SOIL ORGANISMS 317 



fluence the work of these organisms is a question that every one 

 interested in agriculture should know. The micro-organisms in the 

 soil are of two general kinds, injurious and beneficial. 



1. Injurious Organisms. After a soil has been cropped for a 

 number of years it is frequently found to contain numbers of or- 

 ganisms of various kinds, some of which are not only of no benefit 

 to the crop, but are actually injurious. The number and char- 

 acter of these depend largely upon the crops grown and the rota- 

 tion practiced. A single crop system is likely to encourage the de- 

 velopment of organisms injurious to that crop. Hence a rotation is 

 advisable. 



Some of these are the wilt of cotton, flax, cowpeas, probably 

 clover sickness, the scab of potatoes, the rots of many plants. These 

 perish when by rotation they are deprived of their host plants for 

 a few years. 



2. Beneficial Organisms. The heneficial organisms comprise 

 a considerable number of forms, but the group of bacteria is of 

 special importance. It would be impossible to. grow crops with- 

 out these. They are the farmer's best friends. They aid him in 

 getting plant food into the soil in the process of nitrogen fixation 

 and are of vital importance in making plant food available, as in 

 the process of nitrification. 



(a) Fixation of Nitrogen. Some bacteria in the soil live in 

 symbiotic relation with legumes, producing nodules or tubercles upon 

 the roots. Their function as they grow in this connection is to take 

 nitrogen from the soil air and put it into the plant, in this way 

 storing up or fixing nitrogen. These organisms live in this relation- 

 ship with legumes and this explains the importance of this class of 

 plants to the farmer. Turning under the legumes enables the 

 fanner to get a supply of nitrogen into the soil with little expense 

 and in a form that is readily available to other plants which can 

 use only soil nitrogen. In general each legume has its own 

 special bacteria. 



Another class of bacteria known as A/otobacter have the power 

 of fixing nitrogen in the soil directlv or independent of any other 

 plant. To what extent this is done is not definitely known, but no 

 doubt it is of sufficient consequence to justify careful consideration 

 in producing favorable conditions for their activity. 



(b) Nitrification. The nitrogen of soil organic matter cannot 

 he used directlv bv our crops. Tt must first he changed into some 

 readily soluble form, usually nitrates. Some crops, such as rice and 



