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Healthy soil's water holding capacity  RSS feed

 
Jennifer Wadsworth
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Thought I would share this image from the Natural Resources Conservation Service

 
Topher Belknap
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Location: Midcoast Maine (zone 5b)
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While I appreciate the thought, it would have been nice if they had used compatible units. How thick does that increase in material need to be for example? Then I could figure out what the percentage of water would be in good soil, or material needed for a 10' x 30' garden plot. As it stands, all I know is that organic material increases water retention. Which I knew. Thanks for posting.

Thank You Kindly,
Topher
 
Jennifer Wadsworth
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Topher Belknap wrote:
While I appreciate the thought, it would have been nice if they had used compatible units. How thick does that increase in material need to be for example? Then I could figure out what the percentage of water would be in good soil, or material needed for a 10' x 30' garden plot. As it stands, all I know is that organic material increases water retention. Which I knew. Thanks for posting.

Thank You Kindly,
Topher


Let me see if I can help sort out some of the confusion.

First, they are talking about organic matter IN the soil, not on it - thus the "1%". I think this is what's causing you the confusion.

When one does a simple Jar Soil Test, it's easy to see the sand, silt, loam, colloidal materials and organic matter separate into layers. So, if you take a baseline soil test, then work on incorporating more organic matter INTO your soil, you can take another test at a later time to see how much you've increased your organic matter.

Figuring out how much organic matter you'll need for a 1% increase in your soil:

You can figure out how much organic matter to put into your soil by multiplying the square footage of your land by 2 ft depth (the area where most feeder roots are in soil).

So if I'm trying to increase the organic matter in a 10 x 10 garden area I get (10 x 10) x 2 = 200 cubic feet (or 7.4 cu yds) If I want to increase the organic matter 1%, I'd need 2 cu ft or 0.074 cu yards of organic matter.

Now to translate the acres into square feet to see how much that 2 cu. ft of organic matter worked into our 100 sq ft veggie garden nets us in water-holding capacity:

1 acre = 43,560 sq ft.

My 100 square ft garden is 100/43,560 = 0.0023 of an acre

If 1 acre can hold an additional 25,000 gallons of water with a 1% increase in soil organic matter, then my 100 sq ft garden has the potential to hold an additional: 25000/.0023 = 57.5 gallons of water.

Hope that helped.
 
R Scott
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Those are the units farmers are used to. And water holding is the the one that has value to them right now, with the droughts and limiting irrigation and increased cost to irrigate if you aren't restricted.

1 acre-inch = 27,154.285 gallons

So, an increase of 1% in organic matter will hold almost and extra inch of rain, regardless of the area--whether it is a 100 sq ft garden or 1000 acre farm.
 
Jennifer Wadsworth
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R Scott wrote:So, an increase of 1% in organic matter will hold almost and extra inch of rain, regardless of the area--whether it is a 100 sq ft garden or 1000 acre farm.


Yep - an inch of rain is 0.623 gallons. (or 62.3 gallons in my hypothetical 100 sq ft garden above).

I love the way you parsed that out - R Scott!

 
Topher Belknap
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Location: Midcoast Maine (zone 5b)
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The key to my complaint was no mention of depth. Talking about amounts of material in terms of square footage is nonsense. % * ft^2 does not equal ft^3. If there is some well-known thickness which everyone but me knows, so be it, but I rather think such would be climate, etc. dependent. Clearly, 1% of organics in the top 1 inch of soil only, is NOT going to hold 25,000 gallons of water, nor is 1% all the way down 100 feet likely to hold a mere 25,000 gallons. Once I have that number I can do the math. Is two feet definitely the number that the USDA used?

Thank You Kindly,
Topher
 
Jennifer Wadsworth
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Topher Belknap wrote:
The key to my complaint was no mention of depth. Talking about amounts of material in terms of square footage is nonsense. % * ft^2 does not equal ft^3. If there is some well-known thickness which everyone but me knows, so be it, but I rather think such would be climate, etc. dependent. Clearly, 1% of organics in the top 1 inch of soil only, is NOT going to hold 25,000 gallons of water, nor is 1% all the way down 100 feet likely to hold a mere 25,000 gallons. Once I have that number I can do the math. Is two feet definitely the number that the USDA used?

Thank You Kindly,
Topher


The 2 ft number is the one I've seen a host of folks use as it is the generally agreed upon depth of feeder roots of plants (not tap roots). As these folks are talking to farmers, this is probably the depth they're using.

Most of my experience with water and soil comes from the water harvesting field. When we do in-ground infiltration chambers, for example, we try to keep them in the top 24" of the soil profile as that's where the feeder roots are and that's where extra water is going to have the most effect.

You are certainly welcome to contact the NRCS/USDA to ask what measurement they used.
 
R Scott
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I think the USDA considers the top 8 inches (normal field tilling depth). http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_053264.pdf

You should figure the deeper than that, but it is a function of the root depth of the crop YOU want. I would love to get to 2 foot down, but that is a long process. Tilling stuff down that deep is counterproductive, you have to build it from the top down.
 
Jennifer Wadsworth
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R - thanks for that!
 
John Polk
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Since that poster was published by the USDA (a government agency established to help farmers increase their yields), it stands to reason that it is aimed towards the farmers. I believe that the intent was to show farmers the value of incorporating the 'waste' organic material into the soil, rather than mechanically raking it off and then discarding it.

Since the bulk of farmer's cash crops are annuals, that 8 inch tilling depth is where most farmers will be most interested in. With permanent crops, obviously it would be in our best interest to shoot for a much deeper root zone health.

Simple math will help decide how much is needed per square foot for any given target depth.

 
Cj Sloane
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The NRCS has been pushing no-till & cover cropping, which is good. Except... they encourage herbicide instead.

In practice, they want farmers to plant some cover crop in the fall, then spray the cover crop in the spring so they can plant their round-up ready corn or whatever. We talked about this at a seminar I went to in the spring. The only good work around was to crimp the fall cover crop which is not that easy.
 
R Scott
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Farmers are realizing the 8" is limiting yields (profits) so there is progress


If that isn't proof for keyline plowing, I don't know what is.

There is a catch 22, though, because as you increase soil biology it eats more. Gabe Brown (big cover crop no till farmer in ND) and Stefan Sobkowiak (Permaculture Orchard) have both said you can increase the biology to the point you can't maintain organic matter in the soil. As in most things, there needs to be balance.
 
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