MINERAL FERTILIZERS. 



Per cent. 



Calcium oxide (lime) 35-OO 



Carbon dioxide 45-O2 



Oxide of iron and aluminium, \ , 



with traces of phosphoric acid f 



Magnesium oxide 0.66 



Organic matter 7.06 



Sand 9.57 



73 



165. Some marls consist almost entirely of shells 

 more or less broken ; in others, clay or sand predom- 

 inates. Good marl always gives off a large quantity 

 of carbon dioxide on the addition of a strong acid, 

 and in this way some estimate may be formed of its 

 value (31). This fertilizer is particularly well suited 

 for application to sandy lands, supplying the very 

 elements which such lands generally need for the 

 continued production of good wheat and cotton 

 crops. 



166. Gypsum, or calcium sulphate, also called land 

 plaster, is a very important mineral fertilizer and has 

 long been in use as a valuble manure for corn, tobacco, 

 wheat, clover, and the grasses generally. It is com- 

 posed of lime and an oxide of sulphur in chemical 

 combination with a large percentage of water. This 

 water can be driven off by heat leaving a white sub- 

 stance which, ground to powder, forms what is 

 known as plaster of Paris. In this form, it has the 

 property of recombining chemically with water and 

 forming a substance of the same composition as the 



