MINERAL NUTRIENTS AND NITROGEN 203 



plant is not always an index of the mineral elements which it 

 needs. The presence of certain elements or compounds in the 

 plant is not necessarily evidence that they are essential for the 

 growth of the plant. Furthermore, a chemical analysis of the 

 soil does not determine its value for plant growth. It may con- 

 tain compounds in an unavailable form or the proportions may 

 not be correct for the best results in crop production. 



Nitrogen is an element which is necessary for plant growth, 

 and, as we have previously learned, is one of the three most im- 

 portant constituents in commercial fertilizers. It is an essential 

 element in protoplasm and proteins. It constitutes as much as 

 25 per cent, of the dry weight of some plants, especially mem- 

 bers of the family Leguminosw (see page 351). The high 

 percentage of protein in the legume plants makes them valuable 

 as food plants. 



Source of Nitrogen. We have previously learned (page 

 192) that the atmosphere ordinarily contains about 78 or 79 per 

 cent, of free nitrogen. Although this free nitrogen is soluble 

 in water and may enter and leave the plant cells in the sann 

 manner as the carbon dioxide and oxygen (page 195), yet the 

 plant cannot use it in this free form. The green plant obtains 

 its supply of nitrogen from the soil in the form of soluble 

 nitrogen compounds, mostly nitrates of calcium, magnesium, 

 potassium and sodium. These compounds exist in the soil, are 

 dissolved by the soil water and taken up by the rootrhairs by 

 osmosis. Very low percentages of these soluble nitrogen com- 

 pounds means that the soil is low in fertility. Although the 

 amount of nitrogen in the soil is variable, arable soils usually 

 contain about 1 to 3 per cent. The soil also contains nitrogen 

 in the form of nitrites and ammonia compounds, which are 



