This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
It must be a very poor soil indeed which does not contain large supplies of potash. This is locked up with other elements, but fair quantities may be liberated annually by cultivation and the application of organic manures. A fertile soil has been computed to contain about 30,000 lb. of potash to the acre at 9 in. deep, while a soil at Rothamsted which had not been manured for fifty years contained 6750 lb. of potash to the acre at 9 in. deep. The quantity of available potash needed for certain crops varies from 36 to 500 lb. per acre, as may be seen by reference to the figures on p. 144.
Before referring to special artificial potash manures it may be remarked that all organic manures, such as stable manure, the dung of all animals and birds, soot, seaweed, and wood ashes, contain supplies of potash, which are liberated when incorporated with the soil.
Amongst the special potash manures are the following: -