It is a well-known fact that the total phosphorus content of a soil is not a measure of the amount of available phosphorous. Some soils that contain a low total phosphorus content may be supplied with sufficient available phosphorus for present crop needs. On the other hand, some soils that contain a relatively high total phosphorus content may be deficient in available phosphorus.
Proceedings of the Iowa Academy of Science
©1935 Iowa Academy of Science, Inc.
Smith, F. B.; Brown, P. E.; and Millar, H. C.
"Diagnosing the Phosphorus Needs of Soils by Biological Methods,"
Proceedings of the Iowa Academy of Science, 42(1), 85-87.
Available at: https://scholarworks.uni.edu/pias/vol42/iss1/20