IIl6 THE BOOK OF GARDENING. 



The mineral constituents of plants — that is, those ingredients 

 which are found in the ashes of plants when these are sub- 

 mitted to the process of burning — are taken directly from the 

 soil, being absorbed by the roots in the form of solution in 

 water. About thirteen different chemical elements will be found 

 on analysis in the ashes of most garden plants ; but, with the 

 exception of potash and phosphoric acid, and sometimes also of 

 lime, all the other ingredients will be furnished in abundance 

 by practically all fertile soils. 



The various constituents found in plants are combined with 

 each other in certain definite proportions, varying for different 

 genera and species ; and the growth of the plant is measured and 

 limited by the least abundant of these substances required for 

 its sustenance. . 



Reducing these principles to practical use, we may say that a 

 plant will secure a full supply of carbon provided other necessary 

 nutrients are available. The supplies of oxygen and hydrogen are 

 chiefly dependent upon the water-supply. The available nitrogen 

 may be regulated by the use of nitrate compounds or ammonium 

 salts ; or the setting up in the soil of those conditions which 

 favour the development of nitrogen-working micro-organisms. 



Having provided a full nitrogenous supply, the gardener may 

 control the growth of his plants by giving or withholding 

 phosphoric acid, potash, and lime. 



The water-supply to plants is a matter of supreme importance 

 to the horticulturist, for not only does water comprise three- 

 fourths, or more, of the actual weight of all garden plants when 

 growing, but it is the vehicle in which all the mineral and 

 nitrogenous constituents of plant-food are carried to their 

 destination in the fabric of the plant. In the performance of 

 this function, water is constantly passing through the plant, being 

 absorbed by the roots, and transpired by the foliage. It is 

 estimated that more than 3001b. of water must pass through the 

 plant for the deposition of a single pound of dry substance in 

 the plant. Thus the question of the maintenance of the water- 

 supply in greenhouses and conservatories becomes one which 

 cannot be overlooked or neglected. 



The nitrogen supply to plants takes rank next to water in 

 importance, for it is this element which may be most quickly 

 exhausted in the garden soil, and which is the most expensive 

 to replace by artificial means. 



Pood-requirements of Plants. 



Science has taught the horticulturist what nutrients the soil 

 must contain in order to secure thriving plants and the best 

 crops ; the gardener, therefore, should put into the soil he 

 cultivates those fertilising substances only which it requires to 

 render it capable of fulfilling the functions required of it. 



