BULLETIN OF THE BUSSEY INSTITUTION. 49 
The popular notion that crude spent tan is “sour,” or that, in other 
words, it does more harm than good to plants, with which it may hap- 
pen to come into immediate contact, is doubtless true. As Arthur 
Young said long ago, “Spent bark seems rather to injure than assist 
vegetation.” But the very facts that spent tan, as it comes from the 
vats is slightly acid; that it has a slight tendency to destroy plants; 
and that it decays exceedingly slowly, — enable it to smother weeds 
and grass all the more effectually, and thus, for many purposes and 
cases add to its merit as a mulch. 
EeL-GRrass. 
Although, as has just been said, a substance may serve excellently 
well as a mulch, even though it contains nothing which can serve as 
food for plants, it is still true, economically speaking, that in choosing 
a mulch, other things being equal, those substances would commonly 
be preferred which do contain some fertilizing ingredients. In this 
sense, I have sought to compare spent tan and dye-woods, not only 
with sawdust, as above, but with the common eel-grass (Zostera marina) 
of our sea-coast, a substance that is held in little or no esteem by farm- 
ers, either as a fertilizer or as a material for making composts, but 
which is a good deal used for mulching, as well as for protecting cellars, 
cisterns, and the like, from frost in winter. <A fair sample of dead, dry 
eel-grass, taken from a beach in Hingham harbor, where it had been 
deposited by the tide, and kept for some months in a dry loft, on being 
analyzed by the method described on page 29, was found to contain 
14.29% of water, 14.4604, of ashes,* 1.02349 of potash, 0.2256% of 
phosphoric acid, and 1.302% of nitrogen; or, in other words, the ash 
of the eel-grass contained 7.08% of potash, and 1.56% of phosphoric 
acid. ‘The experiments of other chemists show that the ashes are rich 
* From these data, it appears that the percentage of ash in the eel-grass 
dried at 110° was 16.87, a result that agrees closely with a determination made 
by myself, some years since, of the ash in fresh eel-grass. For the experiment 
in question, a quantity of living eel-grass was pulled up, when the tide was out, 
from mud flats in Hingham harbor, Sept. 27, 1857, and was examined two days 
later while it was still wet and perfectly fresh. In two determinations, there 
was found respectively 85.03% and 86.01% of water, the samples having been 
dried at 110°. The amount of ash in the wet weed was 2.62%, or in the weed 
dried at 110°, 17.50%. 
A specimen of living rock-weed (Fucus vesiculosus) collected at the same time 
and place upon rocks near the shore, contained 76.75 to 77.94% of water, and 
8.94% of ash; or 17.88% of ash in the rock-weed dried at 110°. 
VOL. II. 4 
