The Organic Gardening encyclopedia say 30# per 1000 sq ft on sand to 80# per 1000 sq ft on clay will raise the ph one unit. Having put about 4 tons to the ac of quarry siftings about 9 years ago I would say that their unit would be a raise from lets say 6.1 to 6.2. I would be a little more concerned about any change in PH in your case, since 6.8 is at the top of the range for spuds.
Also if your PH is 6.8 and your calcium is low, There's a good chance that the sulfur may be lacking which could also show up as a fungal problem in an off year. Chris > If I understand correctly, it takes a hell of a lot of high calcium > lime to actually change the pH. Am I wrong about this? > > -Allan >
