NITROGEN 6l 



o.io 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. 



75. 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.io per cent, are, as a rule, sterile. 

 Chlorine is present in the soil in soluble forms. It 

 occurs in all plants, although it is not absolutely 

 necessary for plant growth, and its combination in 

 fertilizers is unnecessary. Chlorine with sodium, as 

 common salt, is sometimes used as an indirect 

 fertilizer. 



76. 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 

 grain crops where the phosphates accumulate in the 

 seed, and are sold from the farm. 



77. Nitrogen. This element is present in soils in 

 various forms. As a mineral constituent it is com- 

 bined with oxygen and the base-forming elements as 



