VII MANURES 59 



phosphates are at once acted on by the lime, and their 

 acidity is reduced, and they become dibasic phosphates — Dibasic 

 which is the best form for plant life ; but in soils where fhe ?est for 

 there is not much lime this process cannot take place, and ^^^ ^°^'- 

 agricultural chemists have made reduced phosphates^ which 

 are simply these dibasic phosphates of lime. 



Thomas-Gilchrist Slag. — Within the last few years a 

 valuable phosphatic manure has been made from slag — a 

 substance which Avas formerly a waste product given oiT in Formerly a 



, . . r ' • • r^^ -1 • 111 waste pro- 



the process of purifymg pig iron. This slag is a black duct. 

 blistered material that collects in quantities on the surface 

 of the molten iron. It contains a large proportion of phos- Itscompo- 

 phate of lime, as well as metalUc iron and its oxides, silica, 

 magnesia, and oxide of manganese. The manure called 

 usually " Thomas Phosphate Powder" is made by separating 

 the metallic iron from the slag, and then reducing the re- 

 maining mass to a fine powder — in which form it is applied 

 to the soils as other manures. It has been used with success A valuable 



. . , „ . , . 1 manure for 



on sugar plantations in British Guiana and elsewhere in the soils that do 

 tropics, and it is employed extensively in Europe : but, as it n°uch°iron" 

 contains a large proportion of iron, it is not considered to 

 be suitable for ferruginous soils. 



Calcareous Manures. — Lime is an important con- 

 stituent of all plants ; and, although it is present in abun- 

 dance in most soils, it is deficient in some. There are soils Lime acts 

 also, such as stiff, heavy clays, where the application of lime and me- 

 has a beneficial effect by its chemical and mechanical action. onThe^soii. 

 It renders insoluble substances soluble, and it causes the 

 cold, heavy clay to become warmer, and more friable and 

 porous. Lime is applied to the soil in its ordinary state, and 

 it is then called by the planter " raw lime," or in a crude 

 state as chalk, marl, shells — which are composed mainly of 

 carbonate of lime, and as gypsum or sulphate of lime. 



When coral, or shells, or limestone, are heated in a kiln, 

 the carbonic acid is driven off and pure lime or " quick- 



