178 INSECT PESTS 



these strenuous days the green fallow is usually replaced 

 by a potato crop, which is regarded by many farmers as 

 a cleansing crop for the ground, which then becomes 

 available for wheat again. 



In the previous chapter mention was made of the 

 importance of lime in all soils. Lime is produced by 

 heating chalk, limestone or marble, all of which are of 

 the same chemical composition, viz. one part each of 

 calcium and carbon to three of oxj^gen, called briefly 

 calcium carbonate. When the chalk, which is the cheapest 

 substance, is heated, carbon di-oxide is given off, leaving 

 the calcium oxide or quicklime behind. This, on being 

 slaked with water or exposure to the air, is appUed to the 

 soil. The result is that if the ground be sour and contain 

 an excess of carbon di-oxide from long stagnation, the 

 lime, which is always attracting this gas in its desire to 

 go back into chalk, sweetens the soil in the process. It 

 should be remembered that although carbon di-oxide is a 

 plant food, it is in the wrong place down in the soil, plants 

 breathing in air through their '' lungs " (the leaves) and 

 extracting all the carbon they need above the surface. 

 Beneath it we must endeavour to keep a good circulation 

 of sweet air going, to enable the oxygen to continue its 

 life-giving work by uniting with the elements in the soil 

 to form carbonates, phosphates and nitrates, thus render- 

 ing them available for the support of plant growth. 



