178 HOW CROPS GROW. 
36, p. 391,) and Anderson, (Zrans. High. and Ag. Soc., 
1855-7, p. 349,) that the ash of sea-weeds is, in general, 
as rich, or even richer, in potash than in soda. In 14 
analyses, by Forchhammer, the average amount of soda 
in the dry weed was 3.1 per cent; that of potash 2.5 per 
cent. In Anderson’s results, the percentage of potash is 
invariably higher than that of soda.* 
Analogy with land-plants would lead to the inference 
that the soda of the sea-weeds is in a great degree acci- 
dental, although, necessarily, special investigations are re- 
quired to establish a point like this. 
Oxide of Tron is essential to plants.—It is abundant- 
ly proved that a minute quantity of oxide of iron, Fe, O,, 
is essential to growth, though the agricultural plant 
may be perfect if provided with so little as to be 
discoverable in its ash only by sensitive tests. Accord- 
ing to Salm-Horstmar, the protoxide of iron is indispen- 
sable to the colza plant. ( Versuche, etc., p. 35.) Knop as- 
serts that maize, which refuses to grow in entire absence 
of oxide of iron, flourishes when the phosphate of iron, 
which is exceedingly insoluble, is simply suspended in the 
solution that bathes its roots for the first four weeks only 
of the growth of the plant. (Vs. St. V,p. 101.) 
We find that the quantity of oxide of iron given in the 
analyses of the ashes of agricultural plants is small, being 
usually less than one per cent. 
Here, too, considerable variations are observed. In the 
analyses of the seeds of cereals, oxide of iron ranges from 
an unweighable trace to 2 and even 3°|,. In root crops it 
has been found as high as 5°|,. Kekule found in the ash 
of gluten from wheat 7.1°|, of oxide of iron. (Jahres- 
bericht der Chem., 1851, p. 715.) Schulz-Fleeth found 
17.5°|, in the ash of the albumin from the juice of the 
* Doubtless due to the fact thatthe material used by Anderson was freed by 
washing from adhering common salt. 
