178 HOW CKOPS GROW. 



36, p. 391,) and Anderson, {Trans. 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 Iron is essential to plants. — It is abundant- 

 ly proved that a minute quantity of oxide of iron, Fe^ 0,, 

 is essential to growth, though the agricultural plant 

 may be perfect if pro\ided 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. ( Yersudie, 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 v.ariations 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° 1„ in the ash of the albumin from the juice of the 



* Doubtless due to the fact thattlie material used by AnderBon was freed by 

 wasbing: from adbering common salt. 



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