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Modulus of Rupture as an Index of Crusting of Soil¹

ABSTRACT

Apparatus and procedure are described for measuring the modulus of rupture of soil. Briquets approximately 1 x 3.5 x 7 cm. are formed by placing screened dry soil in brass molds, wetting for one hour by subbing and drying at 50°C. The force required to break a briquet when loaded as a horizontal beam is measured and the modulus of rupture, which is the maximum fiber stress, is calculated by the formula s = 3FL/2bd².

At Riverside, Calif., a fine sandy loam under field conditions gave no diminution of stand for beans with s equal to 108 ± 3 millibars, even though surface crusting effects were markedly in evidence. However, with the exchangable-sodium-percentage artificially increased to 37, it was found that s increased to 273 ± 6 millibars and no bean seedlings emerged. This failure to obtain a stand was presumably due to the increased mechanical strength of the surface crust caused by the presence of exchangeable sodium.

¹ Contribution from the U. S. Salinity Laboratory, Riverside, Calif. and the Division of Soil and Plant Relationships, BPISAE, U. S. Dept. of Agriculture, Beltsville, Md. in cooperation with the Seventeen Western States and the Territory of Hawaii.

² Principal soil scientist. The author is indebted to R. H. Brooks for the preparation of the drawings and to M. S. Mayhugh for making the photographs.

Received for publication May 20, 1953.