?. ; CHEMICAL FERTILIZERS 



Hence the farmer who best knows how this takes place will, 

 other conditions being equal, be the one best fitted to continue 

 to derive a profit under the changing conditions " (see p. 221). 

 All Plants Need Air. Air contains one-fifth of its 

 volume of oxygen, which is needed by all parts of the plant, 

 but especially by the roots. Leaves give off oxygen in sun- 

 light in far greater amounts than is required for their life. 

 Air contains four-fifths of its volume of nitrogen, which is 

 not directly used by the plant, but is absorbed by a few 

 bacteria, some of which live in the soil as free creatures, 

 whilst others live in the nodules of leguminous plants. The 

 small amount of argon in the air has no influence on plant 

 life. Air contains carbon dioxide in amounts ranging from 

 0*03 to 0*04 % ; it is absorbed by the leaves, and other green 

 parts of the plant, in sunlight, giving off oxygen and leaving 

 carbon in organic forms for building up plant tissue. 

 Most of the dry matter of the plant is obtained from this 

 source. 



Air is needed for the roots, and therefore a soil must 

 contain air and be of open structure, though the need for 

 water precludes a structure which is too open. Cultivation, 

 deep digging and ploughing open up the soil so as to admit 

 air. The soil is also opened by the roots of plants and the 

 movements of creatures like earth worms. The depth to 

 which fresh air will penetrate is greatly increased by altera- 

 tions in barometric pressure. The passage of water also 

 facilitates the movement of air in the soil. Air is very 

 important in assisting the oxidation in the soil of those 

 substances which are organic or deficient in oxygen. The 

 rate of oxidation is largely modified by alterations in temper- 

 ature. Below a temperature of 10 C. (50 F.) oxida- 

 tion is very slow, at about blood heat, 37 C. (98 F.), 

 the best conditions occur, whilst above 50 C. (120 F.) 

 very little bacterial action takes place ; at higher temper- 

 atures ordinary chemical changes begin to increase in velocity, 

 and hence oxidation will continue. The carbon dioxide 

 in air is not in sufficient quantity to bring about maximum 

 plant production, as, if it is artificially increased, a greater 



