viii FOREWORD 



appreciated from the fact that this nitrogen is never present in 

 the soil in forms available to plant growth, as ammonia or nitrate, 

 in amounts of more than a few pounds per acre. It is made avail- 

 able to plants only through the constant activity of the microbes. 



The microorganisms, through their various activities, thus 

 enable organic life to continue uninterruptedly on our planet. 

 They keep in constant circulation the elements which are most 

 essential for plant and animal life. They break down the complex 

 organic molecules, built up by plants and animals, into the simple 

 mineralized constituents, making the elements again available 

 for the growth of cultivated and uncultivated plants which in 

 their turn supply further food for animals. 



Just as man and other animals, as well as higher plants, find 

 their habitat on the surface of the soil or immediately below it, 

 so do the microbes live largely within the upper few inches of the 

 earth's crust, where they carry out their important activities, 

 supplying a continuous stream of nutrients in an available form 

 for the growth of higher plants. This surface pellicle of the earth 

 is thus found to be the seat of numerous processes of incalculable 

 importance in the life of man, animals, and plants, enabling them 

 to carry out their normal existence on our planet. Just as man and 

 animals are determined in their development by the supply of 

 plant food, so is the growth of plants determined by the activities 

 of microorganisms in the soil. The microbes were probably among 

 the first living organisms which appeared on our planet millions of 

 years ago. Although their presence in ancient rocks is largely 

 speculative, it is reasonable to assume, from an appreciation of 

 their specific physiological processes, that they may have lived 

 normally on the earth long before it was a fit habitat for higher 

 plants and animals. 



Our knowledge of the soil microbes and their role in soil 

 processes and plant growth has developed in the last fifty years. 

 However, a large body of information has since accumulated 

 which enables us to construct a clear picture not only of the 

 microscopic population of the soil, of its numerous physiological 

 reactions, but also of the relation of these processes to the 

 origin and formation of soil, to the cycle of elements in nature, and 

 to plant nutrition. Selman A. Waksman 



New Brunswick, N. J. Robert L. Starkey 



October 18, 1930 



