604 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



In those rosoarchos it has horn found that the soil instead of being' 

 an inert reservoir is the seat of physieal, ehemical, and vital actions 

 which directly or indirectly inihience soil fertility. The roots of 

 plants and plant debris, with their biochemical activities, micro- 

 organisms, worms, enzymes from various sources, catalj^tic action 

 of orfxanic and inorganic matter of the soil, the relation of the various 

 activities and the effect of fertilizers upon them, and the processes 

 going on in humus, all play an important part in the soil's feitility. 

 These investigations have shown that the roots of higher ])lants are 

 not sim])ly absorbing organs, but possess the power, through oxida- 

 tion, reduction, and otherwise, to make changes in the soil ingredi- 

 ents — changes which are undoubtedly modified by various com- 

 pounds, organic and inorganic. The various fertilizing salts com- 

 monly used, besides serving as nutrients for plants, are doing work in 

 the soil, as are also the various soil constituents themselves, modifying 

 the various reactions in a complex, ever-changing medium, and in 

 particular modifying the kind, the number, and the activities of the 

 microorganisms. These are present in vast numbers, especially in. 

 cultivated soils, associated with the organic matter, and exercise 

 important functions, both good and bad, in connection with the 

 higher plants. The biochemical side of soil fertility investigation 

 has been little studied in the past, but it is only through this that a 

 thorough insight into the problems of soil fertility can be obtained. 

 Whatever adds to our biochemical knowledge of soil, advances and 

 broadens our understanding of the complex problern of soil fertility. 



The organic matter of the soil and the biochemical relationships 

 involved are of the utmost importance. Organic matter is essential 

 to make a soil of what would othei^ise be pulverized and more or 

 less hydrolyzed rock, and while there are some soils capable of grow- 

 ing crops that contain very small quantities of organic matter, on the 

 whole, the quantity of this material in average soils is considerable. 

 The average organic content of soils amounts to 28 tons per acre in 

 the first 8 inches of soil and 50 tons in the soil and subsoil together to 

 the depth of 2 feet. In regard to this organic matter it has been 

 beheved for many years that it consisted chiefly of some such body 

 as humic acid, differing perhaps in different soils, but having the 

 same general properties. One prominent service which these inves- 

 tigations have rendered agriculture has been to show the nonexistence 

 of humic acid and its hypothetical relatives. 



In considering the importance of the organic matter of the soil it 

 should be borne in mind that it is material that is the result of change, 

 and that much, perhaps all of it, is susceptible of still further change; 

 that is, it is in the transition stage. The changes which it has under- 

 gone and changes which it may still undergo are determined by a 

 number of factors, chief of which are moisture, aeration, character 

 of microorganisms, and mutual relation of the organic compounds 

 and the mineral constituents. These factors are many of them 

 influenced or controlled by cultural methods, including fertilizing, 

 drainage, irrigation, inoculation, etc., used in practical agriculture. 

 The operations of irrigation, conserving of moisture by mulches, 

 aerr.tion by cultivation, inoculation with cultures of bacteria, addi- 

 tion of organic and green manures, are all cornmon agricultural 

 methods used by farmers and they are also operations that influence 

 the chemical changes which soil organic matter undergoes. 



