4a RATIONAL FRUIT CULTURE. 



is not, as might be supposed on the evidence of the eye, an 

 inert mass. Its upper layers, those within about six feet of 

 the surface, teem with living things, like a ripe cheese, and 

 these living things, called bacteria, ail so excessively small 

 that they can be seen only under a powerful microscope, are 

 incessantly engaged, so long as the conditions of temperature 

 and moisture are favourable, in breaking up the organic matter 

 in the soil — the manure, the decaying vegetation, and so forth 

 — into simpler compounds, and converting it into plant-food. 

 As we have already seen, there is plenty of air in the groxmd 

 when in good condition, and not water-logged, and some of 

 the bacteria take the nitrogen from it and " fix '■' it — that is, 

 combine it with oxygen, and so make it into nitrate. If it 

 were not for their activities millions of acres which now pro- 

 duce abundant growth would be sterile, while even the best 

 cultivated land is enormously improved by their work. Still, 

 it is not always sufficient. We are obliged to crop the ground 

 so heavily and so continuously that the bacteria cannot keep 

 pace with our requirements, and, if the yield is not to fall 

 off, we must put in much of the nitrate which is t;iken out 

 by the plants. It is essential, too, not to let the soil turn 

 sour, for if it does, bacteria cannot live in it. The remedv 

 is to apply lime, which neutralises the sourness or acidity. 



