JEFFERY ON MECHANICAL ANALYSIS OF SOILS. 87 



A NEW METHOD FOR THE MECHANICAL ANALYSIS OF SOILS. 



J. A. JEFFERY, AGRICULTURAL COLLEGE. 



Ill stnd.viiiji- niaiiy of the soil problems it is desirable to know the 

 size of the soil j;i*uiii. T'pon the size of the grain depends the amount 

 of moisture the soil will hold capilhirily or otherwise, the amount of 

 soluble salts, the rate of solubility of the soil itself, the rate at which 

 the moisturt^ content of the soil will disaj^tear downward by percola- 

 tion or u])ward by cai)illarity and evaporation. Indeed there are few 

 agricultural functions of the soil which do not depend upon the size 

 of the soil grain. 



Here is a soil (a) so coarse that the openings between the grains are 

 so large as to .be perceptible to the naked eye. Here is one (b) which 

 appears to be so closely arranged as to have no openings or pores, and 

 yet this (a) has but 84 per cent of unoccupied space between its grains, 

 and this (b) has 52 per cent of unoccupied space. 



This (a) is a typical truck soil because it allows the passing downward 

 rapidly of the excess of water which may enter it. This in turn 

 allows the rai)id warming of the soil, and the ready entry of air, all 

 of which is desirable for the rapid growth and early maturity of 

 vegetables. This soil (b) is a typical grass soil (1) because it does 

 not allow the rapid passing downward of its moisture, and (2) because of 

 the tiueness of its grain it presents a very large total surface upon 

 which the roots of the crop may feed, and to hold from loss soluble 

 materials already present. 



This soil (b) is so tine that to count the grains contained in a single 

 gram, counting at the rate of one per second ten hours per day, three 

 hundred thirteen days per year, would require five hundred twenty five 

 years. The grains in a cubic inch of the finest half of this soil, if laid 

 grain touching grain would form three rows reaching from San Fran- 

 cisco to Boston, while the total surface of a cubic foot of this soil 

 amounts to 178,700 square feet or nearly four acres. The average 

 diameter of the soil grain in (b) is estimated to be .004956 mm. or 

 1-5000 of an inch. 



Upon the proportion of fine and coarse grains in its texture depends 

 the classification of a given soil, as to whether it be a sandy soil, a 

 clay soil, a loam, a loamv clav, a clavev loam, and so on. 



It becomes evident then that an accurate and ready means of physical 

 analysis is desirable. 



For very coarse soils the micrometer with the microscojje, or the 

 micrometer calipers may be used, and the average of a large number 

 of measurements taken as the average size of soil grains, or the size 

 of the grains may be determined by counting and weighing, in which 

 case the specific gravity of the soil must be known. But no soils of 

 agricultural value can be measured in this way. For these, three gen- 

 eral methods of analysis, or various combinations of these methods, 

 have been employed. In each of these, the soil is separated into groups, 



