No. 4.] NITROGEN AND P^P^RTILITY. l7l 



nitrogen of the soil and plants that they must be taken into 

 account in deciding upon any rational plan of soil manage- 

 ment. 



Nitrogen and Crops. 



Experiments and experience have shown that nitrogen is 

 the most important food material required by plants. It 

 has, however, been demonstrated that a plant cannot acquire 

 nitrogen by absorption through the leaves, nor can the plant 

 take uncom1)ined nitrogen through its roots ; and yet a plant 

 would perish unless nitrogen existed in some combined form 

 in the soil, and in a state readily obtainable by means of its 

 roots. Experience "has, on the other hand, shown us that, 

 though nitrogen is such an important factor and so difficult 

 to obtain by plants, yot there are a number of cultivated 

 plants that olitain it somehow, and that when grown in a 

 rotation or otherwise they leave the land in a better condi- 

 tion in respect to nitrogen content than before. 



The cereals, — wheat, rye, oats, barley and corn, — the 

 roots and the tubers have long been noted as nitroo-en con- 

 suming, because they draw large amounts of nitrogen from 

 the soil, leaving it poor and worn out after repeated crop- 

 pings. It has been found that these plants practically suffer 

 from starvation when the soil does not contain sufficient 

 supply of available nitrogen, and that this lack of plant food 

 must be supplied artificially in order to grow crops of such 

 plants. 



On the other hand, certain plants, such as peas, beans, 

 vetches and clover have been found not to respond to appli- 

 cation of nitrogenous manures, yet apparently are able to 

 obtain nitrogen by some natural means. While this class 

 of plants contains a large amount of nitrogen, in some wa}^ 

 they actually increase the amount of nitrogen in the soil. 

 It was the study of this class of plants, which will be re- 

 ferred to later, that first brought into prominence the germ 

 life in the soil. 



Soil Bacteria. 



It is only within the past twenty years that bacteriology 

 as a science has made its marvellous advance. The word 

 bacteria, which was practically unknown a quarter of a 



