10 ORGANIC CONSTITUENTS OF SOILS. 



cultures. A control in pure distilled water or in an extract from a 

 fertile soil should be run at the same time. In such cases the plants 

 grown in the extract from the poor soil will show many peculiarities 

 not shown by the plants either in the pure distilled water or in the 

 extract from the good soil. The plant will be smaller, with swollen 

 tips, which are sometimes bent into hooks, a phenomenon character- 

 istic of certain toxic action. The growth in this soil extract may 

 even be considerably less than the growth in the distilled water, 

 although the soil extract contains plant nutrients, whereas the dis- 

 tilled water contains none. 



If a separate portion of the original extract be treated with carbon 

 black, made from natural gas by imperfect combustion, much as 

 lampblack is formed on lamp chimneys, agitated, and filtered, the 

 filtrate will be a good medium for the growth of the seedlings. This 

 simple treatment with carbon black has, therefore, removed by 

 absorption the harmful properties of the soil extract, and the growth 

 is now even better than in the distilled water. 



Another experiment with soil and carbon black was made as follows : 

 A layer of moist carbon black was covered with a layer of moist unpro- 

 ductive soil, and this in turn by a layer of moist carbon black. In 

 this experiment the moisture could circulate from the soil to the car- 

 bon black and back again, and thus gradually the soil fluids would be 

 freed of any injurious compounds by absorption into the carbon black. 

 At the end of a day or two of this interaction the soil was freed from 

 the carbon layers, plants were grown in it, and when compared with 

 soil not so treated a very marked improvement was shown, again 

 indicating that a harmful body was originally present and had been 

 removed in whole or in part by this carbon-black treatment. 



Observations of this kind on many soils, together with a study of 

 the properties of the material dissolved in the water, led to no definite 

 isolation of the compound or compounds showing the harmful effect, 

 owing to the fact that the quantities in the water extract were too 

 small for identification, but they did lead to a recognition of the 

 fact that the substances were not mineral in character, but were 

 constituents of the organic matter of the soils. 



That organic substances could produce such effects in such small 

 quantities as must be present in soil solutions was not apparent from 

 the literature, and it became necessary to establish this point. With 

 this in view about 40 substances of organic origin which may get into 

 the soil or be formed therein were tested, and it was conclusively 

 shown that a number of these are decidedly harmful to plants, even in 

 very dilute solutions comparable with the organic content of soil 

 solutions. 



It therefore became essential to make a study of the organic mat- 

 ter of the soil. This, however, was a subject about which the older 



