7 2 SCIENTIFIC AGRICULTURE. 



least one mineral constituent to plants, one, however, 

 that generally exists in soils, and hence its valuable 

 and often surprising effects are not to be attributed 

 to the plant-food it contains so much as to other im- 

 portant purposes which it serves. Its alkaline prop- 

 erties cause it to neutralize acids, which sometimes 

 exist injuriously in soils. It also renders stiff clays 

 light and mellow, and aids in the decomposition of 

 organic substances, and of some insoluble inorganic 

 compounds. In other words, lime corrects " sour- 

 ness " in lands, destroys excess of vegetable matter, 

 lightens heavy clay soils, and releases potash for the 

 use of plants by decomposing silicates. 



163. Marl is a mixture of calcium carbonate 

 derived chiefly from the shells of animals clay and 

 sand in variable proportions. It is generally valued 

 according to the amount of calcium carbonate it con- 

 tains, which may vary from five to ninety per cent., 

 though in almost all marls there are other valuable 

 constituents. The calcium carbonate, or carbonate 

 of lime, as it is generally called, is usually in a finely 

 divided state, and can be readily used as food by the 

 plant. 



164. The celebrated New Jersey green-sand marl 

 contains a large percentage of potash, which gives it 

 great value. Excellent marls are found in many 

 parts of the country, and need no preparation before 

 using. The following analysis by Professor Norton 

 will give some idea of the composition of an excel- 

 lent marl 



