ESSENTIALS OF PLANT UFE I/ 



plant. It has been impossible to exclude soda completely in water 

 culture experiments, owing to its presence as impurities in 

 reagents, and its entrance into solution by the action of water 

 upon glass vessels, but otherwise such experiments show that 

 soda is not essential. Soda does not appear to perform any such 

 highly useful functions as silica. It may, however, take the place 

 of the indifferent essential ash, and so replace potash. 



Definition of Plant Food. Plant food may be defined as any 

 substance which contributes to the building of tissue or is other- 

 wise essential to the life of plants. Carbon dioxide, which is 

 assimilated by the leaves, is plant food, and so is water. But we 

 are more concerned in agriculture with the mineral salts which 

 enter the roots of plants, since these require control and are more 

 or less subject to it, and we have these in mind rather than car- 

 bon dioxide or water when we speak of plant food. By plant 

 food we usually mean potash, phosphoric acid, nitrogen, sul- 

 phates, lime, magnesia, or iron. Often the term is confined to 

 nitrogen, phosphoric acid, and potash, for the reason that they 

 are the only forms of plant food commonly added to the soil. 



\Ye have spoken of the elements essential to plants, but we must 

 bear in mind that the free elements, with two exceptions, are use- 

 less to plants. These two exceptions are oxygen, which is given 

 off by plants to a much greater extent that it is used, and nitrogen, 

 which in the free state can be taken up by leguminous plants, if 

 the bacteria which aid in this process are present. All the other 

 elements in the free state are either useless or injurious to plants. 

 The essential elements must be present in certain forms of com- 

 bination ; other combinations are injurious or useless. These 

 facts have been ascertained by numerous experiments with water 

 cultures and sand cultures. 



Iron is taken up as ferric compounds; ferrous compounds are 

 often injurious. Phosphorus must be present as phosphates, sul- 

 phur as sulphates, chlorine as chlorides, silica as silicates or silicic 

 acid. Sulphides, sulphites, chlorates, and perchlorates, are 

 injurious to plants. Carbon is absorbed as carbon dioxide, 

 and as organic bodies to a much less extent. Oxygen 



