TABLE i 



Constitution of the Soil (Plow Layer) of a Typical Cultivated Silt Loam 

 at or near Optimum Moisture Content 



I. Solid Phase: The soil proper, 50 per cent by volume 



Inorganic portion, 

 95 per cent by weight 



Coarser portion: Sand, Fine clay or colloi- 



silt, and coarse clay, par- 

 ticles >o.2(jl diam., 80 

 per cent by weight. 

 Largely primary miner- 

 als: Feldspars, micas, 

 quartz, and many others 



dal portion: Particles 

 <o.2/i diam., 20 per 

 cent by weight. 

 Largely secondary 

 minerals: Inorganic 



Organic portion, 

 5 per cent by weight 



Largely colloidal: 

 Organic base-ex- 

 change material, 

 lignin, cellulose, and 

 proteins 



base-exchange ma- 

 terial, iron oxides, 

 silica, and kaolinite 



II 



Liquid Phase: The soil solution, 25 per cent by volume 



Largely water solution of salts and gases. Concentration of salts usually 

 ranges from 100 to 1000 p.p.m. of the dry soil. Calcium sulfate and bicar- 

 bonate often predominate. Sulfates, nitrates, chlorides, and bicarbonates 

 of calcium, magnesium, potassium, and sodium present. Traces of many 

 other constituents. 



III. Gaseous Phase: The soil atmosphere, 25 per cent by volume 



Percentage by volume 



In soil air In ordinary air 



Oxygen 20.0 21.00 



Nitrogen 

 Argon 

 Carbon dioxide 



20.0 



78.6 



0.9 



0.5 



100. 



78.03 

 0.94 

 0.03 



100.00 



IV. 



Both soil air and ordinary air may contain traces of hydrogen, ammonia, 

 and oxides of sulfur and nitrogen. Figures for soil air represent a condition 

 of good aeration and, for ordinary air, conditions some distance from cities. 



"Living Phase": The soil organisms, 5 tons of living tissue per acre 



Bacteria 1,000,000,000 or more per gram of soil 



Actinomyces 10,000,000 or more per gram of soil 



Fungi 1,000,000 or more per gram of soil 



Algae 100,000 or more per gram of soil 



Protozoa 1 ,000,000 or more per gram of soil 



Nematodes 10 or more per gram of soil 



Earthworms 1,000,000 per acre in rich soils 



Number of organisms varies greatly with season and kind of soil. Roots of a 

 growing crop may equal or exceed in weight that of the soil organisms. 



