The Soil 39 



of calcium and magnesium carbonates. Oyster shells are prin- 

 cipally a calcium carbonate. Limestones often contain small 

 quantities of clay, iron oxide, silica and nearly always calcium 

 phosphate in comparatively large quantities. The soil left on 

 limestone or chalk consists mainly of these foreign substances, 

 most of the calcium carbonate it-self having been dissolved out 

 by the combined action of water and carbon dioxide. It some- 

 times happens therefore that the soil originating on limestone 

 would be benefited by an application of limestone. 



Limestone only exerts its characteristic and importanf func- 

 tions in a soil when in a very finely divided state. In the form 

 of gravel or sand it is little better than ordinary siliceous sand. 

 In the finely divided state it has two very valuable functions; 

 first, as a source of plant food by virtue of the calcium which it 

 contains, and second, which is more important, as a basic material 

 necessary for the correction of an acid reaction in the soil and 

 for the processes of nitrification. 



Sedentary and transported soils. These terms are convenient 

 in distinguishing between soils which have been made up of the 

 debris resulting from the weathering of the particular rock on 

 which they rest (sedentary soils) and those which owe their 

 origin, not to the rock below them, but to materials brought from 

 a distance and deposited there (transported soils). The rich 

 alluvial soil in the lower reaches of river valleys consists largely 

 of material which has been brought down by^ the river from the 

 higher parts of the valley and since the materials in many cases 

 have been brought from various rock formations, the resulting 

 soil generally possesses a greater fertility than would be shown 

 by soil formed exclusively by the weathering of any one kind of 

 rock. 



Glaciers are also the means of transporting large quantities oi* 

 materials out of which soils may be formed. Large tracts of 

 country are covered with a thick deposit of clay and rock frag- 

 ments, which have been brought from a great distance by glaciers. 

 Such deposits are known as glacial drift, and often quite obscure 



