100 The Sweet Potato 



the production of vegetable growth. A soil overly rich 

 in nitrogen tends to produce an excess growth of vines 

 at the expense of the roots. Phosphorus and potash, 

 while present in the vine growth and more or less es- 

 sential to it, function more largely in the make-up of 

 the edible roots, and in the case of other crops in the 

 formation of fruit or seed. 



Nitrogen, which in commerce is the most expensive 

 of the " essential " elements, exists in three distinct 

 forms ; as organic matter, as ammonia and as nitrate. 



Organic nitrogen occurs in all plant and animal forms 

 and its availability as plant-food depends entirely on 

 the rapidity with which the various forms decay. In 

 any case, its availability as plant-food comes about 

 more slowly than the two other forms of occurrence, as 

 nitrate or as ammonia. The organic forms furnish 

 some of the more valuable sources of nitrogen and are 

 of value where the more quickly available forms would 

 be of relatively little importance. Cotton-seed meal, 

 dried blood, fish scrap and tankage are valuable com- 

 mercial sources of organic nitrogen. 



Nitrogen as ammonia is more readily available than 

 the organic forms. In fact, organic nitrogen in its pro- 

 cess of decay passes through the ammonia stage in be- 

 coming available; it is one of the first products that 

 results from the decay of organic substances. The 

 commercial nitrogen in this form is obtained almost en- 

 tirely from sulfate of ammonia. It is readily converted 

 into the nitrate form and quickly becomes available 

 when placed in the soil. This fprm of nitrogen is es- 

 pecially used as a rainy-weather-available fertilizer for, 

 until it changes into the nitrate form, it has the power of 

 combining with certain minerals and organic substances 



