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Can you have too much organic matter in your soil?

Soil high in organic matterMy findings that adding lots of organic matter actually helped equalize cation levels in the forest garden leads me into a discussion of the most controversial part of Steve Solomon's book.  He agrees with the organic status quo to a certain point, recommending that we add organic matter until our soil contains 4% organic matter in the south and 7% in the north, and until our CEC is at least 7, levels that are achieved through topdressing half an inch of compost per year.  However, Solomon argues that after you reach your organic matter goal, you should back off to topdressing with only a quarter inch of compost per year since adding too much organic matter unbalances your soil.

Organic matter is closely linked to nitrogen, and that was my first concern when I considered Solomon's low-compost policy.  Each 1% organic matter in your soil releases about 15 to 25 pounds of nitrogen per acre per year, and high-demand vegetables need around 200 pounds per acre.  If you run your garden hard, like I do, you might end up growing two low-demand crops and one high-demand crop each year, for a total requirement of 400 pounds of nitrogen per acre, but a more average yearly demand would be 200 to 300 pounds.

Time for some math.  The 8.95% organic matter already in my front garden (which is my best soil) would release 179 pounds of nitrogen per acre annually, but I need another 100 pounds or so.  How do I make up the nitrogen deficit if not with compost?  Solomon uses purchased additions of seed meals, feather meal, and fish meal to bring his nitrogen levels up to par.
Tables

Location %OM pH Ca % sat Mg % sat K % sat
Powerline pasture 2.74 5.1 29.23 13.57 6.96
CP5 4.27 5.9 48.26 16.23 10.63
Mule garden 4.87 6.9 70.58 17.94 5.06
Forest aisles 5.43 6.3 52.66 17.55 12.25
CP3 and CP4 5.48 5.9 52.56 14.77 7.96
Front berries 6.53 7.1 66.25 20.12 8.7
Back garden 7.82 7.1 69.78 19.35 5.92
Blueberries 8.49 6.5 61.16 19.04 6.6
Front garden 8.95 7.3 68.47 19.92 7.08
Forest garden 9.95 7.2 59.45 24.96 10.78


Soil samplesWhile I'm not adamantly opposed to buying nitrogen amendments if it would make my garden healthier, my soil tests bring that assertion into question.  In the table above, higher organic matter levels seem to match up with sweeter soil and with a better ratio of cations.  The exception is the mule garden, which I think must have had the organic matter levels mismeasured this year.  My previous year's results showed the mule garden as having the highest organic matter levels, and my eyes back up that claim.

Granted, there is a limit to how much nitrogen vegetables can use each year, but I estimate you'd have to raise your soil up to 20% organic matter before nitrogen would start washing out into the groundwater.  I think I'll stick to my Rodale approach to nitrogen and organic matter until someone presents a stronger argument for seed meals.

Trailersteading is a look at how several homesteaders used old mobile homes to achieve their dreams.



This post is part of our The Intelligent Gardener lunchtime series.  Read all of the entries:




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I can see seed meal fitting in if you are also growing grains, but I don't think buying input is the answer to a lot of these questions. I could see the power-line pasture playing double duty producing straw and seed meal.

It's too bad hemp is illegal, it could fill a number of needs-deep bedding for the chickens, soil improvement, mulch, seed meal, food, and fire starting material, just to name a few!

Comment by kevin Tue Jan 29 12:22:51 2013
If you have a meat processing plant nearby, bone meal is a good nitrogen supplement. I used to get pulverized bone meal from Valleydale for $200 a pick up load.
Comment by Errol Tue Jan 29 13:42:07 2013

If compost is mature enough, then I'm not sure one can put too much in the soil. Mature compost is, afterall, essentially humus. Immature compost would still contain a good deal of cellulose which could adversely affect the soil structure.

As you know, any organic source of N (amino acids, nucleic acids or urea) has to be digested by soil microbes and turned into the nitrates that are absorbed by the growing plants. Other microbes can take ammonia and make nitrates. One could dilute store-bought "ammonia" and apply it periodically. Although a good deal of this is lost as it out-gasses, it's a pretty cheap source of N. Jerry Baker's home made fertilizer utilizes this source, adding molasses, beer and dish soap, but I don't think those are really necessary.

Comment by doc Tue Jan 29 20:19:14 2013
N
Part of Solomon's love affair with seed meal relates to climate -- it juices up cold soils in the shoulder seasons when soil activity and therefore N availability is low, thus kick-starting early plant growth. Lord knows we need that in the PNW, so I think I might try it, but it appears that your growing season is more optimal.
Comment by Jackie Tue Jan 29 23:05:33 2013

Lots of fascinating comments on this post (and on Monday's, which I got too busy to answer....)

I think that Jackie is spot on that Solomon is a fan of seed meal because of dealing with cold weather. On the other hand, it seems like there are homegrown solutions to low soil availability in cold weather --- like diluted urine.

Comment by anna Wed Jan 30 07:48:52 2013