12 VERTICAL FARMING 



both the water and air move more slowly. The silts and loams 

 are intermediate between the sands and the clays. Loam soils 

 are made up of mixtures of fine and coarse soil particles. If 

 the loam carries a large percentage of sand it is known as a 

 sandy loam ; if the clay particles predominate in amount, it is 

 known as a clay loam. The presence of gravel among the other 

 particles materially affects the texture of a soil and often the 

 fertility as well. When a considerable amount of these particles 

 are present in a loam soil it is usually called a gravelly loam, the 

 difference in clay and sand being maintained as before. The 

 intermediate textures, such as fine sandy loam, silt loam, and 

 the lighter clay loams are usually considered the best, as they 

 tend to be light, well drained and easily cultivated. 



C;$?** 



COMPARATIVE SIZES OF SAND AND CLAY PARTICLES (ENLARGED) 



When the percentage of fine gravel and coarse sand is high, 

 the soil is likely to be too loose, too easily drained, and not 

 likely to withstand drought well. Such a soil, especially in the 

 rainy regions, is likely to be deficient in one or more of the 

 chemical elements needed for the production of plants. Where 

 the percentage of fine silt and clay is high, the soil is likely to 

 be cold, heavy, and sour. Such a soil, unless well tilled to con- 

 siderable depths, resists the ready movement of air and moist- 

 ure. 



Soil Structure. Another important physical characteristic 

 of soils is the way the particles arrange themselves as they lie 

 in the field. A coarse sand is found to have every particle 

 lying separate, and alone with no attachment to the particles 

 which it touches, unless they are cemented together by excesses 

 of lime or similar substances. The clay in a path will be found 



