. Proceedings of the American Society of Agronomy . ber of sizes of particles—separates—to be recognized and the limits of these. Several systems of group-ings are now in use.^*^ The finer the particles the greater is the influ-ence of a given mass of them upon the character of the soil. Manymore divisions should, therefore, be made in the fine material than inthe coarse material. It is an open question whether, as survey workhas been done in the United States, sufficient divisions have beenmade below the sand classes. Undoubtedly field separation of ma-terials by hand examination is not likel
. Proceedings of the American Society of Agronomy . ber of sizes of particles—separates—to be recognized and the limits of these. Several systems of group-ings are now in use.^*^ The finer the particles the greater is the influ-ence of a given mass of them upon the character of the soil. Manymore divisions should, therefore, be made in the fine material than inthe coarse material. It is an open question whether, as survey workhas been done in the United States, sufficient divisions have beenmade below the sand classes. Undoubtedly field separation of ma-terials by hand examination is not likely to be more refined than ispossible with the divisions now generally made. However, finer dis-tinctions in the chemical analysis of material smaller than explain some variations in types of soil not otherwise recognizedand these form the basis for more detailed study of individual types. Briggs, L. J., et al. The Mechanical Analysis of Soils. U. S. Dept. Agr.,Eur. Soils, Bui. 24, 1904. PROCEEDINGS OF THE AMERICAN SOCIETY OF LYON : EXPERIMENTS TO ESTIMATE ERRORS. 89 The structure of a soil as determined by the order of stratificationand the thickness of the layers may also be the basis of type separa-tion. This is independent of general structural differences due to thegeneral mode of formation and the characters of the rock. The soil type is the unit for soil study and should be as nearly alikein all parts as is possible. It is the most important grouping of ma-terial primarily because it does represent the chief physical differ-ences in soils. The next most important grouping is the soil seriesand these two will be most generally identified with particular cropand agricultural interests in practice. This does not minimize thevalue of the larger separations which, as has been suggested, areessential to reasonable accuracy in these last two groups. Of course the final test of a survey must be in the field man whoapplies these principles to a particular s
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