THE SOIL. 13 



I/ 



reference to root development, temperature, and aera- 

 tion. Moreover, since cultivation of the soil is nec- 

 essary for several purposes, such as the covering of 

 weeds, the maintenance of tilth, conservation of mois- 

 ture, etc., the readiness with which this cultivation can 

 be carried on is an important factor in the value of the 

 soil. 



Classes of Soil Particles. When any soil is exam- 

 ined in the field it is noticed at once that it is composed 

 of lumps or granules of various size which in turn are 

 made up of the individual grains varying all the way 

 from a coarse sand to the finest dust. These ultimate 

 grains of which the soil is composed are usually classi- 

 fied into four groups: first, gravel; second, sand; third, 

 silt; and the finest of all is called clay. When a lump 

 of soil is placed in a bottle of water and shaken until 

 all the particles are loosened from each other, and then 

 mixed through a larger volume of water and allowed 

 to settle, it will be found that the gravel and sand will 

 settle almost immediately, while the silt will remain in 

 suspension from a few minutes to several hours, and 

 the largest part of the clay will remain in suspension 

 for several hours, and the finest even for days. Now 

 all soils are composed of varying amounts of these 

 different classes of soil particles. Sandy soils have a 

 large percentage of sand, but also a small percentage 

 of silt and clay. Clay soils have a large percentage of 

 clay and smaller amounts of sand. Relative amounts of 

 these various sized grains in any given soil are spoken 

 of as the mechanical composition of this soil. The 

 sandy soils are those in which the sands predominate, 

 silt those in which the silt size of grains predominates, 

 and clay soils those in which the clay size predominates. 

 In the system adopted by the Bureau of Soils, U. S. 

 Dept. of Agriculture, the following size of soil grains 

 are used: Fine gravel 2-1 m.m. Coarse sand i--5 m.m. 

 Medium sand .5-. 25 m.m. Fine sand .25-.IO m.m. 

 Very fine sand .10-. 05 m.m. Silt .05-. 005 m.m. Clay 

 .005-0 m.m. 



