ABSORPTION OF NITROGEN FROM SOIL AND AIR G3 



but other similar microorganisms are found. The physiological 

 peculiarities which prevent the nitrogen-fixing bacteria from living 

 on the roots of other plants are not understood as yet. Evidently 

 here we have to deal with the same specific adaptability of organ- 

 isms towards each other as is observed in most cases of real para- 

 sitism. 



21. The Nitrogen Cycle in Nature. Artificial Nitrogenous 

 Fertilizers. — The ability of leguminous plants to fix molecular 

 nitrogen of the atmosphere is of enormous importance in the 

 economy of nature in general and in the economy of agriculture 

 in particular. Leguminous plants are widely distributed over the 

 earth's surface. They are quite common in every plant associa- 

 tion. The nitrogen which they accumulate is of such an amount 

 that it is not only sufficient for their own nourishment and for an 

 abundant storage in their seeds, but it also produces a general 

 increase in the nitrogen content of the soil. This enrichment of 

 the soil is due to rotting of fallen leaves, of nodules remaining in 

 the soil, and of other disintegrating parts of the plant. 



The capacity of leguminous plants to accumulate nitrogen in 

 the soil explains their popularity in agricultural practice. The 

 favorable effect of their cultivation on the subsequent yields of 

 cereals was known long before Hellriegel's time. One of the classi- 

 cal experiments carried out by Laws and Gilbert at the Rotham- 

 stead Agricultural Experimental Station in England has shown 

 that when wheat alone was grown for ten successive years on one 

 of two identical fields and wheat in succession with leguminous 

 plants, on the other, five yields of wheat from the second plot were 

 equal to ten from the first. In addition, five harvests of legumi- 

 nous plants had been gathered from the second plot, and each of 

 these contained more nitrogen than in the five crops of wheat. 

 Thus, the effect produced by leguminous plants seems to be almost 

 miraculous. The plants themselves not only are excellently 

 nourished, but in addition the most important nutritive element 

 is left in greater abundance than it was contained in the soil 

 before the experiment. 



In general, plant production may be considered as a practice 

 in which highly valuable food and technical products are created 

 from raw materials, like carbon dioxide, water, and radiant energy, 

 which cost practically nothing. The requisite amount of nitrogen 

 and mineral substances, however, is often quite expensive. 



