XIII.] SUPERPHOSPHATE 255 



oil of vitriol, and yield a soluble acid phosphate, similar 

 in its action to the superphosphate to be dealt with 

 later. These dissolved bones or vitriolised bones, as 

 they are called, form rather a damp mass, which is not 

 very easily sown from a machine ; they offer no special 

 advantage over a mineral superphosphate which is 

 cheaper, even after making allowance for the nitrogen 

 the bone manure also contains. Again, the bones are 

 sometimes submitted to the action of superheated 

 steam before grinding, thus taking out of them most of 

 the material containing nitrogen ; the resulting steamed 

 bone flour is a friable powder rich in phosphates but 

 containing only about i per cent, of nitrogen, and 

 forming a valuable manure on light soils. Bone 

 manures, however, no longer possess their former 

 importance when they were almost the only source of 

 phosphates. For many years deposits of mineral 

 phosphates of lime have been worked for manurial 

 purposes. Sometimes these rock phosphates are 

 ground to a very fine powder, which is practically 

 insoluble in water but which does slowly become 

 available to the plant in soils rich in organic matter and 

 well provided with moisture. In the United Kingdom, 

 however, such ground rock phosphates are rarely 

 employed ; as a rule, the mineral is treated with oil of 

 vitriol and converted into the soluble phosphate known 

 as superphosphate or acid phosphate. Superphosphate 

 of lime, which, for manurial purposes, was invented by 

 the late Sir J. B. Lawes, is the most widely employed of all 

 the phosphatic manures. Being soluble in water, it 

 becomes disseminated throughout the soil, and is there 

 reprecipitated wherever it comes in contact with 

 particles of carbonate of lime or humus. Thus the 

 surface soil gets mixed with precipitated phosphate of 

 lime in a very fine state of division, and it is to the fact 



