The influence of acid-forming and non-acid-forming fertilizer on the development of potato scab

Conclusions

These data demonstrate that it was necessary to study the reaction of the soil in the vicinity of the potato sample in order to determine the relation of the pH to the development of scab.

The soil reaction data for the small areas corresponding to the potato samples indicate that scab did not develop at pH 4.8 or lower with either acid-forming or non-acid-forming fertilizer, but that it increased progressively in amount and severity as the reaction became less acid.

There was no correlation between scab infection and the kind of fertilizer used. The scab index was slightly higher in the non-acid-forming fertilizer than in the acid-forming fertilizer plats between pH 5.05 and 5.35, but it was much higher in the acid-forming fertilizer plats at 4.85–5.05 and at 5.4–5.65.

In view of these results it may be concluded that the occurrence and severity of scab are correlated with the soil reaction and that they are influenced by the type of fertilizer only to the extent that the fertilizer changes the soil reaction. Since scab is able to develop between pH 4.85 and 5.0 but not at more acid reactions it will be advisable in the future for this Station to recommend soil treatment to adjust the reaction to approximately 4.8 instead of 5.0 and then use non-acid-forming fertilizers to avoid making the soil more acid. Data obtained in the liming experiment at Onley, Virginia (I) indicate that there is very little difference in yield at soil reactions between pH 4.8 and 5.0.

The use of acid-forming fertilizers may be advisable on soils where scab has been prevalent in sufficient amount to materially injure the quality of the crop, but their use will not prevent scab until they have lowered the reaction of the soil to at least pH 4.8.