18 THE CHEMISTRY OF THE FAKM. 



made near towns. In tropical rain the proportion of 

 nitrogen as nitrates is generally considerably increased. 



Chlorides are always present in rain, especially in the 

 neighbourhood of the sea. At Lincoln, New Zealand, the 

 chlorides in the rain are equal, on an average, to about 88 

 Ibs. of common salt per acre per annum ; at Cirencester 

 they amount to about 40 Ibs.; at Rothamsted the quan- 

 tity is about 24 Ibs. 



The sulphates found in rain at Rothamsted, 'mean of 

 five years, correspond to about 17 Ibs. of sulphuric anhy- 

 dride per acre, yearly. 



The quantity of chlorides in the rain at Rothamsted is 

 apparently sufficient for the crops on the farm, mangels 

 possibly excepted. The sulphates will also to a consider- 

 able extent meet the demands of most cultivated crops. 



The Soil: 1. Constituents. All soils have been pro- 

 duced by the disintegration of rocks, through the pro- 

 longed action of water, air, and frost. The character of a 

 soil largely depends on the character of the rock from 

 which it has been derived. Primitive and igneous rocks 

 yield soils rich in potash ; fossiliferous rocks produce soils 

 rich in phosphoric acid. The principal ingredients of 

 soils are sand, clay, carbonate of calcium, and humus ; as 

 each of these preponderate the soil is said to be sandy, 

 clayey, calcareous, or peaty. 



Sand is either composed of pure quartz (silica), or con- 

 sists of fragments of more complex minerals mica, for 

 example. When the former is the case, the sand will 

 supply no plant food ; but in the latter case the gradual 

 decomposition of the mineral will slowly increase the ash 

 constituents available for the plant. 



