244 SOIL CONDITIONS AND PLANT GRO WTH 



There is the possibility of much useful work in analysing 

 the phenomena of " sourness," and if acidity be the effective 

 agent, in determining the limits of tolerance of acidity by the 

 common crops and disease organisms. The old recommenda- 

 tion to add sufficient lime to ensure neutrality breaks down in 

 the case of potatoes and of certain market garden crops, 

 rhubarb, and in America blueberry,^ while in other cases it 

 involves an unnecessarily large addition of lime. The object 

 to be aimed at is to keep within certain limits. 



Potato growers, whose scheme of cropping includes clover 

 and potatoes, are frequently in an awkward dilemma : if they 

 withhold lime from the soil the clover fails, if they add lime 

 the potatoes become affected with "scab". The ideal 

 arrangement would be to alter the reaction of the soil with 

 each course in the rotation, making it just. neutral in the clover 

 year, and in the potato year just sufficiently acid to suppress 

 the " scab " organism. Unfortunately, while it is easy to move 

 in the direction of less acidity, it is difficult to effect the con- 

 verse change and increase the acidity.^ 



Are Toxins Present in the Soil? 



A persistent idea that one crop may poison another is 

 current among practical men. Early in the last century De 

 Candolle formulated the hypothesis that plants excrete from 

 their roots toxins that remain in the soil for some time and 

 injure other plants of the same species, but not necessarily 

 plants of different species. He thus explained the well-known 

 fact that a rotation of crops is more effective than a system 

 of continuous cropping ; in a rotation the toxin excreted by 

 a particular crop is innocuous to the succeeding crop and 

 disappears from the soil before the same plant is sown again. 



The hypothesis was tested in a classical research by 

 Daubeny at Oxford (78), but could not be justified. Eighteen 



1 F. V. Colville, U.S. Dept. Agric. Bull, 334, 1915. 



'^ J. C. Lipman has made the interesting suggestion that sulphur could be 

 added to the soil to increase the acidity (Soil Set., 19 19, 7, 181). 



