The progress of the work upon the general problem of the origin of the nitre spots in certain western soils has presented several different points of view. The fact that certain spots in western cultivated soils were rich in nitrates was first observed by Hilgard who attributed their accumulation to the more rapid nitrification of the organic matter of the soil in the warm arid climate of the west when the moisture limit was removed by irrigation.
Stewart, Robert and Peterson, William, "Bulletin No. 150 - Further Studies of the Nitric Nitrogen Content of the Country Rock" (1917). UAES Bulletins. Paper 116.