CHAPTER III 



^rinriplrs nf CCnrn (grninllj and ¥tt bing 



•f HE conditions of plant growth are: I, plant 

 ill food; 2, heat; 3, light; 4, vitality. All of 

 %^ these must be supplied. Plant food is derived 

 from two sources, the atmosphere and the 

 soil. From ninety-five to ninety-nine per cent of 

 the dry matter in the plant is obtained from the 

 atmosphere through the leaves, while only one to five 

 per cent comes from the soil. However, it is sim- 

 ply the plant food in the soil that need be considered. 

 The supply is limited and if removed by continuous 

 cropping, it must be replaced by adding plant food 

 to the soil. On the other hand, the supply of plant 

 food in the air is inexhaustible. But in order to utilize 

 this great source of plant development, it is necessary 

 to secure a proper proportion of the elements of the 

 soil which are combined with the atmosphere elements 

 in the composition of the plant. 



Briefly the elements of plant food are : From the 

 air, comprising one to five per cent of the weight of dry 

 matter of the plant — carbon, hydrogen, oxygen, nitro- 

 gen. From the soil, comprising ninety-five to ninety- 

 nine per cent of the dry matter — iron, sulphur, silica, 

 manganese, magnesia, sodium, potash, phosphorus, 

 chlorine. 



Forms of Atmospheric Food — Carbon is found in 

 a solid state in the earth in the form- of coal and dia- 

 monds. In the atmosphere, it exists in combination 

 with oxygen as carbon dioxide gas. This gas is taken 

 into the leaves of the plant through the leaf pores or 

 stomates, the carbon is separated from the oxygen and 



