FERTILIZERS 223 



application of phosphoric acid is generally more profitable when the 

 material is in a readily available form, hence as a cotton fertilizer 

 acid phosphate is preferable to the less soluble rock phosphate, or 

 the Thomas phosphate, although the availability of the latter ma- 

 terials is increased by mixing or composting them with rotting 

 organic matter, such as stable manure. 



Nitrogen. Although nitrogen usually causes an increase in the 

 yield of cotton, it is the most expensive element of commercial fer- 

 tilizer and its use is not always profitable. Nitrogenous fertilizers 

 must therefore be used judiciously or their actual cost may not be 

 returned in the increased yield which they produce. Generally 

 speaking, the soils to which nitrogen is applied with profit are those 

 of a medium natural fertility. An application of nitrogen to ex- 

 tremely poor sandy soils is wasteful, for the reason that fertilizer 

 alone is not sufficient to raise the productivity of such soils to a 

 profitable standard. On the other hand, the rich bottom-lands are 

 so fertile that nitrogen is not needed, and its addition will usually not 

 result in a profitable increase of the crop. 



Sources of Nitrogen. The organic forms of nitrogen, such as 

 dried blood and cottonseed meal, supply nitrogen more cheaply 

 than do the inorganic forms nitrate of soda and ammonium 

 sulfate. But by far the most inexpensive and profitable source of 

 nitrogen for the cotton crop is the organic matter added to the soil 

 by the growth of leguminous crops cow peas, soy beans, velvet beans, 

 clovers, and vetches. Indeed, the only rational system of cotton 

 farming is one which includes the frequent production of a 

 leguminous crop in its scheme of crop rotation, for this system not 

 only adds in an inexpensive form nitrogen to the soil, but it also tends 

 to conserve the soil's natural store of nitrogen. 



Potash. The southern soils are well supplied with potash and 

 the cotton crop requires but small additional amounts of this ele- 

 ment. Potash is profitable only when combined with nitrogen and 

 phosphoric acid, and then only in small amounts. It has at times 

 been found useful in counteracting or preventing black rust, a 

 disease of cotton. 



Sources of Potash. The commercial fertilizers used as sources 



