290 ' MICROBIOLOGY OF SOIL 



organic matter. In the realm of soil fertility the new discoveries re- 

 vealed the vastness of the task assigned to soil microorganisms in 

 providing available food for crops. It was shown that under the attack 

 of bacteria and of other microorganisms the various organic debris in 

 the soil is split into relatively small chemical fragments; that the 

 carbon is restored to the air as carbon dioxide; that the nitrogen is 

 changed into ammonia, nitrites and nitrates.- It was shown, further, 

 that in this breaking down of organic matter the various cleavage 

 products, and, particularly, carbon dioxide, hasten, to an amazing 

 extent, the weathering of the rock particles and make available thereby 

 the mineral portion of plant food. It was shown, likewise, that apart 

 from accomphshing the iransformation of unavailable into available 

 plant food, microorganisms are concerned also in the addition of 

 nitrogen compounds to the soil. The evidence gathered slowly by 

 many investigators made it plain, therefore, that microbes are an 

 important factor in the growing of cultivated and uncultivated plants. 

 Hence, the important place assigned to microorganisms in the study of 

 soil fertility problems. 



The Soil as a Culture Medium 



Arable soils present so wide a range of conditions as to modify, 

 materially, the development and predominance of different species. 

 Variations as to moisture, temperature, aeration, reaction, food supply 

 and biological relations are important, in each case, in determining 

 the survival or disappearance of any particular species. For this 

 reason, the study of soil microorganisms must reckon with the mechan- 

 ical composition of soils, their ability to retain water and their content 

 of inert and soltible plant food. 



Moisture Relations in the Soil 



Amount and Distribution or Rainfall. — Precipitation in 

 different regions of the earth's surface varies from practically nothing 

 to more than 1,524 cm. (600 inches) per annum. A portion of this 

 water runs off the surface into the nearest stream, another portion is 

 rapidly changed into vapor and is returned to the atmosphere, and the 

 remainder passes downward, into the soil and becomes the medium 

 in which plant food is dissolved. It is estimated that only about half 



