322 HOW CROPS FEED. 



The analysis of the Avell-water sliows that the nutritive 

 solution need not contain the food of plants in greater 

 proportion than occurs in the aqueous extract of ordinary 

 soils. 



The well-water contained, in 100,000 parts, 



Lime, 15.14 



Magnesia, 1.53 



Potash, 2.13 



Phosphoric acid, - - - - 0.16 



Sulphuric acid, .... 7.45 



Nitric acid, 6.03 



We thus have demonstration that a solution containing 

 but one-and-a-hiilf jjarts of jjhosphoric acid to ten million 

 of water is competent, so far as this substance is concern- 

 ed, to support a crop bearing twice as much grain as an 

 ordinary soil could produce under the same circumstances 

 of Aveather. Do we thus reach the limit of dilution ? 

 We cannot answer for agricultural plants, but in case of 

 some other forms of vegetation, the reply is obvious and 

 striking. 



Various species of Fucus, Lam'niarla^ and other ma- 

 rine plants, contain iodine in notable quantities. This 

 element, so much used in photography and medicine, is 

 made exclusively from the ashes of these sea-Aveeds, one 

 establishment in Glasgow producing 35 tons of it annu- 

 ally. The iodine must be gathered from the water of tlie 

 ocean in which these plants Aegetate, and yet, although 

 the starch-test is so delicate thnt one part of iodine can 

 be detected when dissolved in 300,000 parts of water, it 

 is not possible to recognize iodine in the " bitterns" Avhirh 

 remain when sea-water is concentrated to the one-hund- 

 reth of its original bulk, so that its proportion must be 

 less than one part in thirty millions of water! [Otto''s 

 Lehrhich, der Cheni'e^ Ate, A>'Jf., \i\^. 743-4.) 



