SYLVICULTURE. 



Vertical stratification facilitates decomposition and tree growth. 

 The various species of rock differ in hardness, porosity, heat con- 

 duction, and above all in soluble mineral contents. 



b. Quartz sand. Quartz sand is unfertile when pure, since silicic 

 acid fails to be digested by the roots and fails to react with the 

 acids usually found in the soil. Quartz sand is loose, has small 

 hygroscopicity, small capillarity and small heat- retaining capacity. 

 It is hot during the day and cold at night. 



c. Lime. Lime when pure is a poor soil, although not quite 

 as dry and hot as sand. Lime, however, mixed with loam and 

 clay (so-called marl) forms an extremely productive soil. 



d. Clay. Clay has great absorbing and hygroscopic power. It 

 is wet and cold. Main components are aluminum-silicates. 



e. Loam. Loam is a mixture of sand and clay the usual soil 

 in agriculture and forestry. It is usually colored by iron (red 

 loam at Biltmore). We speak of a sandy loam or of a loamy sand 

 according to the prevalence of one or the other component. Loam 

 soil exhibits a happy medium of qualities favorable to tree growth. 



f. Humus. Humus results from the decomposition of vegetable 

 and animal matter under co-operation of bacteria, fungi, rain worms 

 (Darwin), larvae. Humus forms a solvent of mineral plant food. 

 A bad conductor of heat and cold, it prevents rapid changes of 

 temperature in soil, has great hygroscopicity and great water- 

 retaining power and is a preventive to evaporation of soil moisture. 



Mild forest humus shows a basic reaction, whilst the sour humus 

 of the swamps shows an acid reaction. 



Unfavorable is the dust humus formed by many Ericaceae. 



XIII. Physical versus chemical qualities of soil. 



Agriculture withdraws food only from the top layer of soil. 

 It deprives that top layer of its rarest and most valuable com- 

 ponents, by the annual crop of grain excessively rich in nitrates, 

 phosphates and potash. The porosity, and through it the water 

 capacity and the heat capacity of soil, are readily controlled on 

 the field by the plow. It is necessary in agriculture, in the long 

 run, to return to the soil in the shape of fertilizer annually as 

 many pounds of nitrates, phosphates and potash as have been 

 removed in the shape of crops from a given acre of land. 



. The productiveness of agriculture depends, above all, on the 

 chemical qualities of the soil tilled. A crop of trees, on the other 

 hand, takes from the soil very little, since the tree v consists mainly 

 of C, O, H, or since wood is nothing but air solidified by sunshine. 

 The phosphates, nitrates and potash absorbed by the tree are 

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