78 Journal of Agricultural Research Voi. x\'t:i. no. 2 



INFLUENCE OF SOIL FERTILITY 



In the case of soil-infesting parasites which cause the loss of large por- 

 tions of the root system in such a way that it can no longer function nor- 

 mally for the benefit of the plant, it seemed probable that the quantity of 

 available plant food would influence growth in infested soil. It may be 

 supposed, for instance, that, if in a soil low in fertility one-half of the 

 roots are destroyed by disease, doubling the quantity of available plant 

 food would materially reduce the actual amount of damage done in yield 

 of crop. This is in accord with the conclusions of Briggs (j) on this 

 subject. On the other hand, there are the views expressed by many pa- 

 thologists with respect to various diseases, and also by Gilbert {12) and 

 others for the Thielavia rootrot, that an increase of fertilizers, especially 

 those of nitrogenous nature, renders the plant more susceptible to attack. 

 Aside from these theoretical conclusions, we are confronted with the 

 facts that the tobacco rootrot as it occurs in the field is not confined espe- 

 cially to soils in a low or high state of fertility, and that the application of 

 fertilizers, whether as barn manure or commercial fertilizer, seems to have 

 no marked effect upon the relative amount of disease, or on the growth of 

 the plants in infested soil in seasons favorable to rootrot. These con- 

 clusions are based on four years of fertilizer plot experiments carried on 

 at Edgerton, Wis., during the years igioto 1913, inclusive, the detailed 

 results of which can not be given here. These experiments were planned 

 to determine if it is possible to remedy the "worn-out" or "deteriorated" 

 condition of old tobacco soils by the use of fertilizers of various sorts, 

 although the full significance of T. hasicola as the cause of this condition 

 was not recognized at the time the experiments were started, and it was 

 not until after three years of failure to obtain any marked results with 

 fertilizer treatments on a wide variety of old soils, coupled with highly 

 beneficial results on soil new to tobacco that the importance of T. hasicola 

 in crop production was fully realized. 



An experiment to determine more carefully the efifect of plant food 

 applied in the form of pure salts on the severity of the rootrot was carried 

 out in pot tests in the greenhouse during the winter of 1917-18. Twenty- 

 four 2 -gallon crocks with a drainage perforation at the base were each 

 filled with 10 kgm. of soil infested with T. hasicola from the old tobacco 

 field on the station farm. Twelve of these were now steriUzed by steam 

 to destroy the fungus. The cultures were divided into six series of four 

 pots each, two containing infested and two uninfested soil. A complete 

 fertilizer was made up from chemically pure salts according to a formula 

 used for nutrient water cultures, as follows: 



Gtn. 



Calcium nitrate 40 



Potassium chlorid 10 



Magnesium sulphate ." 10 



Tribasic potassium sulpnate 10 



