POTASH FERTILIZERS 179 



230. Amount of Potash in Soils. In ordinary 

 soils there are from 3,500 to 12,000 pounds of potash 

 per acre to the depth of one foot. Many soils with 

 apparently a good stock of total potash give excellent 

 results when a light -dressing of potash salts is applied. 

 The amount of available potash in the soil is more 

 difficult to estimate than the available phosphoric 

 acid. There is a great difference in crops as to their 

 power of obtaining potash. Some require greater 

 help in procuring this element than others. A lack 

 of available potash is sometimes indirectly due to a 

 deficiency of lime or other alkaline matter in the soil, 

 which prevents the necessary chemical changes taking 

 place in order that the potash may be liberated as 

 plant food. 



231. Sources of Potash in Soils. The main 

 source of the soil's potash is feldspar, which, after dis- 

 integration, is broken up into kaolin and potash com- 

 pounds. Mica and granite also, in some localities, 

 contribute liberal amounts. A valuable source of 

 potash are the zeolitic silicates. The amount of 

 water-soluble potash in soils, except in alkaline soil, 

 is extremely small. By the action of many fertilizers 

 the potash compounds undergo changes in composition. 

 For example, the gypsum which is always present in 

 acid phosphates, liberates some potash. The potash 

 compounds of the soil are in various degrees of com- 

 plexity from forms soluble in dilute acids to insoluble 

 minerals as feldspar. 



232. Commercial Forms of Potash. Prior to the 



