186 THE PRINCIPLES OF FLORICULTURE 



ing on these questions, is the use of caustic lime, which 

 material appears to have a certain sterilizing influence." 



155. The availability of fertilizing elements. As has 

 been stated, the mineral constituents of a soil and the 

 amount and character of the organic matter in it have 

 much to do with its fertility. All mineral elements 

 must be rendered soluble before they can be used by plants, 

 and organic substances must exist in correct combinations 

 before they are of value. A soil may contain a quantity 

 of food elements and yet a plant may starve. For this 

 reason, chemical soil analysis is seldom satisfactory. 

 Many samples of soil are sent to experiment stations 

 with requests that they be analyzed, and the grower 

 informed in what particular fertilizing elements they are 

 deficient for the growth of a particular crop. The chemist 

 can determine the minerals which compose the soil, but 

 he is not familiar with the food requirements of the plants 

 and the availability of minerals for them. This is a 

 problem to be determined only by experimentation for 

 each particular crop and for each type of soil. It is 

 essential that the flower-grower know the sources of 

 plant food, and then treat his crops so that they will 

 receive the proper amounts of nutriment. As has been 

 stated, in addition to the food which plants take from 

 the minerals of the soil, many plants absorb from the 

 air an abundance of food in the form of carbon dioxide. 



156. Elements essential for plant growth. The com- 

 position of plant food is so well known by most plant- 

 growers that it is hardly necessary to enter into a detailed 

 discussion of this subject. The elements regarded as 

 essential are hydrogen, oxygen, carbon, nitrogen, po- 

 tassium, calcium, magnesium, phosphorus, iron, chlorin 

 and sulfur. Certain plants make use of other elements, 



