July, 1909.] 



48 



Scientific Agriculture, 



this phosphoric acid manuring is due to 

 the soluble nature of the fertilizer, be- 

 cause of which the additional plant-food 

 is directly available for the crop." A 

 study of the re-action in the soil, how- 

 ever, shows that this theory is in- 

 sufficient ; the soluble phosphoric acid 

 is very rapidly precipitated in the soil, 

 and remains so close to the surface 

 that it is never Avashed out in the 

 drains. Upon such facts as these, 

 Messrs. Whitney and Cameron argue 

 that "the concentration of the soil 

 water for a given plant-food, such as 

 phosphoric acid, must be approximate- 

 ly constant for all soils of the same type, 

 however much or little phosphatic ferti- 

 lizer may have been applied, and since 

 water culture experiments show that 

 this low limit of concentration attained 

 by the soil water is more than sufficient 

 for the needs of the plant, no soil can be 

 regarded as deficient in this or any other 

 element of plant-food. It therefore fol- 

 lows that the action, if any, of a 

 fertilizer must be due to some other 

 cause than the direct supply of plant- 

 food, with which the soil water must 

 always be saturated to a degree which 

 is quite unaffected by the supply of 

 fertilizer ;" and following up this con- 

 clusion they suggest this theory of ferti- 

 lizer action:— "A soil falls off in ferti- 

 lity and ceases to yield normal crops, 

 not because of any lack of plant-food 

 brought about by the continuous with- 

 drawal of the original stock from the 

 soil, but because of the assimilation of 

 injurious substances excreted from the 

 plant itself, These toxins are specific to 

 each plant, but are gradually removed 

 from the soil by processes of decay, so 

 that if a proper rotation of crops is 

 practised, its yield will be maintained 

 without the intervention of fertilizers. 

 The function of fertilizers is to precipi- 

 tate or put out of action these toxins 

 rather than to feed the plant." 



Mr, Hall's criticism of both theory and 

 arguments cannot be considered as any- 

 thing less than destructive. He declares 

 that Messrs. Whitney and Cameron's 

 views cannot have any bearing whatever 

 on the amount of nitrates in the soil 

 water "since they come into a dissolved 

 state as fast as the nitrifying bacteria 

 produce them, and are not in equili- 

 brium with any store of undissolved 

 nitrates in the background. As regards 

 phosphoric acid the theory assumes such 

 as excess of bases that all soils behave 

 alike and immediately precipitate the 

 phosphoric acid in practically the same 

 form ; while as regards potash the 

 argument seems to forget that though 

 the additiou of a soluble potassium salt 

 may throw some of the other sparingly 



soluble potassium compounds out of 

 solution, the total amount of potassium 

 remaining in solution is still greatly in- 

 creased- The function of the carbonic 

 acid in the soil water is ignored, as again 

 the fact that the processes of solution 

 in the soil must be in a constant state of 

 change." The crucial test by analysis of 

 the soil water fails, so far as Rothamsted 

 is concerned, for "When the Rotham- 

 sted soils, with their long-continued 

 difference in fertilizer treatment, are ex- 

 tracted with water charged with carbon 

 dioxide, the nearest laboratory equi- 

 valent to the actual soil water, the 

 amount of phosphoric acid going into 

 solution is closely proportional to the 

 previous fertilizer supply." The new 

 theory also supposes that the plant it^ 

 self exerts no solvent action ; but besides 

 Sachs' famous "etching" experiment, 

 an ingenious device by Kossowitsch 

 seems to prove conclusively that roots 

 have a preponderating influence on the 

 assimilation of phosphoric acid at any 

 rate. "The only factor determining the 

 supply of phosphoric acid and the conse- 

 quent difference in growth," says Mr. 

 Hall in quoting this experiment, " was 

 the solvent action of the roots where 

 they were actually in contact with the 

 calcium phosphate, and this solvent 

 action may most probably be attri- 

 buted to the carbon dioxide secreted by 

 the roots." 



Not Proven. 



With regard to the "toxin" part of 

 the theory, Mr. Hall has some pertinent 

 comments to make. He doubts whether 

 the alleged toxins extracted from the 

 soil by Whitney are really excreted by 

 the roots of the plants and whether they 

 are really toxic in the soil merely be- 

 cause they have been proved so in water 

 cultures. " A body like ammonia, itself 

 a product of protein decay and present 

 in the soil, is exceedingly toxic to water 

 cultures, yet when applied to the soil 

 it increases the growth of the plant." 

 Particularly is it the specific action of 

 tertilizers that is so difficult to explain 

 on this hypothesis. "Why," he asks, 

 " should substances so dissimilar as ni- 

 trate of soda and sulphate of ammonia 

 exert the same sort of action on the same 

 toxin ? Why should phosphates cau^e all 

 classes of plants to develop in one direc- 

 tion, or why should it be appropriate to 

 the toxins of all plants on one particular 

 type of soil, whereas potash answers on 

 another type? Lastly, there is a lack 

 of evidence for the fundamental thesis 

 that the rotation will take the place of 

 fertilizers, and that the yield only falls 

 off when a particular crop is grown 

 continuously on the same land." This 



