246 GEOLOGY AND CHEMISTRY OF SOILS. 



ploughing. If the sub-soil is similar to the soil in composi- 

 tion, the same process may be gone through, but in addition, 

 the ground should be drained, to let the water pass off. 



Crops. Clay soils are best adapted to wheat, timothy and 

 oats ; and where the bottom is dry, to potatoes and clover. 

 Clay loams, containing carbonate of lime, are the best wheat 

 soils known. This arises from the fact that they give stabil- 

 ity to the roots, furnish the requisite alkalies, and absorb 

 gaseous bodies, which are essential to that crop. They are 

 not fit for the tap-roots, although such crops exert a favorable 

 influence upon them by dividing the soil. They should be 

 ploughed in the fall, to be broken down and pulverized by 

 the frosts during the winter, especially if intended for an 

 early summer crop. 



3. Calcareous soils contain large quantitiesof carbonate of 

 lime, under the varieties of chalk, marble, calcareous marl, 

 siliceous, ferruginous and magnesian limestones. It is not 

 necessary for a soil to be composed principally of this earth, 

 in order to render it calcareous, a smaller portion of it being 

 required to give the name, than of the other soils above men- 

 tioned. Calcareous soils originate from the disintegration of 

 limestone rocks, which are most abundant in the secondary 

 formation ; especially from the chalk or cretaceous group. 

 These soils are often washed some distance, and cover over 

 large areas. Some of them contain fossils and some (as those 

 from the primitive limestone) do not. 



Properties. Calcareous soils are either gravelly or sandy, 

 depending upon the degree of comminution. They are more 

 adhesive and absorb more water than siliceous, and less than 

 aluminous soils. But the most striking property is their 

 power of causing the decay of vegetable matters, and of re- 

 taining several gaseous products for the wants of vegetation. 

 Calcareous soil is friable and easily tilled ; not suflering either 

 from drought or too great moisture, provided the sub-soil is 

 not too retentive of water. 



