6o 



TROPICAL AGRICULTURE 



CHAP. 



lime " is left. Cold water poured on quick-lime causes it to 

 swell up, with the evolution of much heat, and then it falls 

 down into a white powdery stuff which is much more bulky 

 Slaked-lime. than the quick-lime was. This powder is called slaked-lime, 

 and it has not such a burning effect as quick- or unslaked- 

 lime. Both forms are used as manure, the unslaked-lime being 

 applied to vegetable soils containing deleterious vegetable 

 acids. It neutralises the acids, and it forms useful com- 

 pounds in the soil. Chalk, broken shells and limestone, 

 and also marly and calcareous earths, are applied to soils in 

 which there is a deficiency of lime. All these manures — for 

 if one soil be added to another to increase the fertility it is 

 still called a manure- — should be kept near the surface, and 

 not be ploughed deeply in, as lime sinks into the soil, and 

 has a tendency, therefore, to run away from the roots of the 

 plants. 



Gypsum^ or sulphate of lime, is often used as a manure. 

 It is an ingredient of all superphosphates, and the phosphatic 

 manures are to a certain extent calcareous ones. Gypsum, 

 which consists of lime, sulphuric acid and water, is found in 

 a native state in various parts of the world. Deprived of 

 the water it forms the well-known " plaster-of-Paris." For- 

 merly gypsum was added to dung-heaps, and spread on the 

 floors of stables, &c., as it was believed to have the power of 

 "fixing the ammonia," that is, preventing it from passing 

 away in the form of vapour ; but now it is applied at once to 

 the land, and it is found to be espectally useful as a manure 

 for potato crops. 



Lime to be 

 applied to 

 surface of 

 soils. 



Uses of 

 gypsum 



Potash Manurp:s. — Potash enters largely into the inor- 

 ganic constituents of plants, as is shown by its being one of 

 the chief of the substances found in the ashes. It is usually 

 abundant in -the soil, and it is only after heavy cropping 

 that it becomes necessary to make up for its loss by the ap- 

 plication of a potash manure. As it is contained in quantity 

 in all plants, it is returned to the soil by the decay of weeds 



