BUREAU OF SOILS. 505 



Isolation in a pure condition of some of the organic constituents of 

 soils has made possible the correct interpretation of soil chancres and 

 the discovery of compounds harmful to crops. This hariiiful effect 

 has already been em])hasized in former reports. This line of re- 

 search has been especially ])rofitable during the last year and has led 

 to the isolation ot more than twenty compounds. The classes of 

 materials found are the fats and oils, fatty acids, hydrocarbons, car- 

 bohydrates, esters and alcohols, as well as many nitrogenous com- 

 pounds. Some of these compounds consist of carbon and hydrogen 

 onlv; some contain carbon, h3^drogen, and oxygen; some carbon, 

 hydrogen, oxygen, and nitrogen; and still others, in addition to these, 

 sulphur and phosphorus. 



Prior to this investigation not a singxe organic constituent of the 

 soil was kno%\Ti. All the hypothetical compounds designated as 

 humic acids have been shown to have no definite existence. In 

 carrying out these researches great difficulties had to be overcome. 

 The*^ results thus far obtained are very gratifying. This knowledge 

 of soil constituents is of the utmost importance to every soil inves- 

 tigator, whether he be a chemist, a bacteriologist, or a plwsicist 

 studying some special problem, or an agronomist, dealing with the 

 general relations of soils to crops. The organic matter of soil is 

 ever changing. On it work bacteria, molds, and other life within 

 the soil, obtaining therefrom their existence and producing in turn 

 other compounds and b3'-products beneficial, harmful, or innocuous, 

 according to the kind of organisms or soil conditions under which the 

 actions take place. The organic compounds modify all other chemi- 

 cal interactions, influencing absorption, the movement of the soil 

 solutions, and the water-holding power of soils, and tliis definite 

 information is therefore of the greatest interest in giving direction 

 and definiteness to many lines of soil investigation and soil man- 

 agement. 



These researches into the nature and properties of soil organic 

 matter have shown conclusively that the soil mvestigator must take 

 into consideration the presence of organic compounds in the soil, 

 which may be beneficial, harmless, or inimical to growing crops. 

 The presence of the harmful compounds may cause, according to 

 their specific action or to the amount in which they are present, all 

 stao;es of crop depression from efl"ects probably in many cases so 

 slight as not to be noticed in actual difference in yield, although per- 

 haps in quality or keeping quality of special crops, to the more 

 marked efiects when |)ractical sterility is observed. A complete 

 solution of the difnculty which the presence of such bodies may 

 occasion the practical tiller of the soil can only be reached by thorough 

 inquiry into the chemical and biological nature of such compounds, 

 by a knowledge of their origin and the causes leading to their forma- 

 tion or accumulation m the soil, and by an investigation into treat- 

 ments of the soil — chemical, such as the addition of fertilizers, lime, 

 etc.; physical, such as drainage, cultural methods, etc.; or biological, 

 such as the influence of crops and their remains on the soil, crop 

 succession or crop rotation, number and kind of bacteria, fungi, 

 etc. — to the end that the best practical means for tlieir removal, 

 destruction, or prevention may be determined and soil treatments 



