Scientific Agriculture. 



234 



[March, 1912, 



But this simple view of the constitu- 

 tion of the soil must in many cases be 

 complicated by another factor because 

 the relative infertility of a soil is often 

 communicated to its aqueous extract. 

 Thus two soils of Cecil clay, very similar 

 in physicial characteristics, show marked 

 differences in fertility, and their aque- 

 ous extracts showed like differences. But 

 we have seen that the soil solution is 

 similar in composition in all soils, and 

 direct analysis proved these two aqueous 

 extracts to be no exceptions to the rule 

 Hence the infertility of the poor soil 

 was not due to any deficiency of food- 

 stuffs, but to some toxic substance. 

 Numerous other cases have been investig- 

 ated, and wherever the relative in- 

 fertility of a soil is transmitted to the 

 aqueous extract it is supposed that some 

 toxin is present. It is further con- 

 sidered that the toxin is organic, 



So great was the toxicity of the solu- 

 tion that in some instauces it was an 

 even poorer medium for plant growth 

 than distilled water. But Livingstone 

 found [14] that its toxicity could be 

 lessened in various ways : by dilution, 

 when indeed it might actually increase 

 plant growth ; and also by shaking with 

 calcium carbonate, precipitated ferric 

 or aluminium hydroxides, cotton wool, 

 shredded filter paper or carbon black ; 

 the solution then became much im- 

 proved as a culture medium. Further, 

 the addition of fertilisers, especially 

 such as were known to benefit the soil, 

 also improved the solution ; the organic 

 matter of dung and of green manure 

 was very useful and had some specific 

 effect not shown by the inorganic con- 

 stituents. In one experiment wlieat 

 plants gave the following results :— 



of Trans- plants, 

 piration. to P 3 onI y- 

 Green. Dry. 



1. Plants grown in soil extract 



alone .. .. 100 100 100 



2, „ „ ,, + dung 



extract ... ... 400 213 151 



,, „ ,, + ash of 

 dung extract + Na N03 

 equivalent to nitrogen 



lost in preparing the ash 196 112 109 



The value of the dung extract is clear- 

 ly not due to its ash ingredients or its 

 nitrogen, because all these are present 

 in Series 3. " That the organic matter 

 is directly of use to plants as nutrient 

 material is not probable," say the 

 authors ; "it appears to be beneficial 

 largely through some action on the soil 

 constituents," It is therefore considered 

 that the infertility of many soils is due 

 to toxic constituents, and as these are 

 probably organic a double set of experi- 

 ments was begun by Schreiner and others 



[16-21J ; the effect on plant growth of 

 organic compounds likely to occur in the 

 soil was studied by elaborate water 

 cultures ; and a careful search was made 

 in the soil itself for such organic com- 

 pounds as could be identified. 



The water culture experiments showed 

 that numerous substances particularly 

 neurine and guanidine were toxic to 

 plants. In general oxidation reduced 

 toxicity : choline was less and retain 

 still less toxic than neurine. Tyrosin 

 again was harmful but the black oxid- 

 ation product it affords was actually 

 beneficial. The toxicity of neurine and 

 guanidine and presumably of the other 

 substances also was reduced by addition 

 of charcoal, pyrogallol, calcium carbon- 

 ate and sodium nitrate. Further, when 

 two crops were grown successively in 

 the same solution the second did better 

 than the first, an effect that may be 

 connected with an oxidising power 

 possessed by plant roots [16]. The com- 

 bined effect of plant roots and of ferti- 

 liser was very potent in overcoming the 

 effects of a toxic body. 



The search for organic compounds in 

 the soil is complicated by the presence 

 of large amounts of sand, slit clay and 

 highly complex organic substances. In 

 consequence the problem has not at- 

 tracted workers; the first serious in- 

 vestigation was made by Schreiner and 

 Shorey [19J, who obtained a number of 

 compounds, including two hydroxy- 

 stearic acids, a piccoline carboxylic acid 

 and several products of protein and 

 nucleprotein hydrolysis. The harmful 

 effects on plant growth of one of these 

 dihydroxystearic acid was investigated 

 at some length [21], 



Several attempts have been made to 

 discover how these toxic organic sub- 

 tances arise in the soil. Whilst some 

 are supposed to be normal decomposi- 

 tion products of the complex organic 

 matter of the soil, others are considered 

 to be excretions from plants. The old 

 hypothesis of de Candolle is revivedi and 

 these excretions are supposed to be 

 harmful to other plants of the same 

 kind but not necessarily, to plants of a 

 different kind. An explanation is thus 

 afforded of what is said to be common 

 phenomenon ; the sterility finally in- 

 duced when one and the same crop is 

 grown continuously on the same ground. 

 Water culture experiments are quoted 

 to show that wheat seedlings exude 

 from their roots something that makes 

 the solution toxic to a subsequent batch 

 of seedlings, 



Lastly, the part played by fertilisers 

 has been studied. The beneficial effects 



