94 THE PRINCIPLES OF SOIL MANAGEMENT 



by figures from the Rothamsted fields. The porosity 

 of the surface nine inches of soil in an old pasture was 

 56.8 per cent, while in the same depth of a cultivated 

 field it was 45.5 per cent. Extensive areas of loam soils 

 in the North Central states have a porosity of from 

 45 per cent to 49.6 per cent. In many of the heavier 

 soils it much exceeds 50 per cent, and in well-granu- 

 lated clays it may reach 70 per cent, or in light sand 

 it may be less than 40 per cent. In general, it may be 

 said that about one-half of the volume of ordinary 

 cultivated soils of intermediate texture is pore space. 



The diameter of the individual pore spaces is of 

 importance, as well as the total volume of pore space, 

 since these determine the capacity of the soil to retain 

 and move water and to permit the circulation of gases 

 in the soil mass, as well as to facilitate the extension of 

 the plant -roots. 



37. Weight. The weight of soil is the result of two 

 factors. These are, first, the absolute weight of the indi- 

 vidual particles, or absolute specific gravity, and second, 

 the volume of pore space in the mass. 



By reference to the table of minerals on page 6, 

 it will be seen that the minerals entering into the soil 

 vary greatly in specific gravity that is, their weight 

 as compared with an equal volume of water. They 

 range from about 2.5 to 6 or 8, but the minerals which 

 make up the great bulk of the soil quartz, feldspars, 

 micas, calcite, etc., all have a specific gravity of from 

 2.6 to 2.8. (See Table I, pages 6 and 7.) Many deter- 

 minations of this property have been made. Fineness 

 does not appear to have any material effect upon it. 



