256 ARTIFICIAL MANURES [chap. 



that this precipitated material is so much finer and 

 more thoroughly mixed with the soil than any ground 

 insoluble material can be, that the activity of the super- 

 phosphate is due. Superphosphate is manufactured in 

 several grades of concentration ; it is adapted to all crops 

 for which phosphatic manure is wanted, except for 

 turnips on an acid soil where " finger-and-toe " occurs. 

 Nowadays the great rival to superphosphate is a 

 fertiliser called basic slag, which is obtained as a by- 

 product in the manufacture of steel from pig - iron 

 contaminated with phosphorus. As it occurs in 

 commerce, basic slag is a very fine dark powder, 

 insoluble in water, containing about 40 per cent, of 

 phosphate of lime and also a certain amount of free 

 lime. This free lime, together with an excess of loosely 

 combined lime, gives the manure its basic character ; it 

 is alkaline in contradistinction to the acid nature of 

 superphosphate, and it adds lime to the soil, whereas 

 superphosphate takes it away. The phosphates of 

 basic slag are insoluble in water, but they are easily 

 attacked by the carbon dioxide contained in the soil 

 water, and it is found in practice that they are readily 

 available to plants, especially upon soils that are damp 

 and sour. Basic slag has proved of particular value as a 

 fertiliser for all pastures on clay land, in which case the 

 free lime that is also present adds greatly to the value 

 of the manure. Its value as a fertiliser on arable land 

 of a light and dry character has hardly been enough 

 recognised in the United Kingdom. Basic slag is also 

 sometimes known as Thomas's phosphate powder or 

 basic cinder. It should be carefully distinguished from 

 the ordinary slag of ironworks, which contains no 

 phosphoric acid to make it valuable. Many of the 

 compound fertilisers to be dealt with later also contain a 

 considerable quantity of phosphoric acid ; indeed some 



