PLANT PHYSIOLOGY 123 
85. Essential Constituents of Plant Food.—There are 
two methods of ascertaining exactly what chemical 
elements are essential for the nutrition of plants. The 
first is to analyse chemically a healthy plant and thus 
discover its composition. The second is to grow the 
plant in a medium of which the constituents are known 
and kept under chemical control. By the second method 
it has been discovered that the following elements are 
indispensable to the healthy growth of all green plants, 
Carbon, Hydrogen, Oxygen, Nitrogen, Sulphur, Phos- 
phorus, Potassium, Calcium, Magnesium, and Iron. 
When analysed, most plants contain in addition to these, 
Chlorine, Silicon, Sodium, and some other elements, but 
only these ten are essential. All except carbon are ab- 
sorbed from the soil, in the form of various chemical 
compounds dissolved in water. The method by which 
carbon is obtained will be considered later. 
86. Water Culture.—It has long been known that a 
plant can be grown in water containing salts in which. 
all the above elements, except carbon, are present. That 
carbon is essential will be proved later; that the other 
elements are essential can be shown by the method of 
water culture. Several plants of the same species and 
size are grown in glass jars, with their roots dipping 
into a culture solution, and if any of the necessary salts 
are omitted there is no healthy growth. Hydrogen, 
and oxygen together form water. Nitrogen is present 
in various salts called Nitrates, Sulphur in Sulphates, 
Phosphorus in Phosphates, the remaining elements 
forming the bases of these salts, e.g. Potassium nitrate, 
Sodium sulphate, etc. 
