SOIL FOR GREENHOUSES 187 



but they are not considered essential. With the exception 

 of potassium, phosphorus and nitrogen, these elements 

 are present in most soils in sufficient quantities to meet 

 the plant's need. 



157. The preparation and use of fertilizing elements. - 

 All fertilizing elements are absorbed by the plants in 

 definite compounds such as water, carbon dioxide, nitrates, 

 sulfates and others. It is the function of plants to take 

 these compounds and to re-combine them into other 

 compounds which are used by the protoplasm to build 

 up plant tissue and to cause growth. For example, 

 carbon is the chief constituent of vegetable substances, 

 and forms about one-half their total dry weight. It is 

 obtained almost wholly from air in the form of carbon 

 dioxide. This carbon also combines with water which is 

 absorbed by the roots, and various starches and sugars 

 result. Water also furnishes the hydrogen and oxygen 

 which enter into various other combinations which serve 

 to build up plant tissue. Nitrogen is one of the chief 

 constituents of the vital cell contents which form the 

 material known as protoplasm. Nitrogen, therefore, 

 stimulates growth. 



The greater number of plants obtain their supply of 

 nitrogen from soluble nitrates in the soil, but some few 

 have the power of assimilating this supply directly from 

 the air. Phosphorus and sulfur assist in the formation 

 of albuminous substances found in protoplasm. They 

 give color to the fruit and also stimulate seed production. 

 Calcium and magnesium seem to be essential in the for- 

 mation of cell wall structure. Potassium assists in the 

 assimilation of food in the cell content, and also gives 

 strength to the cell wall. Iron is essential in the forma- 

 tion of chlorophyll bodies in the cell sap. 



