NUTRITIVE FUNCTIONS OF SALTS 



101 



Of these many elements found in the ash, only twelve are 

 common, — aluminum, boron, calcium, chlorine, iron, magnesium, 

 manganese, phosphorus, potassium, silicon, sodium, and sulphur. 

 These are also among the most common elements present in the 

 soil, so it is not strange that they should be found so often in 

 plants. Up until the latter part of the eighteenth century, it was 

 thought that this small percentage of ash was of minor importance 

 in the plant and accidental to the fact that the plant was growing 

 in a medium containing these elements. Among the first to recog- 

 nize the importance of the ash materials was Lavoisier (1792), who 

 wrote: " Plants take from the air which surrounds them, from 

 the water, and from the mineral kingdom the materials necessary 

 for their organization," and furthermore showed the importance 

 of plants in the general cycle of the elements. De Saussure (1804) 

 insisted that the minerals found in plants were not accidental 

 and pointed out certain ones which were essential. These views 

 were not popular, however, and up until 1840 it was generally 

 believed that the chief importance of fertilizer was its help in the 

 decomposition of humus. 



The Essential Elements. — To determine exactly what minerals 

 are necessary for a plant, the method of analysis is not sufficient. 

 Plants may take up elements which they do not need, and such has 

 been found to be the case. Some further check is necessary, and to 

 settle these questions the method of culture solutions is now com- 

 monly used. Plants are grown either in water solutions of various 

 salts or in clean sterile quartz sand to which salt solutions have 

 been added. The effect of the absence of any element can then be 

 determined and studied. It has been shown in this manner that 

 of the twelve elements listed above as nearly always present, only 

 eight are essential for the growth of the higher plants. These are 

 boron, calcium, iron, magnesium, manganese, phosphorus, potas- 

 sium, and sulphur. Along with carbon, hydrogen, nitrogen, and 

 oxygen they make up the twelve essential elements for plant 

 growth. Until the last decade, only ten elements were considered 

 essential, but as methods of the purification of salts have improved, 



