126 



MASS. EXPERIMENT STATION BULLETIN 276 



From limed plots, in the absence of acidifying treatments, alfalfa re- 

 moved as much calcium and magnesium oxides as did tobacco (but no more), 

 and timothy did not remove half as much as did tobacco. Tobacco was sim- 

 ilarly more effective than alsike clover on plots not limed. 



Much more of the calcium and magnesium oxides was removed by tobacco 

 on limed plots acidified with sulfur or with sulfuric and orthophosphoric acids 

 than by tobacco, alfalfa, or timothy on limed plots not acidified. 



The effects of tobacco, alfalfa, and timothy on the pH values of soil in 

 limed pilots in the years 1926 to 1930, inclusive, may be compared by reference 

 to Tables 11-15. The pH values of the soil in all of these limed plots were 

 about 6.0 or 6.1 in 1930, and in no case were they lower in 1929 and 1930 

 than in 1926; in fact, they were all somewhat higher in the last two years. 



The substitution of alfalfa or timothy for tobacco for two or three years 

 did not result in soil reactions any less favorable to ThleJavia basicola. If, or 

 when, these crops have any effect on black root-rot of tobacco which may 

 follow them in a rotation system, this effect is ajiparently exerted through 

 some other factor than soil reaction. 



Alfdlfa and Black Root-Rot 



Black root-rot caused by Thielavia basicola has been reported (2, 19) as 

 t'ccurring on several species of legumes, including alfalfa. There is an opinion 

 (12, 17), therefore, that legumes should not be grown preceding tobacco in a 

 cropping system. According to Major (22) and Burkholder (10), alfalfa is 

 suscej)tible to infection by Thielavia basicola: but in experiments by Johnson 

 (19) the suscejitibility of alfalfa to this fungus was clas.sed as low (compared 

 with that of other hosts). Thielavia has been reported (2) as occurring on 

 alfalfa in only three states, although the roots of alfalfa have presumably 

 been examined by plant pathologists in many more. 



