June. 1940] Experiments With Potatoes 23 



The difference between the highest organic carbon level and the low- 

 est at the end of the experiment, represented the range of possible changes 

 which may be effected by varying the fertilizers. This statement assumes 

 the experiment has been carried on sufficiently long to level out original 

 existing dift'erences in organic carbon levels. This difference of 3000 

 pounds of organic carbon might be particularly important on some soils. 

 particularly if accelerated erosion was impending. Since this difference of 

 3000 pounds of organic carbon is much less than original differences, it is 

 concluded that the combined effects of the rotation and cultivation were 

 greater than those exerted by the various fertilizers. The crop response 

 is conditioned by the organic carbon level. A given treatment applied 

 to an area of high humus content may show a reduction in organic carbon 

 level, whereas the same treatment applied to a similar area of low humus 

 may have a beneficial effect. 



Potato Scab Control 



Attempts have been made to reduce potato scab on certain Jackson 

 plots which were heavily limed, and on limed plots on Wdienal and Ire- 

 land farms. Greenland, by the use of aluminum sulphate and sulphur. 



Considerable data have been accumulated on this phase of the work. 

 While certain trends are indicated, such as the increase in acidity due to 

 their use, and some reduction in scab, the results are not wholly con- 

 sistent, due probably to the complex factcjrs involved. 



The scab organism is the most widely distributed of any p(;tato 

 disease. ]Many workers have noted that this organism increases in de- 

 structive action between pH 5.5 and 7.0, but decreases above or below 

 these points. Blodgett and Howe (2) noted in a study of potatoes on 

 313 farms in New York that the least scab occurred in soils with pll -1.3- 

 5.4, more scab on soils with pH 7.5-8.5, and the most scab on soils rang- 

 ing from pH 5.45-7.4. This work was later confirmed in general by Smith 

 (22) in an experiment in which soil acidity was controlled in pH by lime 

 or sulphuric acid. 



In the studies reported here, none of the soils were limed to the neu- 

 tral point, although because of imperfect spreading and mixing, there 

 doubtless were localized areas of soils with four-ton lime applications 

 which reached that point. For this reason most of the lime applications 

 used doubtlessly placed the soil within the optimum range for scab develop- 

 ment. 



Data covering W'henal farm potatoes in 1935 and 1936 are reported 

 in Table XXIII, and Jackson farm in 1936 and 1937 in Tables XXIV, 

 XXV", and XXVI. These tables cover the soil pH, and scab counts. For 

 convenience in listing, notations for scab cover bad, medium, and slight 

 or no scab ; and the figures given are percentages of the total number of 

 tubers counted. Bad scab is defined as being severe enough to throw the 

 tuber out of grade so that it would have to be sold on its merits as a 

 scabby potato. Medium scab is defined as being severe enough to be 

 noticeable, and if only a few such potatoes were present, they might be 

 retained in the number one grade, while those only slightlv scabby would 

 under any condition go into the number one grade. 



