There have been numerous attempts in recent years to develop a method which would indicate accurately the phosphorus needs of the soil. Biological methods and chemical methods have been suggested, but at some time or other all the methods have failed to give satisfactory results.
Proceedings of the Iowa Academy of Science
©1931 Iowa Academy of Science, Inc.
Schlots, F. E.; Smith, F. B.; and Brown, P. E.
"Aspergillus niger as an Indicator of Available Phosphorus in the Soil,"
Proceedings of the Iowa Academy of Science, 38(1), 303-307.
Available at: https://scholarworks.uni.edu/pias/vol38/iss1/108