8 SOILS AND MANUBES 



By further experiment it can be ascertained that the omis- 

 sion of sodium, silicon, chlorine and fluorine compounds 

 makes no apparent difference to the growth of the plants, 

 and, of course, these elements would not then be found 

 in the plants when grown. It is for this reason that they 

 are deemed non-essential. Neither does the omission 

 of carbon compounds from the mixture retard the growth 

 in any way, and, as the usual amount of carbon is found 

 in the plants, it must be concluded that this element is 

 derived from the air. If, however, nitrogen, sulphur or 

 any one of the elements of the ash, other than those men- 

 tioned above, be entirely withheld, the result will be the 

 same as if none of them at all were present. The plant 

 will die when the nourishment stored up in the seed is 

 exhausted. It appears, therefore, that nitrogen is derived 

 from the soil not from the air l and that nitrogen and 

 the other elements, viz., potassium, magnesium, calcium, 

 iron, sulphur and phosphorus are absolutely indispensable. 

 There is evidence of another kind that they perform impor- 

 tant physiological functions. 



It can also be demonstrated by experiments on these 

 lines, that if any one of the essential elements be supplied 

 in insufficient quantity the growth of the plants will be 

 correspondingly limited and cannot be maintained by 

 increasing the quantities of any or all of the others. 



Water Cultures. Similar experiments may be con- 

 ducted, by another method known as " water culture." The 

 principle is the same, but sand is dispensed with and water 

 alone is used as a medium or vehicle to convey the nourish- 

 ment to the roots of the growing plant. The various sub- 

 stances are dissolved in pure distilled water and the solu- 

 tion put into a clean wide-mouthed bottle. The seedling 



1 This is a question of the highest importance. There are certain 

 exceptions to the general rale stated in the text. The subject is more 

 fully discussed at a later stage, 



