June, 1940] Experiments With Potatoes 25 



A higher degree of consistency is shown in Table XXIII, Whenal 

 farm data, than in Table XXIV. Both acidulating substances reduced the 

 pH value of the soil and tended to reduce the amount of potato scab. In 

 the main, aluminum sulphate appears to have had more effect than sulphur 

 in reducing scab, as judged by the percentage of potatoes in both years 

 that were entirely free from scab. It is impossible to correlate these re- 

 sults definitely with pH values, so it is not clear whether the variations in 

 scab amounts are entirely due to the substances used or to soil conditions 

 over which no control was exercised. 



Table XXIV lists scab counts on three plots which had been limed 

 at the rate of four tons per acre eight years prior to the potato crop of 

 1936. Before the potatoes were planted in 1936, the plots were divided 

 and one end treated with 900 pounds, the other with 300 pounds of alum- 

 inum sulphate per acre. 



The results do not appear very consistent, mainly because of plot 115 

 which exhibited dift'erent conditions on either side of the plot, whereas in 

 our acidulating work the plot was divided crosswise. One side of the plot 

 exhibited 100 per cent bad scab regardless of treatment. The soil was not 

 sampled in the spring to cover this variation, but the sampling at digging 

 time showed a much higher pH value on that side of the plot on which 

 the bad scab occurred than on the other. Aside from this factor, pH 

 values were lower on all tests after treatment than before, and the heavy 

 application reduced the amount of scab to an appreciable extent more than 

 the light one. 



In Tables XXV and XXVI scab counts are presented from plots on 

 Jackson farm \\-hich had previously been limed or had received lime-bearing 

 substance in the fertilizer. It will be noted in Table XXVI that check plot 

 counts in 1937 showed only 78.3 per cent of tubers free from scab although 

 very little bad scab was present. Potatoes from plots treated with basic 

 slag showed considerably more scab infection than those treated with 500 

 or 1,000 pounds of lime, wdiereas the most scab appeared on the heavily 

 limed plots even though that lime had been applied eight years before 

 the data were taken, and were acidulated in addition. 



From the data it is apparent that land can be acidulated and scab 

 injury reduced by the application of aluminum sulphate or sulphur, al- 

 though in no instance was perfect scab control obtained. In fact, it seemed 

 that the costs of acidulating soils by the use of one of these substances was 

 too great to warrant their use in actual farming practice; and from the 

 growers' point of view it would be preferable to plow under green manure 

 crops, or use a broadcast application of sulphate of ammonia to achieve the 

 results desired. 



