130 



MASS. EXPERIMENT STATION BULLETIN 229 



on such a soil the location of tlie critical region would be lower. On the 

 basis of the yields of the last three years it may be stated that, as the acidity 

 of the soil becomes less above 5.9, the loss caused by black root-rot becomes 

 greater. (The upper limit of alkalinity has not been determined.) Plots on 

 the limed areas in this experiment may be placed in four groups on the basis 

 of the pH values of their soils. These are correlated with average yields as 

 follows : 



pH 5.9 to 6.1 — yield 1233 pounds 



" 6.1 " 6.2 — " 1183 



" 6 2 " 6.3 — " 1120 



" 6 3 " 6.4 — " 1039 



We have, then, quantitative evidence of the increasing injury from black 

 root- rot which results when by the addition of the lime the soil is made less 

 and less acid to a point beyond pH 5.9. 



Huw Muck Lime Moy De Used with Safety? 



If the soil has reached such a pomt, and if it is seriously infested with the 

 black root-rot fungus, what then may be done? If no further lime is applied 

 and if tobacco is continuously grown on such soil, the loss of lime by leach- 

 ing and by removal in the plant will gradually result in a more acid condi- 

 tion. Of course the loss from black root- rot may be so great as to make to- 

 bacco growing on such land unprofitable. Continuous tobacco, no further lim- 

 ing, and the use of acid fertilizers, such as sulfate of ammonia and sulfate 

 of potash, are all the remedial measures to be suggested at this time. A 

 quicker method of increasing soil acidity is desirable, and this matter is now 

 imder investigation. 



The Effect of Timothy 



Since timothy is extensively used and widely recommended as a cover crop 

 for tobacco soils, it was assumed at the outset that it was known to increase 

 the yield of tobacco by virtue of the organic matter which was added to the 

 soil or the plant food which was saved from leaching through its use. It was 

 included in the present experiment to see whether it influenced the damage 

 from black root-rot and as a check against the clover cover crop which was 

 supposed to increase root-rot because it is a host plant of Thielavia. At the 

 end of the iirst season, therefore, it was a surprise to find that both the in- 

 oculated and uninoculated timothy plots showed a reduction in yield — 13 per 

 cent and 18 per cent on the inoculated and uninoculated plots respectively. 

 It could not have been a result of the timothy depleting the soil water be- 

 cause 1922 was an extremely wet year. Losses from the use of timothy both 

 on the inoculated and uninoculated plots, but in a smaller degree, were also 

 recorded in 1923 and 1924. Certain plots at times showed small increases 

 over adjacent plots without cover, but considering the experiment as a whole, 

 the conclusion is inescapable that the effect of timothy has been to decrease 

 both the yield and value of the tobacco. This is in agreement with data re- 

 cently published by Jones (8) who reports an average decrease in yield of 13 

 per cent associated with the use of the timothy cover crop. The percentages 



