76 Fruitgrowing under Irrigation 



and where this has been going on for ages large 

 deposits of decayed vegetable matter have accumu- 

 lated. Humus, consisting of elements drawn from 

 the air and soil, and having formed the substance of 

 living plants must necessarily contain all the ingre- 

 dients of plant life. A soil rich in humus must 

 therefore be a fertile one, and few of the higher 

 classes of plants will thrive if this is deficient in the 

 soil. 



As the fertility of a soil is determined by the 

 availability of its ingredients to the action of plant 

 roots, humus, as containing all plant requirements 

 in a readily available form, must therefore be one of 

 the chief factors controlling soil fertility. 



SOIL BACTERIA. 



In addition to its direct value as plant food, humus 

 is the home of numerous small organisms called 

 soil bacteria. These bacteria have the property of 

 taking up the nitrogen from the air that penetrates 

 the soil into their own bodies, and thereby increasing 

 the nitrate contents of the soil. 



These bacteria seem to be most numerous in the 

 top layers of the soil, and are seldom found deeper 

 than two feet from the surface of the ground. They 

 also appear to like a considerable amount of heat, 

 and are more active that is, increase more quickly 

 during summer than winter, and in a hot climate 

 than in a cold one. This is probably one of the rea- 

 sons why the soils of hot, dry climates generally 

 appear to be better supplied with nitrogen than the 

 soils of wetter and colder countries. 



Cultivation, by allowing the heat and air to freely 

 enter the ground, has the effect of greatly stimulat- 

 ing the growth of these bacteria, and thereby in- 

 creasing the nitrogen contents of the soil, provided 

 always that the land is sufficiently supplied with 



