FERTILIZERS 133 



In spite of the great progress that has been made, however, 

 there has been little to encourage the hope of directly increas- 

 ing the nitrogen supply of the soil for the wheat crop by 

 means of bacteria. The more practical solution seems to be 

 the indirect one of growing in rotation with wheat leguminous 

 crops aided by artificial cultures. Success in this has been 

 pronounced and practical. In 1904 the United States depart- 

 ment of agriculture made a very extensive experiment with 

 artificial inoculation of leguminous crops. About 12,500 tests 

 were made under all sorts of conditions and in almost all of 

 the states in the union; 74 per cent of the tests properly made 

 proved successful. 1 Not only was nitrogen thus fixed in avail- 

 able form for subsequent grain crops, but the leguminous crops 

 sometimes yielded five times as much as non-inoculated ones 

 grown under similar conditions, the usual increase ranging from 

 15 to 35 per cent. One result of the success of the experiment 

 was a demand for cultures far beyond what the department 

 could supply. A great improvement was made in 1906 by 

 abandoning dry cultures for pure liquid cultures hermetically 

 sealed in glass tubes. 



Perhaps the best method of distributing and applying the 

 organisms is by inoculating the seed of the legumes used. 

 This way is thoroughly effective and costs but a few cents per 

 bushel of seed treated. One gallon of liquid culture will inocu- 

 late 2 bushels of seed. Soil may be inoculated and then dis- 

 tributed as fertilizer would be, or earth may be transferred 

 from a field containing the bacteria. Both of these methods 

 are expensive, less certain of success, and weeds or pests may 

 be transferred with the soil. 



The nitrifying bacteria are parasitic plants that penetrate 

 the roots of legumes to obtain food carbohydrates. After the 

 roots are from 2 to 4 weeks old, the bacteria are unable to 

 enter them. It is now known that tubercle formation is not 

 essential to successful inoculation, and that the bacteria may 

 be present in an efficient state in the absence of tubercles. 2 

 Humid soil and a temperature of 60 to 80 F. are most favor- 

 able to the growth of soil bacteria, 35 F. being the lowest, 

 and 98F. the highest temperature at which growth is possible. 



1 Bu. of Plant Industry, Bui. 70, p. 41. 



2 Yearbook U. S. Dept. Agr., 1904, p. 49. 





