MANURING WITH SEA-WEED. 431 



covered with wood. The winds bear along particles of earthy mat- 

 ter (see note. p. 427,) which they deposit again in the still forest.^ ; and 

 thus gradually form a soil even on the most naked places. This slow 

 general cause of accumulation may not be without its effect, and 

 should not be forgotten, but it evidently affords no explanation why, 

 in the same range of country, the soil which is covered by forests o^^ 

 one kind should improve more rapidly than those which are shelterer. 

 by trees of another species. 



§ 9. Of the use of sea-weed as a manure. . 



Among green manures of great value and extensive application 

 there remains to be noticed the sea-weed or sea-ware of our coasts. 

 The marine plants of which it consists differ from the green vege- 

 tables grown upon land, — 



1°. By the greater rapidity with which they undergo decay. When 

 laid as top-dressings upon the land they melt down, as it were, and in 

 a short time almost entirely disappear. Mixed with soil into a com- 

 post or with quick-lime, they speedily crumble down into a black earth, 

 ill which little or no trace of the plant can be perceived. 



2°. By the greater proportion of saline or other inorganic matter 

 which these plants, in their dry state, contain. It is these substances 

 which are obtained in the form of kelp when dry sea-weeds are burn- 

 ed in the air. 



We have seen (Lee. X., § 3,) that the quantity of ash left by 1000 

 lbs. of our more usually cultivated grasses, in the dry state, varies 

 from 5 to nearly 10 per cent, but the fucus vesiculosus^ which is 

 reckoned the most valuable for the manufacture of kelp, gives up- 

 wards of 160 lbs. of ash from 100 lbs. of the dry plant. This ash, 

 Jiccording to Fagerstrom, consists of — 



Gypsum 63*4 lbs. 



Carbonate of Lime 34-1 " 



Iodide of Sodium 2-7 " 



Other Salts of Soda 29-9 " 



Sihca, Oxide of Iron, and earthy Phosphates.31-1 " 



161-2* 

 This ash contains less potash, but more soda and gypsum, than 

 those of the grasses, (Lee. X., § 3,) and hence, as you will readily 



' Berxelius Arsberdttelse, 1S24, p. 225. — If we compare the composition of Ihia asli with 

 that of the several varieties of Ae/p, given in page 366, it will be seen to differ from them 

 very considerably. But kelp is always manufactured from a mixture of different plants 

 in varying proportions, and hence om cause of the diversity of composition among diffar- 

 ent samples of this substance. 



Sprengel states (Lehre vom Dunger, p. 277,) that the fucus vesiculosiis contains only 16 

 per cent, of water. I do not l^now whethar this is the result of experiments of his own, 

 but I have not introduced it into the text, because it appears to me inconsistent with the 

 remarkable manner in which sea-weed shrivels up when dried, and with its little perma- 

 nence as a manure. *' If an acre of land is completely covered with it, after a few days 

 of dry weather, the whole would not weigh 500 lbs. The fibrous parts reduced fo mere 

 threads alone remain — so that it is like manuring land with cobwebs" (Dr. Walker.) This 

 would seem to imply the presence of a larger quantity of water in fresh seaweed than in 

 preen gras.s, and consequently a less efficacy as a manure wlien applied in equal weights. 

 According to Boussingault, the fucus digitatus contains 40 per cent, of water, and the 

 fucus sacc/iarinus 76 per cent, when newly taken from the sea, and 40 per cent, after being 

 dried in the air. 



