57- Besides the ordinary division into tilth and subsoil, 

 layers known as pans sometimes occur. These are of three 

 kinds : (i) Moor-band pan which exists as an impervious deposit 

 a few inches below the surface. Salts of iron combining with 

 dead plants washed down by rain, oxidise and form a cement 

 which require to be broken up by a strong subsoiler. 

 (2) Calcareous pan is the result of long continued shallow 

 ploughing of soils rich in lime, the lime sinking gradually 

 and forming a cement ; (3) Hard pan. The cementing 

 material in this case may be oxide of iron or alkaline silicates 

 or calcium carbonate. Pans should be broken up by deep 

 ploughing. 



CHAPTER V. 



CHEMICAL CLASSIFICATION OF SOILS. 



7I)LANTS derive the bulk of their food from the air and 

 from water. The largest proportion of a plant consists 

 either of carbon or of water. Potatoes contain as much as 75% 

 of water, carrots and beet 80 or 90%, a tree felled when the 

 leaves have shed in the cold weather contains from 30 to 50% 

 of water, and when it is in leaf it contains 40 to 60% of water. 

 The carbon or the charcoal portion of a plant also varies 

 very much but it usually comes next in importance to water. 

 The carbon is fixed in plants with the help of sunlight acting 

 on chlorophyll granules, out of the Co 2 of the air. Air con- 

 tains about 4 parts of Co 2 in every 10,000 parts and the car- 

 bon of plants is therefore derived without any trouble on the 

 part of the cultivator. The nitrogen of plants is partly 

 derived from the atmosphere by means of rainfall without 

 any trouble, but it is also derived mainly from the soil and 



