Last night at least 21 Haines gardeners gathered at the library - called together by Master Gardeners George Figdor, Alexandra Feit, and others. It was fun. There were prizes - (where there is George, prizes are nearly always nearby!) - generously donated by Haines Home Building.
This question came up: Should seaweed be considered a nitrogen source (a "green" in lasagna gardening lingo) or a carbon source ("brown")? Opinions differed.
Feeling feisty, I was quite willing to offer my first born to defend my position that seaweed does not belong in the "green" group. But I did and do feel guilty about being so opinionated backed up only by memory of previous research. So here is a little new news about seaweed and it's role in the development of the nitrogen content of your soil.
W. A. Stephenson, in his classic book, Seaweed in Agriculture & Horticulture states:
Basically, the addition of seaweed leads to a temporary diminution of nitrogen available for crops, then a considerable augmentation of the nitrogen total.So, seaweed, through the bacteria it adds to your soil, facilitates nitrogen, but it, in and of itself, doesn't "contain" nitrogen in any appreciable amount. The quote also adds the cautionary note about that period of time when the carbon you add to the soil takes up nitrogen and your soil, if tested during that period, will test "nitrogen deficient." It's really not. It's just busy. But it also explains why fresh seaweed put on your garden during the growing season is not a soil amendment. It's good for mulch; but as an amendment, let it compost first.
When seaweed, or indeed any undecomposed organic matter, is put into the soil, it is attacked by bacteria which break the material down into simpler units -- in a word, decompose it. To do this effectively the bacteria need nitrogen, and this they take from the first available source, the soil. This means that after seaweed has been added to the soil, there is a period during which the amount of soil nitrogen available to plants is reduced. During this period seed germination, and the feeding and growth of plants, can be inhibited to greater or lesser degree. This temporary nitrogen deficiency is brought about when any undecomposed vegetable matter is added to the soil. In the case of straw, for example, which is ploughed in after harvest, bacteria use up soil nitrogen in breaking down its cellulose, so that a 'latent' period follows. Farmers burn stubble after harvest to avoid this latent period, and the short-term loss of available nitrogen which causes it. But such stubble-burning is done at the cost of soil structure, soil fertility, and long-term supplies of nitrogen which ultimately would have been released from the decomposed straw.
Stephenson's book, and other neat books about seaweed, is available at Acres subtitled, "The original Eco-Ag bookstore."
Check it out. There are at least 4 reasonably priced items on seaweed.
