210 THE PLANT WORLD. 



have brought out a number of valuable contributions. In Bulle- 

 tin No. 41 of that Bureau, Frank K. Cameron and James M. 

 Bell present the results of a study of "The Action of Water and 

 Aqueous Solutions upon Soil Phosphates." The discussion lies 

 mostly in the realm of physical chemistry, as must naturally be 

 the case, but direct application of the facts brought out is made 

 to the theory of fertilizer practice in agriculture. ''The phos- 

 phates of the soil " " * are of such a nature as to yield 

 a solution containing very small quantities of phosphoric acid. 

 "■ * * The least soluble phosphates are the ones which will 

 be formed and will control the concentration of the soil moisture. 

 This fact, together with the well known phenomena of absorp- 

 tion, gives a satisfactory explanation of the observation that the 

 concentration of the soil moisture is low and varies but little for 

 different soils and with the total amount of phosphoric acid in 

 the soils. For the same reason, the addition of phosphatic fer- 

 tilizers can not be expected to influence materially the concen- 

 tration of phosphoric acid in the soil moisture. The action of 

 phosphatic fertilizers is, therefore, on the soil and not primarily 

 on the plant ; for tlie concentration in plant food constituents 

 of the solution on which the plant feeds is not materially 

 altered by the addition of phosphatic fertilizers in the amounts 

 used in ordinary field practice." B. E. L. 



Few scientific theories have stood the test of time and exper- 

 iment so long and gained such universal acceptance as the Liebig- 

 theory of soil fertility. jSTevertheless, recent investigations seem 

 to indicate that this conception is likely to undergo considerable- 

 modification in the near future, suggesting that the ''exhaustion"' 

 of many soils may be due, not so much to the withdrawal of min- 

 eral constituents as to the accumulation of certain organic toxie 

 substances. This idea is not a new one, for as long ago as 1832 

 DeCandolle stated that many plants give off in their growth sub- 

 stances which are injurious to themselves and to closely related 

 plants, but harmless to others. This conception, based on insuffi-^ 

 cient evidence, as it was, did not gain acceptance, but lias smouK 



