PHYSICAL CONSTITtJENTS OF SOIL ^1 



British Guiana, and Madras does not exceed that 

 found at Rothamsted. The amount of nitrogen is 

 considerably larger in rain collected near towns. 



Chlorides are always present in rain, especially in 

 the neighbourhood of the sea. At Georgetown, British 

 Guiana, the chlorides in the rain are equal, on an 

 average, to about 186 lbs. of common salt per acre 

 per annum ; at Cirencester they amount to about 

 3G lbs. ; at Rothamsted the quantity is about 24 lbs. 



The sulphates found in the rain at Rothamsted, mean 

 of five years, correspond to about 17 lbs. of sulphuric 

 anhydride per aci^e, 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 considerable extent, meet the demands of most 

 cultivated crops. 



The Soil. — 1. Physical Constituents. — If a soil con- 

 sisted of spherical particles, all of the same size, the 

 empty spaces between these particles would amount to 

 about 47 per cent, of the volume with the loosest 

 packing, and to nearly 26 per cent, with the closest 

 packing. The total empty space would be the same 

 whatever the size of the particles. If the interspaces 

 with the closest packing v/ere occupied by another set 

 of smaller spheres they would be reduced to 6*7 per 

 cent, of the volume. If this process was again repeated 

 they would become 1"7 per cent. With loose packing 

 the proportion of interspace would, in all cases, be 

 much larger. 



In a natural soil the particles are of very various 

 sizes, and of irregular shape ; the first condition tends 



