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Kimmsr and others, what % of OM for my silty loam

Posted by kristimama SF East Bay Zn 9 (My Page) on
Thu, Jan 26, 12 at 19:13

Hi Kimmsr and others, so I wanted to ask specifically, now that I've done Kimmsr's soil in a jar test... what I should do now?

When the soil settled, I ended up with what looks like
50% ish sand
45% ish silt
1-2% organic material
a nominal, very thin film of clay (less than 1%)

The statistic of an ideal 6-8% OM is for a sandy loam and this obviously the *ideal*, but I don't think what I have fits into the definition of "Sandy Loam." It's more like, silty loam, or silt/sand mixture?

Are the recommendations for the % of needed OM adjusted when there is less clay, as mine is?

The bottom line is this, I need to fill these boxes by about 10 inches. To get the the *ideal* of 6-8%, that's only an inch or 2 of compost.

If I add 10 inches of compost, I'm clearly going to exceed the 6-8%.

I am assuming that if I add the 10" of OM, it'll eventually just shrink back down again.

And just to clarify, this was premixed soil that I brought into my yard from a local company calling it a mix of "sandy loam and compost"... I think more accurately it should have been called sand and silt, as there's not much clay in it to call it accurately a "loam." But, oh well. Lesson learned.

What should I do now?


Follow-Up Postings:

RE: Kimmsr and others, what % of OM for my silty loam

Over the years as I have read I have found most all of our Agricultural Schools have been saying soils need 5 percent organic matter. Conversations with many Soil Scientists tell me that comes mostly from knowing that a good loam contains about 5 percent organic matter. In the various soils I have worked in in many parts of the USA and in conversations with organic gardeners all over he world I have reached the conclusion that the 6 to 8 percent level of organic matter is optimal. In the jar test that equates to about 1/4 inch of organic matter floating on top, when the amount of soil put in the jar is 4 inches.
I have not seen that amount change in clay soils, loam soils, or sandy soils, and any organic matter you add will, depending on how active a Soil Food Web you have, "disappear", be digested by that Soil Food Web and will need to be replaced quite often. Since that organic matter is what feeds the Soil Food Web and the Soil Food Web is what feeds your plants that is just a necessary part of gardening.
How much organic matter is needed? Depends on many things, but usually starting with a bit too much is probably better then starting with not enough. The organic matter fills in the voids in your sandy loam which can raise the level of that soil, but probably will not raise the level of the bed 10 inches, 2 inches of compost will probably only raise the level of that bed about 2 inches.

Here is a link that might be useful: The Soil Food Web Primer

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