62 NITROGEN FIXATION 



the nitrogen contained) as the rapidly disappearing saltpetre. 

 Calcium cyanamide is now being extensively manufactured on the 

 Continent and it is not to be questioned that the construction of 

 nitrogen compounds will be so extended and the process so im- 

 proved as to meet the rapidly increasing demands of the culti- 

 vators of the soil. 



Much study has also been given to the nitrogen-fixing bacteria 

 as a further source of supply for nitrogen compounds. Failure 

 attended the numerous attempts to cultivate these important 

 plants with practical results, although our own department of 

 agriculture has met with a limited success in making cultures of 

 bacteria that are sold to farmers for mixing with their clover or 

 other leguminous seeds. See Bulletin 71, 1905, U. S. Dept. of 

 Agriculture, There are many difficulties in the way of intro- 

 ducing these forms, such as keeping them alive, supplying the 

 proper conditions for their growth and the variety of forms that 

 must be dealt with. Although there is possibly but one species 

 of these bacteria, they have become so changed through associa- 

 tion with the various kinds of leguminous plants that there are 

 numerous varieties, each adapted to one or a few species of plants. 

 Soils in one section may have the forms suitable for one kind of 

 plant and be totally deficient in forms required for other plants. 

 At present the only sure way of inoculating sterile soils is by 

 mixing with them soils containing the desired bacteria — a labori- 

 ous and expensive method. Without doubt means will be devised 

 for cultivating the various forms of these bacteria so that they 

 may be sold for mixing with the soil or seeds. Thus the farmer 

 will be able to raise a crop that contains a large amount of 

 nitrogen derived from the air and by plowing under this crop a 

 valuable nitrogenous fertilizer is added to the soil and the way is 

 thus prepared for another crop, as wheat, which is dependent for 

 nitrogen upon nitrogenous compounds. This alternation of crops 

 is of prime importance in successful farming. Each kind of plant 

 takes from the soil certain elements in large amounts and for this 

 reason the alternating or succeeding crop should be of such a kind 

 as to require other elements. As these elements are removed 

 from the soil suitable fertilizers containing nitrogen, phosphorus, 



