GARDEN PESTS IN NEW ZEALAND 



nothing of the other protozoa, present in the weight of soil above- 

 mentioned,, would be capable of destroying about 172,000,000 of the 

 bacterial population. Since the partial sterilisation of soil by steam 

 results in an increase of fertility, it is thought, on account of the 

 sterilisation destroying the protozoa, being more susceptible, and not the 

 bacteria, that protozoa inhibit the activities of the bacteria to such an 

 extent as to reduce the fertility of the soil; but this is a subject as yet 

 open to argument. Apart from the bacteria-destroying protozoa, there 

 are other forms that are thought to have something to do with the 

 decomposition of organic substances. 



The fungi, alga? and bacteria are amongst the lowest forms of 

 plant life, and hold somewhat the same position in the plant kingdom 

 as the protozoa do among animals; they are, especially the fungi and 

 bacteria, of primary importance in the maintenance of soil fertility. 

 The role of alg;ae lies mainly in increasing the organic content of the 

 soil, and they are invaluable in developing favourable conditions for 

 the establishment of vegetation on purely mineral soils. The fungi and 

 bacteria are responsible for setting up the intricate reactions involved 

 in the decomposition of organic matter, the bacteria being concerned 

 in practically all of the chemical processes going on in the soil. Both 

 fungi and bacteria are of two kinds : those that bring about decomposi- 

 tion, and those that live in a reciprocal relationship with plants upon 

 the roots of the latter. Such relationship, which benefits both organisms 

 and plants, is called symbiosis, the fungi being known as mycorrhiza, 

 while the bacteria, form nodules on the roots of such plants as the 

 legumes. 



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