May IS, 1919 Influence oj Soil Environment on Rooiroi of Tobacco 53 



in this paper with other environmental conditions. The soils have 

 been kept as nearly as possible uniformly watered, never approaching 

 saturation for any considerable period of time and never relatively 

 dry, so that it is not believed that the results have been vitiated by 

 this factor, though it is admitted that they may have been responsible 

 for many of the individual variations in results always occurring in 

 experiments of this sort, and which is planned to be overcome by mass 

 of data rather than by the most careful attention to a single experiment. 

 If the above-ground symptoms of the rootrot are considered, it is at 

 once realized that the reduction of the water supply is probably the 

 most important one, partly as a result of reduced food supply to the 

 plant brought about by the gradual but effective depletion of the root 

 system. It is therefore usually quite impossible to judge from the 

 above-ground portion of plants alone as to whether lack of available soil 

 moisture or lack of roots or both are responsible for a reduced yield. 

 It is only when growers obtain a greatly reduced yield on land known 

 to be in a high state of fertility that they begin to suspect other troubles. 

 It is said with confidence, therefore, that fully nine-tenths of the damage 

 by the rootrot is attributed by the growers either to a deficiency or to 

 an excess of soil moisture. Sixteen moisture determinations of the field 

 plots (the same soil as used in the pot experiments) at about 3-day 

 intervals between June 19 and August 6 in the summer of 1917 showed 

 a range of moisture content from 25.3 per cent on June 25 to 16.2 per 

 cent on August 2. These determinations showed that during the entire 

 season the moisture content was practically between the limits of one- 

 half to three-fourths saturation. In these plots White Burley tobacco 

 made no growth whatever during this time because of T. basicola, and 

 Connecticut Havana made only half a crop. It is clear that the moisture 

 content was not excessive for the best growth of tobacco, and yet the 

 parasite was almost at its maximum of activity. 



INFLUENCE OF SOIL REACTION ON ROOTROT 



The reaction of the soil has been considered to play a part in the 

 severity of parasitism in practically all plant diseases having their origin 

 in the soil. The reasons for these rather widespread calculations are 

 perhaps manifold. Among the earliest chemical agents applied to soil 

 with the hope of checking plant pests was lime, and experiments too 

 numerous to mention here have since been conducted with it in the 

 hope of checking the diseases and insects attacking plants. Where lime 

 has proved efficacious, however, pathologists have considered it both as 

 influential as a sterilizing agent against the parasite and as a neutralizer 

 of soil acidity favorable to parasitic action. The beneficial action of 

 lime to the growth of green plants and to bacterial activity in soils has 

 also no doubt served to stimulate its use in phytopathological problems. 

 No advantage is to be gained by reviewing the rather extensive study of 



