September 30, 1904.] 



SCIENCE. 



429 



in the making of a farm. They absorb and 

 utilize the carbon dioxide of the air and 

 when they die the carbonaceous compounds 

 so formed are in part left as a component 

 of what we call the humus element of soil. 

 These bacteria are thus actively engaged in 

 reducing the mineral substances to a pul- 

 verized form and as actively helping to 

 provide a store of organic matter towards 

 the formation of soil suitable for higher 

 plants. It is remarkable, too, that these 

 organisms are capable of accomplishing 

 this work in the dark. As they do not 

 derive their energy from light, as is the 

 case with green plants, they must utilize 

 some other radiant force or form of force 

 for this purpose, possibly heat rays, or 

 others not so well understood; or it is sug- 

 gested that they derive their energy direct- 

 ly from the oxidizing processes they set up. 

 At all events, they are wonder-working 

 little creatures. 



But our story concerning these partic- 

 ular kinds is not all told. Most micro- 

 organisms acting upon nitrogenous matters 

 are destroyers of these compounds in such 

 sense that the substances are rendered less 

 available as food for higher plants. They 

 are denitrifying agents. These peculiar 

 species do just the reverse. They put 

 nitrogenous compounds which are little or 

 not at all utilized by higher vegetation in 

 available shape and so contribute to soil 

 fertility in another way. They can absorb 

 ammoniacal gas washed by rains from the 

 atmosphere or they can act upon insoluble 

 salts of ammonia existing in the soil and 

 form nitrites first, and then nitrates, in 

 which latter condition common plants can 

 use them. This is called nitrification, and 

 the bacteria nitrifying agents. There are 

 two groups of these workers. One set of 

 species oxidize ammonia or its compounds 

 to nitrites and another set further change 

 these nitrites to nitrates. A very curious 



thing is that these bacteria are not able to 

 exert their activities in the presence of 

 soluble organic matter— the very condition 

 most favorable for most other micro-organ- 

 isms. Even ammonia in a free state is an 

 antiseptic to the nitrate formers. Soil that 

 is liberally supplied with actively decom- 

 posing organic matter may have an abun- 

 dance of these latter bacteria, but they are 

 practically inactive until the decomposition 

 has passed certain stages of its process. In 

 the meantime this soil may be poorly suited 

 for crops until later when the nitrifying 

 processes are resumed. Denitrification can 

 go on in a compact soil, but nitrification re- 

 quires air. It seems that the stimulating 

 effect of tillage is very considerably due 

 to nitrifying activity, so induced; that is, 

 we cultivate soil in part to favor these 

 peculiar bacteriological friends. We see 

 too how tile drainage may indirectly im- 

 prove the productive capacity of the soil by 

 securing better penetration of air and so 

 again aid our friends, the nitrifiers. Again, 

 these latter can not work in an acid me- 

 dium. Sometimes the application of lime 

 to certain soils proves to be greatly advan- 

 tageous, not perhaps because there is too 

 little of this substance for plant food, but 

 because it corrects the acidity and permits, 

 among other things, the nitrifying bacteria 

 to work. Possibly in the full development 

 of agricultural science there wiU be found 

 more direct methods of securing the volun- 

 tary services, to their highest efficiency, of 

 these minute friends of man. 



It should be said that these nitrifying 

 bacteria appear to be always present in all 

 soils in sufficient numbers, hence there is 

 little reason to suppose that anything can 

 be gained by the addition of artificial cul- 

 tures or of impregnated earth— processes 

 which may be decidedly helpful in the case 

 of certain other bacteria to be spoken of 

 later— but it seems evident enough that 



