76 



Journal of Agricultural Research 



Vol. XVII. No. 2 



on the basis of percentage loss on ignition, has not consistently increased 

 the amount of disease seems to warrant the conclusion that the growers 

 have little to fear in the way of increased infestation of the soil as a result 

 of plowing under green manures or applying lo, 20, or 40 tons of manure 

 to the acre. 



INFLUENCE OF THE CLAY AND SAND CONTENT OF THE SOIL 



The value of sand or sandy soils in reducing the severity of the rootrot 

 and its increased severity in clay soils has been especially suggested by 

 Benincasa {2) and Gilbert (12). 



A pot experiment with pure quartz sand and with the purest clay 

 obtainable was carried out with the hope of throwing more light upon 

 this subject. Theonly factor which it is desirable to vary in such an experi- 

 ment is the size of the soil particles. Although this is not practicable, 

 the relative proportion of sand and clay particles no doubt resembles soil 

 conditions equally well. Superior red clay was obtained from the sub- 

 station of the State experiment station located at Ashland, Wis. This 

 is a very "heavy" pure clay soil containing very little organic matter and 

 is low in fertility. The sand used was a medium to coarse ground quartz. 

 The mixtures of sand and clay made were those shown in Table IX. 



Table IX. — Influence of relative amount of clay and sand on rootrot of tobacco 



The experiment in this case was confronted with practically the same 

 difficulties and complications as occurred in the tests with organic mat- 

 ter. The clay soil alone, or in mixtures with sand, seemed to have an 

 "injurious" action upon the growth of tobacco which was not remediable 

 with nutrient solutions applied. The yield, therefore, was low in all 

 cases. Two pots of each series were inoculated with equal volumes of 

 spore suspension from cultures of T. hasicola on agar, which were thor- 

 oughly mixed with the soils. White Burley tobacco was then trans- 

 planted into them. The first experiment was started November 18, 19x6. 

 The result of this experiment was not recorded by weight, but some of 

 the crocks were photographed (Pi. 5, I), and serve to illustrate the re- 

 sults obtained. The conclusion drawn from this experim.ent was that 



