91 



as disinfectants, either due to particular compounds produced 

 within them or to their natural acidity. Our own results in thit^ 

 and previous years poitit strongly to the conclusion that either the 

 acidity or alkalinity of soils or else the presence of carbonate* 

 within them, determines in a great measure the injury which may be ex- 

 pected ^Vom potato scab when the germs ol the disease are once intro- 

 duced into them. ThediflPi 3ulty of settling satisfactorily the pttrticular point 

 as to whether it is the p esence of carbonates or the reaction of the soil 

 as concerns acidity and alkalinity, will be more readily apparent when 

 we bear in mind that such substances as are capable of counteracting 

 the acidity of soils are either carbonates or are changed into carbonates 

 by natural processes within the soil, subsequent to their application. 



General Summary in Relation to the Potato Scab. 



1. Experiments for three years show that the growth of the potato 

 scab fungus is promoted by the presence of air-slacked lime. 



2. Calcium sulphate (known as land plaster and gypsum) is the 

 only form of lime employed which has not injured the growth of the 

 crops, and which has at the same time failed to promote with certainty 

 the development ot the scab. 



3. Barnyard manure, owing to its alkalinity or the production of 

 carbonates from it, has probably in and of itself increased the scab. 



4. By the use of air-slacked lime, wood ashes, barn-yard manure^ 

 soda ash (sodium carbonate), or double carbonates ot potash and mag- 

 nesia, the production of scab would be favoured. 



5. If favourable for its development, the fungus seems to multiply 

 in the soil independent of the continual presence of potatoes or 

 or other root crops, though how long this is possible without the occa- 

 sional intervention of some root crop is unknown. 



6. The results show the danger liable to follow even if a few germs 

 are introduced into the soil, provided it is of itself, or has been made by 

 injurious fertilisation, highly favourable to the development of the scab. 



7. It is shown in a striking manner that the corrosive sublimate 

 or some other satisfactory treatment of the seed tubers should always be 

 resorted to on soils which are favour-able to the development of the 

 potato scab. 



8. Upon our acid soil practical immunity from scab has been secur- 

 ed upon three successive crops by the use of fertilisers representing our 

 ordinarv commercial fertilisers, even when slightly scabbed untreated 

 seed tubers have been employed. 



9. The results with calcium compounds also accord closely with 

 those of 1894 and 1895. A scabless product was produced where calcium 

 chlorid or land plaster (gypsum) was used. Calcium chlorid had a 

 marked poisonous effect upon the potato plants and nearly destroyed 

 them. Land plaster appeared not to have increased, and it may have 

 lessened, the yield slightly. Where calcium at the same rate as in the 

 calcium chlorid and land plaster was applied in the form of wood ashes, 

 air-slacked lime, calcium carbonate, calcium oxalate and calcium acetate, 

 the vit£our of the plants and the vield of tubers were wonderfully 

 increased, but the crop was so badly scabbed as to be worthless. 



