AVAILABILITY OF PLANT NUTRIENTS 347 



solution being similar in all soils, it follows that the rela- 

 tive productiveness of different soils bears no relation to 

 the supply of soluble nutrients, but must be due to other 

 factors. Hence soluble fertilizers increase plant growth, 

 not by supplying a greater quantity of plant nutrients, 

 but through other effects on the soil — as, for instance, 

 their favorable influence on tilth, or through the de- 

 struction of toxic matter. 



250. Discussion of the theories regarding soil solu- 

 tions. — The difficulty in securing a true sample of the 

 soil solution as it exists in situ complicates any attempt 

 to ascertain how these theories comport with the actual 

 condition of the soil solution. A number of attempts 

 have been made to throw light on this subject, but none 

 of the data obtained is of a nature to definitely prove the 

 correctness of either theory. The evidence, so far as it 

 goes, indicates that the water extract of soils differs in 

 concentration in different soils, and is increased, under 

 some conditions, by large and continued applications 

 of soluble fertilizers. There can be no doubt, more- 

 over, that plant growth in properly balanced nutrient 

 solutions increases with the concentration of the solu- 

 tion up to several thousand parts to the million, as has 

 been demonstrated by many experiments. 



One rather convincing experiment may be quoted. 

 Hall, Brenchley, and Underwood ! analyzed the water 

 extract from certain plats on the Rothamsted Experi- 

 ment Station farm, the fertilizer treatment and the 

 yields of which had been recorded for a long term of years. 



iHall, A. D., Brenchley, W. E., and Underwood, T. M. 

 The Soil Solution and the. Mineral Constituents of the Soil. 

 Philosoph. Trans. Royal Soc. London, Series B, Vol. 204, pp. 

 179-200. 1913. 



