60 THE MECHANICAL ANALYSIS OF SOILS [chap. 



market gardening or wheat growing, but the more 

 refined points of difference connected with the manage- 

 ment of given soils, which become known by experience 

 to a good practical farmer, cannot as yet be deduced 

 from the analysis. It is necessary to accumulate more 

 data, until we possess the mechanical analysis of a 

 large number of soils whose texture and amenability 

 to cultivation have been ascertained by long practice ; 

 then we shall be able to assign any soil by its mechanical 

 analysis to a known type. 



The power of a soil to retain moisture and resist 

 moderate drought depends on a predominance of the 

 finer particles and of humus ; good wheat land or 

 land that will form sound permanent pasture will 

 contain at least 30 per cent, of silt and clay. The 

 ease with which a soil suffers the rain to percolate 

 depends upon the relatively low proportion of silt and 

 clay rather than on the amount of coarse - grained 

 material ; the fine particles pack in among the larger, 

 and the soil is equally resistent to the passage of water, 

 whether the finest material is diffused among coarse 

 sand and gravel, or among the finer grades of sand. 

 The shrinkage of a soil on drying, and its tenacity when 

 dry, are even more dependent on low proportions of 

 coarse sand, humus, and chalk, than on the actual 

 amount of clay and silt which cause the shrinkage. 

 The really difficult soils to work are those containing less 

 than 20 per cent, of sand above 01 mm. in diameter. 



The table on page 61 will serve to illustrate these 

 points. 



Soil No. 1 represents one of the lightest of sands, 

 about the extreme limit of cultivation a soil, indeed, 

 which had been found unfit for ordinary farming, and 

 had been planted with conifers. 



It will be seen that more than 83 per cent, consists 



