 grower must make a study of his individual soil re- 

 quirements by noting " results " rather than by theoret- 

 ical calculations. For this reason it has been thought 

 best to give as briefly as possible the fundamentals un- 

 derlying fertilization and with this knowledge each 

 grower can be guided in the intelligent ^manuring of 

 specific fields. ^ 



In the commercial sections of northern production 

 areas where the growing season is comparatively short, 

 more liberal application of manures will be profitable 

 than in the South. When rapidity of growth, earliness 

 and high quality are important factors, even the most 

 fertile soils contain too little quickly available plant- 

 food for maximum yields. In such cases, more liberal 

 and careful application of commercial manures must 

 be resorted to. 



Hurmis. 



It has been said that " humus is just as necessary to 

 make soil fertility as water is to make lime and sand 

 into plaster." This is especially true of land on which 

 the best yields of sweet potatoes are to be secured. Al- 

 though the chemistry of humus is not thoroughly under- 

 stood, it is known to be a very powerful factor in. ren- 

 dering soil loose and friable (when well-drained) ; it 

 supplies plant-food in the form of nitrogen ; it combines 

 with many of the other fertilizing elements, making 

 them available and effective ; it is very important in con- 

 trolling the moisture^content of the soil, in furnishing 

 food for friendly bacteria, in enhancing the physical 



