Research Bulletin (Iowa Agriculture and Home Economics Experiment Station)


Various chemical and biological methods have been suggested for the determination of the fertilizer needs of soils but all have been found to be of limited practical value. The search for a suitable method continues, however, and recently Winogradsky3 suggested a biological method which, by more nearly duplicating the conditions existing in the field, would give more accurate results. He devised what has been called the spontaneous culture test. This depends upon the growth of Azotobacter and possibly other aerobic, non-symbiotic, nitrogen-fixing microorganisms on the surface of the soil, treated with a a carbohydrate, calcium carbonate, a phosphate and a potassium salt in varying amounts, separately and in combination and placed in plates. A greater development of colonies on a soil treated with calcium carbonate, phosphorus or potassium is assumed to indicate a need for the same constituent in the field. Some success has been reported4 by the use of this test for determining the fertilizer needs of soils, and further studies on it seem desirable.



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