BLACK ROOT-HOT OF TOBACCO 11!» 



PRELIMINARY EXPERIMENTS 



A lirief sunimary of tlie results of some of these jireliiiiiiiary experiments 

 lias already been published (13). 



The soil used, exce})t as otherwise indicated, is an ice-deposited stony 

 loam, heavil}' infested with ThieJavia hnsirola. To determine the quantity of 

 each acid necessary to cause the desired lowering- of the pH value of tlie 

 soil, a known number of cubic centiiueters of a normal acid was applied in 

 ])reliminary experiments to each 100 grams of air-dry soil, together with 

 enough water to bring the soil up to 60 per cent of its water-holding capacity. 

 Tiie ])!! values of the soil were determined at various intervals of time 

 thereafter.- 



With Sulfuric and Nitric Acids 



Sulfuric and nitric acids were alxiut equally eii'ective in increasing the 

 hydrogen-ion concentration of this soil and in reducing the intensity of infec- 

 tion of tobacco by Thielavia; but nitric acid was so toxic to tobacco j)lants 

 that it was not used in subsequent field experiments. 



The growth of tobacco could be correlated with pH value of acidified 

 soil only as each acid was considered sejiarately. For example, an applica- 

 tion of nitric acid by which the pH value of the soil was lowered to 5.-5 (from 

 (i.O) prevented practically all growth of tobacco, while an application of sul- 

 furic acid by which the pH value of the soil was lowered to 4.6 (from 6.0) 

 permitted good growth of tobacco. The hydrogen-ion concentration of the 

 soil is not the only factor involved, and as Carr and Havercamp (11) have 

 pointed out, the toxicity to plants of an acid applied to soil is possibly at- 

 tributable to easily soluble salts (as of iron and aluminum) which the acids 

 form in the soil. Be that as it may, it appears that the oi^timum pH value of 

 soil for the growth of tobacco (and presumably other plants) is not fixed but 

 depends partly on the means by which the soil reaction is adjusted. 



With Orthophosphoric Acid 



A greater increase in growth of tobacco resulted from tiie application of 

 orthophosphoric acid than from either sulfuric or nitric acid; although or- 

 thophosi)horic acid had much less eifect in lowering the pH value of the 

 soil than either of the others, and did not at all reduce the intensity of in- 

 fection by Thielavia. Whenever orthophosphoric acid was applied to soil in- 

 fested with this fungus, black root-rot was severe or rather more severe, al- 

 though the pH value of the soil was thus lowered as much as to 5.0; but 

 when orthophosphoric acid was applied to soil not limed and not heavily in- 

 fested with Thielavia, no increase in black root-rot resulted. 



In one series of pot experiments, orthophosphoric acid, in the amounts 

 named in Table 1, was applied to limed soil infested with Thielavia, and to- 

 bacco plants were grown in soil so treated. The initial pH value of this soil 

 was 5.9. The application of orthophosphoric acid lowered the pH value of 



^ In the preliminary experiments and in the field experiments in 1926 and 1927, 

 the pH values of the soil were determined colorimetrically by a method previously 

 described (3). The soil pH values in the field in 1928, 1929, and 1930 were deter- 

 mined electrometrically. 



