64 SOILS AND FERTILIZERS 



73. Sulphur occurs in all soils mainly in the form 

 of sulphates, as calcium sulphate, magnesium sul- 

 phate, and sodium sulphate. It is an important ele- 

 ment of plant food. There is generally less than one- 

 half per cent, of sulphuric anhydride in ordinary soils, 

 but the amount required by crops is small and there 

 is usually an abundance in all soils. 



74. Chlorine is present in all soils, generally in 

 combination with sodium, as sodium chloride. It may 

 be in combination with other bases. Soils which con- 

 tain more than o. ioper cent, are, as a rule, sterile. Chlo- 

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

 all plants, although it is not absolutely necessary for 

 plant growth, hence its combination in fertilizers is 

 unnecessary. Chlorine with sodium, as common salt, 

 is sometimes used as an indirect fertilizer. 



75. Phosphorus, one of the essential elements for 

 plant growth, is combined with both the volatile and 

 non-volatile elements of the soil. Plants cannot make 

 use of it in other forms than those of phosphates. 

 Phosphorus is usually present in the soil as calcium 

 phosphate, magnesium phosphate, or aluminum phos- 

 phate, and may also be combined with the humus, 

 forming humic phosphates. The form in which the 

 phosphates are present, as available or unavailable, is 

 an important factor in soil fertility. Soils are quite 

 liable to be deficient in phosphates, inasmuch as they 

 are so largely drawn upon by many crops, particularly 



