150 Fertilisers [pt. hi 



season is shorter or an early start more necessary the 

 high soluble slags have proved more effective. 



Basic slag is sometimes said to contain 22 to 44 per 

 cent, of tricalcic phosphate, but this is incorrect: the 

 actual nature of the phosphorus compound is not yet 

 known but it is probably a complex silico-phosphate^ : 

 the method of expression as tricalcic phosphate is con- 

 venient as it allows instant comparison with other phos- 

 phatic fertilisers. It contains a certain amount of free 

 lime, usually about 2 per cent., which gives a distinct 

 alkaline reaction, in addition a considerable quantity of 

 the combined lime can probably act as a base in the 

 soil (see p. 219). 



Basic slag is not soluble in water but it dissolves in 

 carbonic acid which occurs in the soil-water, and there- 

 fore readily comes into solution in the soil. The action 

 is hastened by its fine state of division, at least 80 per 

 cent, being guaranteed to pass a sieve with 100 meshes 

 to the linear inch. 



It has given remarkable results on clay grass lands, 

 and has probably been the cause of greater improve- 

 ment on these than any other single factor, its action 

 being to bring on the white clover which then so in- 

 creases the nitrogenous organic matter of the soil that 

 greater growth of grass becomes possible. As its effect 

 is at a maximum when the herbage is most scanty it is 

 best to begin with a heavy dressing, say 8 to 10 cwt. 

 per acre in the first year, followed by 5 cwt. at a later 

 date. This is an apparent exception to the general law 



1 Morison (Journ. Agric. Science, 1909, ni. 161-170) shows that it is 

 probably a compound of the type (M0)6M'0, SiOa, P2O5. He actually 

 found (CaO)5FcO, SiO.,, PoOg. There is no evidence for the statement 

 often made that basic slag is a tetra -basic phosphate. A further paper on 

 the subject is by Krol, Journ. Iron and Steel Institute, 1911, p. 126. 



