THE CHEMICAL COMPOSITION OF SOILS 75 



plants. It is not necessary to apply carbon fertilizers 

 to produce the carbon compounds of plants, because 

 the carbon dioxide of the air is the source for crop 

 production. It is estimated that there are 30 tons 

 of carbon dioxide in the air over every acre of the 

 earth's surface. 22 The carbon in the soil is an indirect 

 element of fertility, because it is usually combined with 

 other elements, as nitrogen and phosphorus, which are 

 absolutely necessary for crop growth. 



83. Sulphur occurs in all soils mainly in the form of 

 sulphates, as calcium sulphate, magnesium sulphate, 

 and sodium sulphate. It is an essential element of 

 plant food. There is generally less than o.io per cent 

 of sulphuric anhydride in ordinary soils, but the amount 

 required by crops is small and there is usually an 



abundance. 



/ 



84. Chlorine is found in all soils, generally in com- 

 bination with sodium, as sodium chloride. It may be 

 in combination with other bases. Soils which contain 

 more than 0.2 per cent are, as a rule, sterile. Chlorine 

 is present in the soil in soluble forms. It occurs in all 

 plants but is not absolutely necessary for plant growth. 

 Its use in fertilizers is unnecessary, although chlorine 

 with sodium, as common salt, is sometimes used as an 

 indirect fertilizer. 



85. Phosphorus, one of the essential elements for 

 plant growth, is combined with both the volatile and 



