[12] It has recently been discovered that under some conditions these bacteria are preyed upon by tiny one-celled animals (protozoa) living in the soil and are so reduced in numbers that they cannot do their work effectively. If, then, the soil is heated artificially or treated with antiseptics so as to kill the protozoa, the bacteria which escape multiply so rapidly as to make the land much richer than before.
[13] That crop rotation is not primarily a process to conserve the fertility of the soil, but is a sanitary measure to prevent infection of the soil, is the latest belief of the scientist.
Reference Books
elementary
Hunter, Laboratory Problems in Civic Biology. American Book Company.
Bigelow, Applied Biology. The Macmillan Company.
Coulter, Plant Life and Plant Uses, Chaps. III, IV. American Book Company.
Mayne and Hatch, High School Agriculture. American Book Company.
Moore, The Physiology of Man and Other Animals. Henry Holt and Company.
Sharpe, Laboratory Manual in Biology, pp. 73-87. American Book Company.