GARDEN PESTS IN NEW ZEALAND 



CHAPTER II. 



Soil Organisms and Soil Fertility. 



IX the first chapter the plants were referred to as the primary 

 producers of life, and the animals as the consumers; the former not 

 only furnish nourishment for their own growth., but also for the support 

 of the animal world as a whole. Living plants (in reference to green 

 plants) utilise the sun's energy in the manufacture of their complex 

 food materials from comparatively simple chemical compounds. These 

 latter compounds are carbon dioxide, derived from the air through the 

 agency of leaves, and a weak solution of various chemical compounds in 

 water, derived by means of the roots from the soil, and carried up 

 through the plant to the leaves, where the elaboration into the complex 

 compounds to be utilised by the plants as food takes place. 



These comparative!} 7 simple compounds from which the plants 

 elaborate their nourishment are the raw food materials, and that they 

 must always be available -for plant growth, is evident when one considers 

 the vast areas of vegetation that cover, with the exception of desert 

 regions, the surface of the earth. Under moist climatic conditions it 

 has been calculated that some 500 tons of carbon dioxide and 1,000,000 

 tons of water, having the raw food materials in solution, are used 

 annually by one square mile of dense forest. For their development, 

 therefore, plants require : 



(1) Sunlight as the source of energy for the carrying on of their 

 life functions; 



(2) Air for the supply of carbon dioxide, oxygen, and, indirectly, 

 nitrogen ; 



(3) An ample supply of water required for the living tissues and 

 as a vehicle for the transport from the soil of 



(4) The raw food materials, in the form of various chemical 

 compounds. 



\Yith the exception of the carbon dioxide derived from the air, all 

 the raw food materials water, nitrates, phosphates, sulphates, potas- 

 sium, calcium, magnesium, iron, etc. are present in the soil, though 

 only a part of them is in a form suitable for imbibition by plants. In 

 the formation of these food materials, which render the soil fertile, 

 physical forces and the activities of living organisms play a leading 

 part. Our immediate concern is with the influence of these organisms 

 upon soil fertility, but it is advisable to give some consideration to the 

 soil itself, since it is the environment in which the organisms live, and 

 with which their existence is intimately associated; in this respect 

 attention will be confined to the type of soil usually cultivated by the 

 horticulturist, and to the uppermost layers that is, approximately, 

 within one- foot of the surface. 



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