SOILS AND FERTILIZERS 99 



deplete the land of those elements, (b) The soil, 

 through lack of proper cultivation or drainage, loses 

 its capacity for holding moisture and so is unable to 

 bear crops, although rich in plant food elements, as 

 these elements enter the plant only in solution, (c) Wind 

 and water bear away much of the fertility of the 

 soil, (d) The condition of the soil may be such that 

 the microscopic organisms (see page 76) cannot thrive, 

 and therefore the plant does not thrive, (e) The sup- 

 ply of humus may be exhausted, which will result in a 

 decreased supply of nitrogen and mineral plant food 

 associated with the humus (see page 32), will render the 

 soil less able to hold moisture, and will lessen the forma- 

 tion of organic acids which help to make plant" food 

 available. 



Sooner or later one or more of these causes will 

 operate to render the richest land barren if the farmer 

 does nothing to counteract them. 



Elements of the Plants. Chemical analysis of the 

 tissues of many plants has revealed the presence of 

 some fourteen elements in varying combinations 

 Only two of these are derived directly from the air. 

 carbon and oxygen, in the form of CO 2 , and hy- 

 drogen, and these three form about nineteen twentieths 

 of the tissues of plants, the remaining one twen- 

 tieth being composed of elements supplied by the 

 soil. Of these soil elements, only four need our special 

 attention, nitrogen, phosphorus, potassium, and cal- 

 cium, for these are the only elements which must 

 be supplied by man, nature's supply often being in- 

 sufficient. 



Note. Recent investigation by Professor Hart of the Wisconsin 

 Experimental Station seems to indicate that sulphur may also be deficient 



