The Azotobacter Test of Soil Fertility Applied to the Classical Fields at Rothamsted

1. The kneaded plate (plaque moulée) method of detecting deficiency in lime and available phosphate was applied to seventy-nine soil samples taken from the classical Rothamsted arable plots, and the Azotobacter population from some of these samples was estimated by counts on silica jelly. 2. The silica jelly counts showed that Azotobacter cells were very much reduced in number, or even absent in soil receiving 86 lb. per acre or more of mineral nitrogen. It is suggested that this is due to competition with other organisms whose growth is stimulated by added nitrogen compounds. 3. The kneaded-plate test correctly indicaṫed whether phosphate had been applied in soils receiving little or no nitrogen manures. 4. In those soils receiving 86 lb. or more of mineral nitrogen, the kneaded-plate test usually showed little or no Azotobacter growth even in the presence of phosphate and calcium carbonate. This failure was probably due to the paucity of Azotobacter cells originally present in such soil samples. In some cases the test was modified by inoculating the sample with a culture of Azotobacter and it then gave correct indications as to phosphate content.