146 SCIENCE PROGRESS 



manures, whilst non-nutrient salts are not in use for this 

 pupose, is strong ground for allowing nutritive functions to 

 fertilisers ; nor, since their complex action is admitted, do we 

 see what is gained by denying them such functions. 



Among the numerous organic compounds of the soil that 

 Schreiner and his co-workers are isolating, it is not surprising 

 that some possess toxic properties. But it does not follow 

 that a substance toxic in water culture is toxic in the soil. 

 Powdered charcoal reduces or removes the toxicity of the 

 solution apparently by absorbing the toxic substances ; soil 

 also is known to possess similar absorptive powers, indeed 

 in one of Livingstone's experiments soil was actually found 

 to reduce the toxicity. We cannot therefore take it as proved 

 that toxic substances isolated from the soil have been acting 

 as toxins. There is, however, indirect evidence of the presence 

 of toxic substances in the soil under certain conditions. It 

 has already been stated that soils can be divided into two classes 

 in their relationship to plant growth according as they do or 

 do not contain calcium carbonate. Soils deficient in calcium 

 carbonate are known to be relatively infertile ; the practical 

 man calls them "sour" and writers usually explain matters 

 by saying that acids are present which plants will not tolerate. 

 There is reason to suppose that the micro-organic flora of 

 the soil is altered when the calcium carbonate falls below a 

 certain limit and we can take it as certain that the course 

 of decomposition in such soils differs from that going on in 

 presence of sufficient calcium carbonate. Unfortunately no 

 investigations have been made in this country of "sour" soils 

 in spite of the interesting problems they present, but there are 

 indications pointing to the presence in them of toxic substances. 



In their disregard for chemical analyses, the Bureau of Soils 

 do not generally state whether or not the soils under investiga- 

 tion are rich in calcium carbonate. But from their general 

 description it appears that the soils giving the toxic extracts 

 are in the "sour" category, especially as calcium carbonate or 

 lime commonly improve the soil and the extract. So far as 

 " sour " soils are concerned, therefore, we agree with Whitney 

 that toxic organic substances may be a cause of infertility. 

 We have no evidence, however, that such substances are 

 present in soils sufficiently provided with calcium carbonate, 

 in which class are included all our well-farmed soils. 



