426 SOILS: PEOPEETim AND MANAGEMENT 



sary. The use of lime is beneficial 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 number of dis- 

 eases. The breeding of plants immune to the disease af- 

 fecting its particular species has been successfully carried 

 out in the case of the cowpea and cotton plants, and can 

 doubtless 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 

 experience 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 BoUe}^ ^ in large measure to a 

 diseased condition 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 has experimented with 

 pure cultures taken from wheat grains, straw, and roots, 

 and has demonstrated that certain strains or species of 

 Fusarium, Helminthospormm, Alternaria, 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. 



349. Plant microorganisms not injuriotis to higher 

 plants. — The vegetable microorganisms of the soil all 

 take an active part in removing dead plants and animals 

 from the surface of the soil, and in bringing about the other 

 operations that are necessary for the production of plants. 

 The first step in the preparation for plant growth is to 

 remove the remains of plants and animals that would 



^BoUey, H. L. Wheat. North Dakota Agr. Exp. Sta., 

 Bui. 107. 1913. 



