stituents will be treated hereafter. These paragraphs give 

 the most important facts as to structure and growth by 

 which the fruit grower has to guide his practice. 



The Soil-constituents of Plant-food. 



1 1 . We have now to consider what substances form 

 the food of plants, and which of them the soil must contain 

 in order to supply their wants. When the wood of a tree 

 is burned slowly with very small access of air there results 

 a mass of charcoal or carbon. All the moisture and a 

 little of the carbon originally in the wood-substance have 

 been driven off like gas by the heat. Whence did the 

 carbon come ? Wholly and entirely from the atmosphere, 

 which contains a variable amount, from 3^ to 6 measures, 

 of carbonic acid gas in every 10,000 of air. The foliage 

 absorbs this carbon -containing gas, and turns the carbon 

 to account in building up the tree's tissues. Let us sup- 

 pose the log of charcoal is allowed to burn slowly away. 

 All the carbon passes into the atmosphere again, being re- 

 converted into carbonic acid gas. But there is left behind 

 a small quantity of ash of a mineral nature. This handful 

 of ash is the sum total of what the tree has drawn out of 

 the soil in a dissolved state by means of its root-hairs, and 

 without which constituents it could not have maintained a 

 healthy life. If we analyse this ash we shall clearly get 

 to know what substances a soil must contain in order to 

 be fruitful, and what earth- elements go to make up the 

 food of plants, independently of those contained in the 

 essential air and water. They are as follows : Nitrogen, 

 sulphur, phosphorus, potassium, calcium, magnesium, 

 iron, perhaps also chlorine. Of course, very small quan- 

 tities are required, but they cannot be omitted. Plant life 

 cannot go on continuously without a sufficient proportion 

 of each one. Two other substances are always present, viz., 

 sodium and silica, but they are not essential for healthy 

 growth. Alumina, the base of clay, is also very widely 

 diffused in most soils, but is not taken up by plants. 



Nitrogen is presented in the form of ammonia and 

 nitrates, and it seems probable that the ammonia is decom- 

 posed and converted into nitrates before absorption takes 

 place. Not only is this element added to the soil by 



