MICROORGANISMS AS A FACTOR IN SOIL FERTILITY. 227 



organic matter. In the realm of soil fertility the new discoveries revealed 

 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 are 

 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 accomplishing the transformation 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 uncul- 

 tivated 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 mechanical composition 

 of soils, their ability to retain water and their content of inert and soluble 

 plant food. 



MOISTURE RELATIONS IN THE SOIL. 



AMOUNT AND DISTRIBUTION OF RAINFALL. Precipitation in different 

 regions of the earth's surface varies from practically nothing to more 

 than 1,524 cm. (600 in.) 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 the total rainfall 

 percolates through the soil. Where the soils are open and nearly level 



