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| How much organic matter do I need to add to 100 sq ft of my garden each year to maintain 5% organic matter in the soil. Let's assume the garden started out at 5%. I realize the answer is: 'it depends on', but what are some of the main considerations involved? |
Follow-Up Postings:
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- Posted by toxcrusadr (My Page) on Wed, Aug 29, 12 at 15:02
| It's going to vary a lot by climate, type of soil, crops, etc. etc. But you knew that. A good rule of thumb that I hear spoken a lot is to add an inch of compost to the garden each year. I can't point to any studies showing that it's approximately the right amount, maybe others can. |
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| 10'X10'X 1' foot deep = 100 cubic feet of soil. 100 cu ft X 5%(100 x 0.05 = 5)= 5 cubic feet of compost. |
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| Thanks guys. I see the calculation for adding 5% of organic matter, but how much of that would decay in a year and need to be replaced. I'm really wondering how much needs to be added to the garden to maintain it at 5%. Would all that compost just disappear, i.e. turn into inorganic matter in just one year? My principal soil amendment is seaweed and eelgrass, and it definitely stays in the soil more than one year. |
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| You must have sandy soil. Peat moss is known to break down slowly, so that might be a helpful amendment for you. Since you have seaweed and eelgrass available, I would be using these, in your situation. If the amount of organic matter is too high, you might encounter rotting. That would be a sign to back off on adding any more grass and seaweed. |
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- Posted by toxcrusadr (My Page) on Thu, Aug 30, 12 at 11:00
| Compost does continue to decay after it's added to soil. The decomposition process gives off carbon in the form of CO2, but a small fraction of it eventually becomes humus, the complex organic molecules that have reached the end of the line and are indigestible to microbes. Note, it's very common for compost to be half moisture, and a fraction of the dry material is inorganics - salts and minerals. So if you add a pound of compost, you may only be adding 1/3 lb or less of organic matter. There are so many variables that unless you get a soil test every year to check it, it's going to be hard to know exactly. It's probably not worth it for a home garden, but if you've got an acre sized truck farm, that's a different situation. A better approach for the home garden might be to just feel and observe your soil, as well as your plants, and act accordingly. Lots of plants grow in very high percentages of organic matter. Heck, some of the best veggies voluteer in compost piles. So it's hard to overshoot, unless you have drainage problems that can lead to anaerobic conditions (as ericwi mentioned). |
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| I remove my seasonal crops & add compost, even if I am not planting new plant for a few weeks. You can use cover crops/green manure between crops, too. The only bad time to add compost is when you be not have any to add. That is why I have at least 3 pile of 3 different ages going at all times. |
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| How much organic matter you need depends on how active your Soil Food Web is as well as what kind of organic matter you put in there. In your area of the world that SFW will rest about 3 to 4 months of the year and be active, depending on soil temperatures, the rest of the year digesting, and converting into plant food, which means any organic matter in the soil is being converted and it "disappears". There are mathemtical formulas that can hint at quantities but none I have seen takes into account the differences in soil types or soil biology. 5 percent would be the bare minimum, 6 to 8 percent would be better. |
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| SOM is more than just a percentage. There are different components of SOM so stating X amount is a minimum is pretty useless. Lloyd |
Here is a link that might be useful: What is organic matter?
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| Thanks for the link to the aticle, Lloyd. It answered exactly what I wanted to know. |
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| Excellent article, Lloyd. Which raises the question of what materials lead most quickly to SOM. Clearly most of what is traditionally added falls into the "active" category, stuff that is pretty easy to come by. Charcoal would be an example of ready-made SOM. SOM is also what adding humate is about. Arguably materials like calcium carbonate and carbonatite are SOM as well. |
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- Posted by toxcrusadr (My Page) on Thu, Sep 6, 12 at 11:15
| As a chemist (not even a soil chemist) I would consider carbonates to be inorganics with not much in common with organic matter (in terms of cation exchange capacity, water holding characteristics, etc.). Not to split hairs. :-] |
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| Aren't they ultimately the result of organisms taking carbon out of the air? I think the carbon content is part of why calcium carbonate has such a good effect on acidic low-carbon soils. Certainly I would not suggest it as a total substitute for SOM, but an important component in some soils. |
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- Posted by toxcrusadr (My Page) on Thu, Sep 6, 12 at 13:20
| There would be multiple sources of carbonates in soil or rock, but I think limestone (where most of it is sequestered) is mainly formed by CO2 dissolving in the ocean, transforming to bicarbonate and carbonate, and precipitating out with dissolved calcium. I don't know enough about the biochemistry going on in soil to know for sure whether the *carbon* content of carbonates plays a role in and of itself, it's not something I've heard anything about. We've wandered off into the soil geek corner again! :-D |
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| Yep! Where's gargwarb? |
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| My eyes glazed over long ago. ;-) Lloyd |
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- Posted by toxcrusadr (My Page) on Thu, Sep 6, 12 at 16:27
| "Could we get some brandy over here in the corner for our friend? He heard too much chemistry." |
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| When my soil test (UMASS) lists x% organic matter, are they talking about SOM or overall organic matter? |
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- Posted by strobiculate none (My Page) on Tue, Sep 11, 12 at 9:04
| The carbonates are part of soil organic matter, arguably? Need more alcohol. Don't see how. My understanding is that the carbonates are a mineralized form, and i've not seen a reference to applying calcium carbonate as any meaningful or trivial amount of organic material. But then, when I took chemistry...i was years from understanding how much I should have been paying attention. The % is any part of soil that is not derived from a parent source of mineral origin. Organic matter is good, humic acids are "where it's at", which is why the compost teas are considered so effective...essentially, you are brewing a form of humic acid, the end result of the entire composting process. The inch a year rule is probably about what you should be looking at...not everything is going to break down as quickly, so once you establish a cycle, keep it going, renewing the old. So many things about growing things are about experience. You can research the topic, but until you do it, you still have no more than a vague idea. And then you realize, the full comprehension of all the details isn't necessary...close enough is good enough. There is one question I'd like to ask...where does the original 5% number come from? What is it you are trying to do? |
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| 5% was just a reference level. I figured for a higher level I'd simply multiply the annual OM addition by the appropriate ratio. My last soil test came back with an OM of 7%, and I'm wondering what I need to add annualy to maintain it at that level. |
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- Posted by toxcrusadr (My Page) on Tue, Sep 11, 12 at 11:40
| josko, the %OM test does not count carbonates. If I remember my lab days correctly, what we used to do was add acid to drive off the carbonates as CO2, leaving all the organic matter behind. If one wanted to quantitate the mineral carbon, it was counted by running it through a CO2 analyzer. Then the sample is heated red hot with oxygen, which burns all the organic material to CO2 which is then quantitated by the instrument. That second number is what is reported as %OM. I've been out of the lab for awhile but I would bet this is still the basic method. |
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| If you're on Cape Cod, you need all the organic matter you can get. For those who don't know, Cape Cod is a sand pile with a sprinkling of pine needles on the top. My brother had great gardens in P-town and Wellfleet many years ago. He used a thick eelgrass mulch, constantly replaced, but build up the organic matter in the soil. I remember him breaking off parsnips at 36 inches. |
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- Posted by toxcrusadr (My Page) on Tue, Sep 11, 12 at 17:25
| What I wouldn't give to struggle with sand for awhile instead of clay! |
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| What %OM should I be shooting for given my Cape sandy soil? I was pretty happy to have it test at 7%. It's been built up over the years largely with eelgrass and compost. What's an ideal %OM for a very sandy vegetable garden? |
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- Posted by strobiculate none (My Page) on Wed, Sep 12, 12 at 8:48
| tox...given the choice, i'll take clay, every time. And when I run across an article that complains about clay, I never again read anything by that author. I do understand certain construction practices makes soil more difficult to work with...but I'll still take clay. There is no "ideal". 7% on coastal sand is an accomplishment. But for all the work you'be done, skip a couple years, and it will literally disappear. And there is not a formula of how much to add, because there are too many variables...how deep is the sandy topsoil, condition and depth of subsoil, amount of precipitation/irrigation, length and intensity of seasonal change, length and depth of ground freeze over winter, what kind of material you are adding and how quickly it breaks down. Vegetables. you have to know your soil, your weather, and your crop. are there certain crops that do well? are there certain crops you are having trouble with? A soil organic matter % over five is decent. you should be able to grow most things with good success. I tried an experiment with onions this year...1, no fertilizer. 2, monthly. 3, every two weeks at a low rate. No.3 returned the best results, by far. Garlic similar, and corn...i think you could overfertilize, but you'd have to make a concerted effort. One of the problems with sand is that sand is a chemically inert material. Lots of smooth edges, not a lot of free outer shell electrons to allow an exchange of minerals between the soil matrix and the roots. Organic matter helps wth that, adding a spongy material that can absorb water, has nnoks and crannies on the surface for air pockets, and lots of freely available outer shell electrons to allow plant roots to exchange. The bad news, sand doesn't have a lot of nutrients in it of it's own, that chemically inert thing. Not sure if this helps or not. |
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