67 

 fiSecondar-- Soils . 



(aX Sg-nA including gravel. Sand is alraost the opposite of clay. 

 It is porous, but its food value is poor l^ecause of Ite make-up. 

 Sand is made up of particles of alraost insoluble minerals as quart -^s. 

 Water percolates througli it so rapidlj it has alraost no cohesion and 

 no water capacity. It heats and cools with greater rapidity than 

 other soil, hence it is most xerophytic of soils. 

 (Id) Clay . Heterogeneous soil. Host northern clays represent 

 pulverized rock left by glacier. Chief element Alg^{Si 0i^y3}, 

 Clay may or may not be rich in food stuffs, according to its origin. 

 It consists of more soluble materials and retains them. Siaaller th^ 

 particles, less the porosity. A swamp is never foiuid on a sand hill 

 but often on a clay. 



(c) Humoia a soil derived from decay of organic matter where there 



is not complete oxidation. Better than sand or clay chemically and 

 also because it contains soluble avids. Plants require a certain 

 amount of inorganic su"') stances. Presence or absence of water largely 

 determines presence of humis. Three kinds of humis : mould, peat, 

 schlaum. Schlaura is entirely formed below the water. Organic mud. 



Color of soil depends largely upon the amount of oxidation. 

 Blacker and finer humis, better the soil, as in a forest. Underground 

 animals add greatly to the value of soil, as earthworms and bacteria. 

 Peat is eliaravt eristic of a northern climate from coolness and moistDro 

 Recent studies show that physiographic youth of region also determines 

 the formation of peat. Best explained by absence of oxidation and 



