GEOLOGY OF AGRICULTURE. 71 



this ingredient is concerned. More may exist in some 

 soils, but oftener less. It is generally found combined 

 with carbonic, or. some other acid, as a salt of potash. 



111. Soda (NaO) exists in soils, varying perhaps 

 from one-tenth to one-half of one per cent. 



112. Xime(CaO). — Some soils contain not less than 

 8 or 10 per cent, of lime ; while a soil may be excel- 

 Tent, and yet not contain more than one per cent. It 

 is generally in combination with sulphuric, phos- 

 phoric, and carbonic acids, forming sulphate, phos- 

 phate, and carbonate of lime ; or with silica, forming 

 silicate of lime. 



113. Magnesia (MgO). — One per cent, of this would 

 be a large allowance. Soils generally contain much 

 less. More would be injurious rather than other- 

 wise. 



114. Alumina (Al'O^). — This is a fine white pow- 

 der. It is the basis of clay, which is a silicate of alu- 

 mina, composed of about 40 per cent, of alumina and 

 60 of silica. Good soils contain all the way from 2 

 to 10 per cent, of alumina. Those containing more 

 than 10 are apt to be too adhesive, and those having 

 less than 2 are too porous and open. If a soil is too 

 clayey, it is difficult to cultivate ; if not sufficien4;ly 

 clayey, it lacks the power of retaining the food of 

 plants, and allows them to escape by both evapora- 

 tion and filtration. Many a sandy soil would be more 

 benefited by 10 loads of manure and 10 of clay, than 



