6o 



TROPICAL AGRICULTURE 



Lime to be 

 applied to 

 surface cf 

 soils. 



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. 



, 



POTASH MANURES. 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 



Uses of 

 gypsum 



