PHOSPHATIC MANUKES 231 



possesses a doubly basic character. It contains basic 

 (tetracalcic) phosphates, and it also possesses the basic 

 character of the original slag in virtue of the excess of 

 lime which remains in the free or uncombined state. 



The amount pf " free lime " in slags as now produced 

 rarely exceeds 6 per cent, and in some cases is less than 

 2 per cent., 1 but a considerable proportion of that which 

 is actually in combination with phosphoric acid and silica 

 acts potentially as a base and can liberate ammonia from 

 neutral salts. 



Manurial Effects. Basic slag acts best on clay soils ; 

 these generally contain plenty of moisture, and owing 

 to the comparative insolubility of the phosphates, 

 this appears to be indispensable for the proper action 

 of basic slag. Clay soils again are often deficient in lime, 

 and though the total quantity of lime in an ordinary 

 dressing of slag is small, it is finely ground, mixes well 

 with the soil, and produces both physical and chemical 

 changes (p. 113) which are of great benefit to the crops. 

 Basic slag may give very good results on both sandy and 

 humous soils, but not if they are at all dry, and it has even 

 been found beneficial on chalky soils if the natural deficiency 

 of potash is made good. It is particularly valuable as a 

 means of supplying a readily available phosphate to soils 

 that are inclined to acidity, and therefore to anbury, or 

 other troubles arising directly or indirectly from that cause. 



Basic slag has been very largely used as a dressing for 

 pastures in which it tends to encourage the growth of clovers, 

 probably owing to its limey and basic character. The 

 improvement in quality is often more striking than the 

 increase in quantity of the herbage. Of course, on poor 

 land or even in soils of moderate fertility, basic slag or 

 other phosphatic manure will generally give a profitable 

 increase. For the same reason it is perhaps the most 

 1 Hendrick, J.S.C.I., July 1909. 



