48 VEGETABLE FORCING 
it contains. Classification is based upon mechanical 
analyses, excluding stones, gravel and fragments of rocks 
which do not pass through a 2-millimeter sieve. 
Classification of soil material 
The figures in the following classification,* represent 
per cent; the minus sign (—) less; plus sign (+) more; 
the hyphen (-) when used between two numbers, as 20-50, 
should read from 20 per cent to 50 per cent. 
Soils containing —20 silt and clay: 
Coarse sand: 25-+ fine gravel and coarse sand and less than 
50 any other grade. 
Sand: 25+ fine gravel, coarse and medium sand, and less 
than 50 fine sand. 
Fine sand: 50+ fine sand, or —25 fine gravel, coarse and 
medium sand. 
Very fine sand: 50+ very fine sand. 
Soils containing 20-50 silt and clay: 
Sandy loam: 25+ fine gravel, coarse and medium sand. 
Fine sandy loam: 50+ sand, or —25 fine gravel, coarse and 
medium sand. 
Sandy clay: —20 silt. 
Soils containing 50+ silt and clay: 
Loam: —20 clay, —50 silt. 
Silt loam: —20 clay, 50+ silt. 
Clay loam: 20-30 clay, —50 silt. 
Silty clay loam: 20-30 clay, 50+ silt. 
Clay: 30+ clay. 
It is seen from the foregoing classification that soils 
vary greatly in the proportion of the different-sized 
mineral particles. In the coarse sand the particles are the 
largest; in the clay they are the smallest. 
The proper soil texture is an exceedingly important 
matter with reference to the production of crops under 
glass. The heavier types, such as the loams, silt loams 
and clay loams, are universally regarded as pre-eminently 
adapted to the culture of the staple farm crops. Like- 
wise, the value of the sandy types has been recognized for 
trucking and market gardening, although many classes of 
* Bulletin 78, Bureau of Soils, United States Department of Agriculture. 
