It is quite generally believed now that nitrification may occur in acid soils, at least to some extent. The addition of lime, however, results in practically all cases in an increased activity of the nitrifying flora. This is especially true when soils are strongly acid in reaction and low in organic matter and when ammonium sulfate is used to measure nitrification.

Probably it is not merely because lime neutralizes acid soil conditions that it increases nitrification. There may be very important indirect effects of liming which are responsible for greater nitrifying action. Lime makes the soil conditions more favorable for the development of the nitrifying organisms and at the same time makes them less satisfactory for what may be called "acid-tolerant organisms" and an entirely different soil flora undoubtedly results. 'l'his new soil flora probably has a much less injurious effect on the nitrifying bacteria and may even stimulate their development. There are also many important physical and chemical effects of liming which indirectly affect nitrification. Undoubtedly the influence of lime on nitrification in general represents the combined effects of the material on the chemical, physical and biological soil conditions, all of which are very closely related.