THE WORK OF ROOTS 151 



and distributed just where the roots can get at them 

 instead of being in coarse particles on the surface, 

 as is the case with ground bone, burnt bone, etc. 

 Consequently the superphosphates give far better 

 results than ordinary ground bone. 



Bone meal is usually ground bone, steamed to 

 render it more soluble. It contains both the earthy 

 and the gristly constituents (the latter contain nitro- 

 gen), but on account of its greasiness decomposes 

 slowly and dissolves very little. Bone-black is burnt 

 bone; it contains practically no nitrogen. Certain 

 kinds of slag and rocks rich in phosphoric acid are 

 also used as fertilizers. 



How may lime be supplied to the soil ? Aside from 

 the bones, we have as sources of lime, marl, marble, 

 shells, land -plaster^ (also called plaster of Paris, gyp- 

 sum and sulphate of lime), limy soils, etc. Lime acts 

 much more quickly if burnt, since it then dissolves in 

 water, while if air- slaked it is more slowly dissolved 

 by the acids in the soil -water. The amount of lime 

 in the soil can be judged by the condition of the well- 

 or spring- water. If this contains much lime it is 

 "hard" and deposits a scale (composed principally of 

 lime) on the in sides of tea-kettles, boilers, etc. It 

 may be softened for cooking purposes by boiling, or 

 for washing purposes by adding soda (or other alkali). 



^A story is told of Ben.i. Franklin, that he strewed fjypsum on a Clover field 

 in such away that the words "This has been plastered" appeared conspicuonsly, 

 owing to the more luxuriant growth where the gypsum was applied. 



