EFFECT OF SEASON AND CROP GROWTH ON THE PHYSICAL 

 STATE OF THE SOIL 



By D. R. Hoagi.and and J. C. Martin, Division of Agricultural Chemistry, California 

 Agricultural Experiment Station 



Investigations previously reported by this laboratory x have shown 

 definitely that the soil solution is extremely variable in its composition 

 and concentration, as indicated by water extracts or by the freezing- 

 point method of Bouyoucos and McCool. 2 Recently McCool and Millar 3 

 in an extensive series of field studies have upheld this conclusion. In all 

 these investigations it has been demonstrated that the absorption of 

 solutes by the plant may have a very pronounced influence on the soil 

 solution at certain periods and may bring about a very striking decrease 

 in the concentration of nitrates and other constituents. Moreover, this 

 condition may persist for a long time. During the course of our experi- 

 ments it was noted that the state of dispersion of the colloidal matter in 

 the various soils fluctuated in a most decided manner under the influence 

 of the different treatments. It was decided, therefore, to make a sys- 

 tematic series of observations relating to this point. 



The soils used were kept under controlled conditions in tanks as 

 described by Stewart. 4 Both cropped and uncropped soils were com- 

 pared under otherwise identical conditions. The principal measurements 

 were made on a number of tanks of silty clay loam soil, clay in which vari- 

 ous crops were grown — namely, corn, barley, potatoes, beans, and beets. 

 There were three tanks of barley, containing, respectively, 24, 50, and 71 

 plants. All soils were kept at approximately optimum moisture con- 

 tent by the addition of distilled water. At frequent intervals during 

 the growth of the crops samples of soil were taken for examination. 



In order to study conveniently the changes in the water-soluble con- 

 stituents, conductivity measurements were made on water extracts of 

 the soil. These were made by thoroughly mixing 1 part of moist soil 

 with 2 parts of distilled water and filtering through filter paper. This 



1 HOAGLAND, D. R. THE FREEZING-POINT METHOD AS AN INDEX OF VARIATIONS IN THE SOU, SOLUTION 



DUE TO season and crop growth. In Jour. Agr. Research, v. 12, no. 6, p. 369-395, 8 fig. 1918. Literature 



cited, p. 394-395- 

 McCool, M. M., and Millar, C. E. soluble salt content of soils and some factors affecting 



IT. Mich. Agr. Exp. Sta. Tech. Bui. 43, 47 p., 4 pi. 1918. 

 Sharp, L. T. salts, soil-colloids, and soils. In Proc Nat. Acad. Sci., v. 1, no. 12, p. 563-568. 1915 

 Stewart, Guy R. effect of season and crop growth in modifying the son. extract. In Jour. 



Agr. Research, v. 12, no. 6, p. 311-368, 24 fig., pi. 14. 1918. Literature cited, p. 364-368. 



2 Bouyoucos, George J., and McCool, M. M. the freezing point method as a new means of meas- 

 uring the concentration of the son. solution directly in the son,. Mich. Agr. Exp. Sta. Tech. 

 Bui. 24, p. 592-631, 2 fig. 1916. 



8 McCool, M. M., and Millar, C E. op. cit. 

 4 Stewart, Guy R. op. err. 



Journal of Agricultural Research, Vol. XX, No. 5 



Washington, D. C Dec. 1, 1920 



v w Key No. Calif. -26 



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