322 HOW CROPS FEED. 
The analysis of the well-water shows 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, - - T45 
Nitric acid, - - - - - 6.02 
We thus have demonstration that a solution containing 
but one-and-a-half parts of phosphoric acid to ten million 
of watcr 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 weather. 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, Laminaria, 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-weeds, one 
establishment in Glasgow producing 35 tons of it annu- 
ally. The iodine must be gathered from the water of the 
ocean in which these plants vegetate, and yet, although 
the starch-test is so delicate thit 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” which 
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 
Lehrbuch der Chemie, 4te, Aufl., pp. 743-4.) 
