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March, 1912.] 



287 



Scientific Agriculture, 



action is admitted, do we see what is 

 gained by denying them such functions. 



Among the numerous organic com- 



Eounds of the soil that Schreiner and 

 is co-workers are isolating, it is not 

 surprising that some possess toxic pro- 

 perties. 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 experi- 

 ments 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 relation- 

 ship 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-organio flora of the soil 

 is altered when the calciam 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 Soil9 do not generally 

 state whether or not the soils under 

 investigation are rich in calcium carbon- 

 ate. But from their general descrip- 

 tion it appears that the soils giving the 

 toxic extracts are in the "sour" cate- 

 gory, 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 sub- 

 stances are present in soils sufficiently 

 provided with calcium carbonate, in 

 which class are included all our well- 

 farmed soils. 



When we turn to the question of plant 

 excretions we can find no evidence what- 

 soever in favour of their existence. At 

 Rothamsted plants have been grown 

 continuously in water cultures, in sand 

 cultures and in soils without showing 

 any sign of suffering from excretions of 

 previous generations. Six crops of rye 

 were grown in succession in sand to 

 which only nutrient salts were added so 

 as to maintain the food material at a 

 constant amount. A seventh crop was 

 then taken, and at the same time a crop 

 was grown on perfectly fresh sand on 

 which nothing had grown before, sup- 

 plied with an equal amount of the same 

 nutrient salts. There was no significant 

 difference in the two crop yields, they 

 agreed within the 5 per cent, experi- 

 mental error. A similar experiment was 

 made with buck-wheat, another with 

 spinach, and a parallel series was made 

 in soil cultures. In all cases the result 

 was the same ; the 1910 weights were as 

 follows :— 



Cropped six times previously... 

 Fresh sand 



1910 Crops: Mean of Four Pots. 

 Sand Cultures. 

 Weight of dry matter, 

 grams. 



R * e - whea k t. S P ina <*- 

 30 4 5-4 33-3 

 31-3 13-5 29-5 



Both sand and soil contained 2 per cent, of calcium carbonate. 



Soil Cultures. 

 Weight of dry matter, 

 grams. 



Buck- 

 wheat. 

 23-9 

 25-2 



Rye. 



26-4 

 271 



Spinach. 



20-0 

 20 '8 



If either the rye, buckwheat or spinach 

 excreted any toxin the amount accu- 

 mulating during the growth of six suc- 

 cessive crops waB insufficient to cause 

 any appreciable depression in yield in 

 the crop; the exceptional result given 

 by buckwheat in sand could not be 

 confirmed. 



Thus we are once more up against a 

 question of fact ; it is asserted by the 

 American workers that the continued 

 growth of one and the same crop on the 



same soil leads to a low crop-production, 

 whilst we on this side are unable to 

 obtain any evidence to this effect. 



But again the experimental difficulties 

 are considerably greater than they 

 appear at first sight. Water cultures do 

 not long remain sterile but soon contain 

 a bacterial population which clearly 

 produces some change in the solution, 

 because the nitrates quickly disappear. 

 Sand cultures rapidly develop a vigorous 

 growth of algee and bacteria ; here also 



