October 13, 1911] 



SCIENCE 



495 



SPECIAL ARTICLES 



A COUNTING METHOD FOR THE JIECHANICAI, 

 ANALYSIS OF SOILS* 



Many investigators have worked on the 

 problem of the mechanical analysis of soil, 

 notably, Osborne,^ Hilgard," Hopkins,* Briggs,° 

 Toder" and Atterberg,' and the final result 

 seems to indicate that some form based on 

 sedimentation in water is the most serviceable 

 for practical purposes. However, it may be of 

 interest to soil workers to note a counting- 

 plate method which approaches the problem 

 from a different angle. 



The counting-plate method has been used 

 for the determination of soil particles in an 

 attempt to save the time of centrifuging and 

 evaporating the silt and clay suspensions. 

 The sand is obtained by subsidence as in the 

 regular method. The total weight of the silt 

 and clay is determined by difference. The 

 relative amounts are then obtained as follows: 



The method is based on counting the num- 

 ber of silt and clay particles on a counting 

 plate and from the relation thus established 

 to determine the amount of clay and silt in 

 the soil. This is an adaptation of a method 

 employed in many different lines of work 

 where both the number and diameter of small 

 grains are to be obtained; as in the examina- 

 tion of blood, starch, etc. This method is not 

 recommended for general use with soils, and 

 should be used only where time is especially 

 important or the facilities are not available 

 for the determination of silt and clay as de- 

 scribed above. 



A definite amount of soil is weighed out, put 

 in a sterilizer bottle with water and ammonia, 



^ Published by permission of the Secretary of 

 Agriculture. 



= Ann. Eep., Conn. Expt. Sta., 1886, p. 141; 

 1887, p. 144; 1888, p. 154. 



'Ann. Eep., Cal. Expt. Sta., 1891-92, p. 243. 



* Proc. Asso. OflE. Agrl. Chem., Bull. 56, Division 

 of Chemistry, U. S. Dept. of Agr., 1898, p. 67. 



= TJ. S. Dept. of Agr., Bureau of Soils, Bull. 24. 



= Bulletin No. 89, Utah Expt. Sta., 1904. 



'Veihand. d. II. Inter. Agrogeol. Konf. Stock- 

 holm, 1910. 



and shaken in a mechanical shaker for at least 

 seven hours. With most soils one half gram 

 material and 120 cubic centimeters , water 

 give a good dilution for accurate counting. A 

 compound microscope with a micrometer eye- 

 piece and a counting plate are necessary. In 

 the micrometer used one scale division corre- 

 sponds to 0.005 millimeter, the superior limit 

 of the clay, and ten divisions to 0.05 milli- 

 meter, the superior limit of the silt. The 

 counting plate is marked off in squares of 0.1 

 millimeter a side. After removing from the 

 mechanical shaker, the contents of the bottle 

 are thoroughly shaken by hand and a sample 

 for examination immediately taken from the 

 center of the bottle. A drop is placed on the 

 counting plate and the number of silt and 

 clay particles in ten squares counted. The 

 sand need not be considered. In most cases it 

 settles quickly and escapes being taken in the 

 subsample. If a sand particle appears in the 

 subsample it is disregarded. 



As one silt particle is miich heavier than 

 one clay particle, a factor must be used in ob- 

 taining a ratio to express the relative total 

 weights of silt and clay in the soil. This fac- 

 tor was determined approximately by com- 

 paring the results obtained by counting with 

 those obtained from analyses made in the reg- 

 ular way. For instance, soil No. 5,862 gives 

 a count in ten squares of 26 silt particles and 

 2,020 clay particles. The regular analysis 

 gives 23.06 per cent, silt and 45.Y8 per cent, 

 clay, practically twice as much clay as silt. 

 So first the number of silt particles (26) is 

 multiplied by 2, giving 52. Dividing the 

 number of clay particles (2,020) by 52 gives 

 the factor 38 plus, or shows that 38 clay par- 

 ticles weigh as much as one silt particle. 

 Therefore in this soil if we multiply the num- 

 ber of silt particles (obtained by counting) by 

 38, the result wiU be to the number of clay 

 particles as the weight of the silt is to the 

 weight of the clay. In 26 soils of widely vary- 

 ing texture, lately analyzed by the counting 

 method and checked by the centrifugal analy- 

 sis, an average of the factors was 35. The 

 factors varied, however, widely enough to pre- 



