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Results from Cunninghamella Plaque Tests for Available Phosphorus in Calcareous Soils¹

ABSTRACT

Studies were made on the adaptability of the Cunninghamella plaque method for determination of available phosphorus in calcareous soils with pH values lying between 7.7 and 8.2.

In making the plaques, a moisture content of 45 to 50 per cent of the water-holding capacity of the soil was most desirable.

The diameter of the fungus colony increased consistently with increased amounts of phosphate added to the plaques. In a field soil, the only fertilizer treatments which brought about significant changes in colony diameter were those containing phosphate. Laboratory storage of the soil samples for a period of six months did not cause any appreciable change in the diameter of the Cunninghamella growth. Samples taken from the same plots in early summer and late fall gave only slightly differing results.

In tests with field soils, the Cunninghamella method agreed closely with field tests for phosphate need on soils differing widely in their available phosphate content. On soils differing less widely, agreement with field results was less pronounced. On these soils, selected largely because of the difficulty of determining their phosphate requirement by a chemical method, the results of the biological method tended to agree with the chemical test rather than with field test results. The data indicated distinct possibilities for the Cunninghamella plaque method as a test for available phosphorus in calcareous soils.

¹ Contribution from the Colorado Agricultural Experiment Station.

² The author wishes to acknowledge the helpful assistance of Robert Gardner of the Colo. Agr. Expt. Sta. and of H. W. Dahlberg, R. J. Brown, and of A. C. Maxson of the Great Western Sugar Company, in securing samples of soil for test.