24 THE HANDBOOK FOR PRACTICAL FARMERS 



and contains twenty to twenty-three per cent of nitrogen in a 

 readily available form. It should not be used on soils that are 

 deficient in lime. 



Market gardeners and others engaged in intensive farming 

 need to make liberal use of nitrogenous commercial fertilizers, 

 but the general farmer will find it much more economical to 

 depend upon clovers, stable manure and green manuring crops 

 to furnish nitrogen, supplementing these sources by the purchase 

 of coimuercial forms of nitrogen when it can be done with profit. 



Potash carriers. — Previous to the war with Germany, practi- 

 cally all of the potash used for fertilizers came from that country 

 in the form of Strassfurt salts. Some of these salts were 

 imported in their crude form, such as kainit, but the great bulk 

 of the material was brought over as either muriate or sulphate 

 of potash, both of which contain about fifty per cent of actual 

 potash in a soluble form. During the war several sources of 

 potash were developed in the United States. The more impor- 

 tant domestic supplies come from the brines of old lake beds in 

 some of the western states, from kelp or giant sea-weeds, and 

 from flue dust collected from blast furnaces and cement mills. 



Most soils, especially clays, contain much more potash than 

 nitrogen or phosphoric acid but the greater part of this native 

 potash of the soil is very insoluble and not readily available for 

 the use of crop plants. Even clay soils respond to light appli- 

 cations of soluble potash, and for light, sandy soils and muck, 

 potash fertilizers are absolutely necessary for profitable crop 

 yields. Potatoes, tobacco, and root crops are heavy consumers 

 of potash and give generous response to the use of fertilizers 

 high in potash. 



Phosphoric acid carriers. — The phosphorus found in ferti- 

 lizers is derived mainly either from the minerals of certain 

 rocks, or from the bones and the tankage from slaughterhouses. 



Mineral phosphates are obtained from extensive phosphate 

 rock deposits found in the Carolinas and in Tennessee and 

 Florida. These rocks contain from eighteen to thirty-two per 

 cent of phosphoric acid and are sometimes marketed as fine 

 ground raw phosphate rock or floats, the material being pre- 

 pared for fertilizer purposes by grinding the raw rock to a very 

 fine powder without previous treatment. Since this material is 

 not readily soluble it should be used only in connection with an 

 abundant supply of manure or where green manuring crops are 

 being plowed under. The rotting of the manure and the decay 

 of the green manuring crops in the soil produce acids which 



