Tillage, Fertilizing and Rotation 101 



in the soil which prevent its loss by leaching. Commer- 

 cial nitrate of ammonia contains about 20 per cent ni- 

 trogen. 



In the form of nitrate, nitrogen is directly available 

 to the plant as food. It is the most soluble form. Ni- 

 trate of soda, containing an average of about 15 per cent 

 nitrogen, is the most common commercially. Neither 

 nitrogen as ammonia nor as nitrate is so widely used for 

 sweet potatoes as are the organic forms. There are in- 

 stances, however, when the more available forms are 

 used, either because of their cheaper cost or because of 

 some specific purpose ; as giving the plajit a. rapid start 

 by the use of a quickly available form on very poor 

 land.^ Cotton-seed meal, which contains on an average 

 from 6 to 7 per cent nitrogen, is by far the most com- 

 monly used form of nitrogen for sweet potatoes. When 

 the cost of nitrogen a pound in this form is not greatly 

 in excess of the cost in other forms, authorities are unan- 

 imous in recommending it. The long hot season during 

 the growing period furnish conditions best suited to its 

 rapid decay. 



Phosphorus exists in the form of " organic " phos- 

 phates and as mineral phosphates. The latter are so 

 termed in contradistinction to the organic forms be- 

 cause of their lack of organic or animal matter. As the 

 organic phosphates are seldom used in sweet potato cul- 

 ture, discussion will be confined to the mineral forms. 



The chief sources of mineral phosphates in this coun- 

 try are the raw phosphate rock of Florida and South 

 Carolina, phosphates from the mines of Tennessee and 

 Thomas slag which occurs as a by-product from the 



J- For more exhaustive study of forms of plant-food, see Farmers' 

 Bull. 44. 



