90 Rkport S.A. a. Advanckmext of Scikxce. 



Wliile there has been of late j-ears a gradual approach to uui- 

 formitv of nomenclature, a sufficiently wide variation still exists to 

 render the use of mere names valueless unless the method of analysis 

 is also distinctly stated. 



The importance of silt and clay as component parts of any soil 

 is one of the chief reasons for the systematic mechanical analyses of 

 soils. Nf)t only does the clay bind together soil particles which would 

 otherwise collapse into drift sands, but its own fineness of texture 

 renders soils which contain much clay all the more retentive .of 

 moisture, and of the nutritious substances therein, when sands, 

 although abounding in mineral plant food, may be unfitted for 

 cultivation simply through the lack of soil moisture. 



Moreover, the finest particles of the soil are those which contain 

 the largest amount of the elements of plant food in an available form ; 

 so that in every respect a soil containing a large proportion of silt and 

 clay is thereby the bettei- fitted for plant sustenance, and, altogether 

 apart from any question of direct chemical analysis, the mechanical 

 analysis of a soil will f)-equently be a good index to its probable 

 fertility. Above and l)eyond all this it is to be remembered that 

 a fine-grained soil not only presents its plant food in a form better 

 fitted for the plant to assimilate, but it generally has more of it to 

 present. 



By clay, it need hardly be said, is meant not only the term as 

 used in the strictly chemical sense, namely, silicate of alumina : but, 

 in a wider signification, it denotes that finely divided material which, 

 when stirred up with water, does not settle down readily, but remains 

 suspended in the water for a long time, generally to be measured by 

 days— sometimes by weeks.* 



Now the effect of rain beating upon the surface of the soil is 

 to stir up the soil particles, and, as the water percolates down through 

 the soil, it constantly carries with it in suspension a quantity of da}'. 

 Naturally, therefore, in a rainy district the subsoil becomes in time 

 more clayey than the surface soil, and as clays contain more plant 

 food than coarser-grained soils, it also follows that the subsoil will 

 as a rule be richer than its surface soil. Hence, too, subsoils are more 

 retentive of moistiue, and less easily penetrated by the rain owing 

 to the accumulation of clay. 



The mechanical analyses of soils are thus of far-reaching im- 

 portance : not only does such an analysis ser\e its direct purpose 

 of determining the proportion of particles of various sizes in the soil, 

 but, indirectly, it sheds light upon the soil's power to retain moisture, 

 and also upon its permeability to plant roots, and it even aids — as 

 stated above — ^in forming a conjecture as to its chemical potentia- 

 lities, since the soils that contain most silt and other fine grades 



* " Ah used in a pliysical sffiise day niay l>e silica, felspar, limestone, mica, 

 knulin, or any otlier rock or mineral wliich lias pulvcriseti until the particles 

 are less than •0<>r> mm. diameter" (Hnyder. Thr Chnnittni of Snih mid 

 FfHiii^er.i, 1K99, p. 13). 



