FERTILIZERS 171 



Soils slowly become acid because plants remove a little 

 more of the alkaline than of the acid elements, and the alka- 

 line elements leach from the soil more rapidly, leaving acid 

 residues in both cases. The balance of alkaline to acid ele- 

 ments is referred to as pH which means the ratio of hydrogen 

 ions (acid) to the alkaline or basic ones. A pH of 7 means 

 that they are in equal amount and neutralize each other. 

 When acid ions increase to ten times as many in proportion 

 to the basic ones the pH is 6 and when alkaline ions increase 

 to ten times as many in proportion to acid ones, the pH is 8. 



A few plants, such as the azaleas, rhododendrons, most 

 ferns, and orchids, are adapted to an acid soil of a pH ion 

 concentration of 5.0 or lower. Those plants that naturally 

 grow in an oak or evergreen forest are likely to grow better 

 when the soil is kept acid. Soils may be made more acid by 

 aluminum or ammonium sulphate. In the former the alumi- 

 num becomes insoluble and in the latter the ammonia is 

 absorbed by the plant, leaving the sulphate part as an acid 

 in the soil. 



Most plants grow best when the soil is only slightly acid, 

 as explained in Chapter 21. Lime, in any one of three forms, 

 may be used to neutralize the soil acids; as finely ground 

 limestone, calcium carbonate (CaCOs), as the burned lime- 

 stone or quick lime called calcium oxide (CaO), or as the 

 hydrated lime (Ca(0H)2) where one molecule of water 

 unites with a molecule of the calcium oxide. The ground 

 limestone although slow in its reaction is a little more con- 

 venient to handle. The hydrated form is commonly used. 

 Most lime contains some magnesium, but it should not 

 exceed 14 per cent, since it is toxic in excessive amounts. 



Calcium, Hke phosphorus, has been found recently to 

 diffuse very slowly through the soil while the other elements 

 diffuse more rapidly. In order to have accessible to a large 

 part of the root system a favorable supply of the calcium and 

 phosphorus it is best to mix large amounts of it with at least 



