SOIL ORGANISMS 397 



tion of orops is effective for some diseases, but entire absence 

 of the host crop is often necessary. The use of lime is bene- 

 ficial in the case of certain diseases. Chemicals of various 

 kinds have been tried with little success. Steam sterilization 

 is a practical method of treating greenhouse soils for a num- 

 ber of diseases. The breeding of plants immune to the dis- 

 ease affecting its particular species has been successfully car- 

 ried out in the case of the cowpea and cotton, and can doubt- 

 less be accomplished with others. 



In regions in which farming is confined largely to one 

 crop or to a limited number of cereals, it is the common ex- 

 perience that yields decrease greatly in the course of a score 

 of years after the virgin soil is broken. The cause for this 

 is attributed by Bolley, 1 in large measure, to a diseased con- 

 dition of the plants, due to the growth of various fungi that 

 inhabit the soil and attack the crops grown on it. He reports 

 that he experimented with pure cultures taken from wheat 

 grains, straw, and roots, and has demonstrated that certain 

 strains or species of Fusarium, Helminthosporium, Alter- 

 naria, Macrosporium, Colletotrichum, and Cephalothecium 

 are directly capable of attacking and destroying growing 

 plants of wheat, oats, barley, brome-grass, and quack-grass, 

 and that within limits the disease may be transferred from 

 one type of crop to another. 



217. The beneficial influences of soil organisms. — While 

 the macro-organisms of the soil are usually beneficial to 

 higher plants, the more important relationships are occupied 

 by the micro-organisms. The micro-organisms of the soil take 

 an active part in removing dead plants and animals from the 

 surface of the land, and in bringing about the other oper- 

 ations that are necessary for the production of higher plants. 

 The first step in preparation for plant growth is to remove the 

 remains of plants and animals that would otherwise accumu- 

 late to the exclusion of higher plants. These are decomposed 

 1 Bolley, H. L., Wheat; N. Dak. Agr. Exp. Sta., Bui. 107, 1913. 



