GARDEN PESTS IN NEW ZEALAND 



Soil is the product of disintegrated and weathered rocks with which 

 are mixed the residues of organic matter. Apart from the particles of 

 disintegrated rocks, which form the matrix, soil contains chemical com- 

 pounds of two kinds: those of a purely mineral nature derived from 

 the inorganic components of the original rocks, and those of an organic 

 origin derived either from the ancient remains of organisms, which, in 

 the case of sedementary deposits, became incorporated in the rocks at 

 the time of their origin, or from the remains of present-day plants and 

 animals decomposed by soil organisms. In addition, there is the humus, 

 which has a fundamental physical influence, and for the production of 

 which soil organisms are responsible. 



In the initial stages of soil formation during the disintegration 

 and decomposition of rocks, the first type of soil to be formed is suitable 

 for the growth 'of only certain plants ; it is of a purely mineral nature, 

 containing raw food materials derived mainly from the rocks and not 

 from organic matter, unless from such organic residues as were incor- 

 porated in the rocks during their formation in ancient times. Such 

 soil cannot sustain the higher types of green plants, no*r is it populated 

 by soil organisms; it furnishes suitable pabulum, however, for the 

 nourishment and growth of the more lowly types of vegetation, which 

 are able to convert to their benefit the limited supply of food materials 

 available. The complex organic compounds that such primitive plants 

 elaborate from these food materials of purely mineral origin, and 

 incorporate in their tissues, are, after death, returned to the soil, which 

 becomes correspondingly enriched, and a favourable environment for 



Figure 2 



THE THREE MAIN TYPES OF SOIL PROTOZOA. 

 Magnified 300-400. 



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