THE ART OF THE SECOND GROWTH 



XII. Species of soil. 



a. Rock. Most important rock formations are: Granite, gneiss, 

 limestone, sandstone, slate and trap. 



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 unproductive when pure, since 

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

 the acids and radicals 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 during 

 the 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 to the trees is the dust humus formed by many 

 Ericaceae. 



XIII. Physical versus chemical qualities of soil. 

 Agriculture withdraws food but from the top layer of the soil. 



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

 ponents, by removing an annual crop of grain excessively rich in 

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

 water capacity and the heat capacity of the soil, are readily con- 

 trolled 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 aS 

 many pounds of nitrates, phosphates and potash annually as have 

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

 19 



