SOILS FERTILITY 403 



pound in 19 1G, while in the rock phosphate the phosphorus cost 

 from two and one-half to three cents per pound, depending upon 

 the distance from the mines, in material containing fourteen per 

 cent of the element phosphorus or 32 per cent of phosphoric acid. 



If rock phosphate of the same money value as acid phosphate 

 or hone meal were applied and the conditions were at all favorable, 

 the results obtained would compare well with those from the other 

 forms and the phosphorus content of the soil would be increased, 

 as so much more of the element would be added. 



Potassium. As may be seen from the tables, pages 392, 393, 

 and 394, soils vary a great deal in their content of potassium. Clay 

 and silt soils contain the most, while peats and sands have least. 

 Many peat soils are so deficient in this element that applications of 

 potassium are necessary. Notwithstanding the large amount in 

 soils, it is sometimes so unavailable that crops fail to obtain the 

 amount necessary for good yields. Potassium is usually locked up 

 in silicate minerals and the action of acids of some kind is necessary 

 to liberate it. This may be accomplished by the acids of decaying 

 organic matter which attack the minerals and free the potassium. 



In soils such as peat the potassium may be supplied by applica- 

 tions of potassium sulfatc or chloride, each containing about eight 

 hundred fifty pounds of the element per ton, or kainit. containing 

 two hundred pounds (Fig. 191). Wood ashes contain five per cent 

 of potassium. Annual applications of one hundred to two hundred 

 pounds of the sulfate or chloride per acre are sufficient for most 

 crops. Manure may be used, but a ton contains only eight pounds, 

 and the nitrogen of manure has a much greater value upon other 

 types of soil. 



Other Elements. While several other elements are required 

 for crops, the supply in the soil is so large, or the amount used by 

 crops is so small, that there is little danger of a deficiency. Sul- 

 fur is required in small amounts, and probably will need to be 

 applied only in the case of crops such as turnips, cabbage, etc., which 

 require large amounts. Iron is used only in small amounts and 

 the soil contains an abundance. Calcium and magnesium aro low 

 in some soils, especially acid ones, and may bo easily supplied in 

 limestone, which has been discussed in Chapter Nil. 



Lime, Limestone. All soils should contain some carbonate, but 

 more especially calcium carbonate or limestone. Its presence is 

 very important in the functioning of nitrifying bacteria and the 

 production of available nitrogen. A base must bo present to unite 

 with the nitrous and nitric acids formed, or the presence of these 



