120 MASS. KXPKIUMKNT STATION lUM.l.KI'lX 27fi 



Tabi.k 1. — Ep'fects of oicriioi'iiosi'jrouic acid ox pH valfks of sou. and ox 



GUOWTH AND lU.ACK IIOOT-ROT OF ']()HAC'CO IN LIIMKI) SOIL. 



Orthoi)hosphoric acid 



per 100 grams dry soil pH values 



c.c. of N/1 acid of soil 



None 5.9 



15 5.0 



20 5.0 



25 5.0 



tlie soil from 5.9 to 5.0, but black root-rot of tobacco was nevertheless severe — 

 rather more severe at pH 5.0 with orthophosphoric acid than at pH 5.9 with- 

 out this acid. The degrees of infection may he seen by reference to Figure 1. 

 Plants were larger, however, with orthophosphoric acid than withcmt it. 



It is evident from this that other factors than or in addition to the hydro- 

 gen-ion concentration of the soil are involved. This soil with its pH value 

 lowered by orthophosphoric acid far below the usually accepted danger point 

 for black root-rot was no less, in fact it was little more favorable to infection 

 of tobacco by Thielavia than the same soil with the initial pH value of 5.9 

 but without treatment with orthophosi>horic acid. 



.Some i)hosp]i()rus is of course necessary for the growth of tobacco, al- 

 though according to Anderson and his co-workers (4) tobacco on old tobacco 

 land in Connecticut may be grown without any carrier of j)hosphorus in the 

 fertilizer mixture, since large quantities have accumulated in such soils. 



The work here described was confined to pot experiments, but so far as 

 these results are indicative, they suggest applying no more phosphorus than 

 is necessary for the best growth of tobacco when one object is to hasten the 

 dying-out of Thielavia in over-limed and infested soil. 



As described in a later part of this i>aper, however, there Mas no increase 

 in black root-rot when sidfuric acid and orthophosphoric acid were used to- 

 gether in the field. 



With Aluminum Sulfate and Sulfur 



It is ])ossible that certain soluble toxic substances, for instance salts of 

 aJuiiiinmn, in addition to (or instead of) hydrogen-ion concentration, inhibit 

 the growth of Thlehivia Ixialrola in acid soil, and that orthophosphoric acid 

 as well as lime inactivates these toxic substances with consequent benefit to 

 the fungus. This is in line with the exi>lanation by Carr and Havercamp {loc. 

 cif.) of the beneficial efl'ect of ])hosi)horic acid on growth of ])lants, i. e., 

 ]>hosi)horic acid forms less available and less toxic conqiounds with iron and 

 aluminum in the soil. 



Sulfur, sulfuric acid, and alumimuii sulfate (.VL (SOi), .ISII.O), sepa- 

 rately or together, were added to soil infested witli Thlcldvia haaicoht, and 

 tobacco ])laiits were grown in this soil (in 2-gal]()n crocks). The effects of 

 each treatment on soil ])II value, dry weight of ])laiits, and black root-rot 

 are recorded in Table 2. 



The pi I value of the soil was lowered by all treatments, but more by 

 almniniun sulfate with sulfuric acid or sulfur than by either of these chemicals 

 alone. All of the acidification treatments reduced the se\erity of black root- 

 rot, although none of them eliminated it. 



