374 Soil Factors on Disease Resistance 



subject by a systematic examination of the root-system 1 . The results 

 obtained are summed up in the present paper. They are admittedly in- 

 complete but are put forward with a definite purpose, namely, to suggest 

 that much more attention should be paid in future investigations of 

 diseases to the general facts of root-development and to the condition 

 of the absorptive areas of the root-system both before the actual advent 

 of the parasite and during the period when the disease is actually 

 established. Up to the present, attention has been paid chiefly to the 

 influence of soil-aeration and soil temperature on disease-resistance. 



II. Soil-Aekation. 



The importance of the soil-aeration factor in the growth of crops 

 has been obscured by several causes. In the first place, the chief agri- 

 cultural experiment stations at which soil problems have been studied 

 have been situated in humid regions where the soil obtains frequent 

 applications of highly oxygenated water in the form of rainfall. In the 

 second place, the discovery of artificial manures has influenced agri- 

 cultural science just as profoundly as it has revolutionised practice. 

 When most soils are found to respond at once to applications of combined 

 nitrogen, phosphates and potash or of various combinations of these 

 substances and when artificial manures are purchasable in any market 

 place of the country, it is natural to regard such soil deficiencies as due 

 to exhaustion and to find in applications of artificial manures the natural 

 remedy. Under circumstances such as these no stimulus to the study 

 of factors like soil-aeration is likely to occur. When, however, we push 

 over cultivation into the desert and endeavour to make up for defective 

 rainfall by irrigation which often produces impermeable crusts on the 

 surface, the importance of soil-aeration soon becomes manifest 2 . De- 

 prived of regular applications of dissolved oxygen in the form of rainfall, 

 the crop and the soil have to rely on other means of obtaining new 

 supplies of this gas and of getting rid of accumulations of carbon dioxide. 

 Under such circumstances the physical condition of the soil takes on 

 a new meaning and any cause which affects gaseous interchange between 

 the pore spaces and the atmosphere is found to affect the crop imme- 

 diately. Not only does soil-aeration influence the amount of growth but 

 also the development of the root-system and the resistance of the crop 

 to disease. In some cases defective soil-aeration actually causes disease. 



1 Ayr. Jour, of India, Special Indian Science Congress Number, 1917, p. 17. 



2 Bulletins 52-61, Agr. Research Institute, Pusa, 1915-16. 



