92 SOILS AND FERTILIZERS 



deficient in this element. Soils which contain 0.5 per 

 cent or more of lime carbonate will produce good crops 

 on a smaller working supply of potash than soils which 

 are deficient in lime. As a rule the best agricultural 

 soils contain from 0.3 to 0.6 per cent of potash. Sandy 

 soils of good depth may contain less plant food than the 

 figures given, and not be in need of fertilizers. 



The best results are obtained from soil analysis when 

 an extended study is made of the soils of a locality. 

 Then a soil of that region which fails to produce good 

 crops can be compared with a productive soil of known 

 composition. An isolated soil analysis, like an isolated 

 analysis of well water, frequently fails in its object 

 because of a lack of proper normal standards for com- 

 parison. Where extended series of soil analyses have 

 been made, much valuable information has been obtained. 



The term ' volatile matter ' of a soil is sometimes incor- 

 rectly used for organic matter. The volatile matter in- 

 cludes the organic matter and also the water which is held 

 in chemical combination, as in the hydrated silicates. 

 A soil may have a high per cent of volatile matter and 

 contain very little organic matter. Indeed, all clays 

 contain from 5 to 9 per cent of water of hydration. 

 The per cent of humus, as will be explained in the next 

 chapter, does not represent all of the organic matter. 



105. Total and Available Plant Food. Suppose a 

 soil contain 0.40 per cent of acid-soluble potash and 

 field experiments indicate there is a deficiency of 



