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Soil repelling water

Posted by beets_me USDA 10 / SS 23 (My Page) on
Thu, Apr 25, 13 at 1:04

What makes soil repel water and how does one go about fixing it?

I'll apply the same amount of water at one end of a level border bed as at the other, but where the soil in one place becomes saturated, the soil in the other only looks wet on the surface while it's actually bone dry just under the surface. The only thing I know about the area is that it was tilled several years ago and at that time was mulched with compost. A gal at the garden center suggested adding peat or coconut coir but I'm hesitant to do anything until I know more about what is going on.

Any info or leads to info you can provide would be greatly appreciated.


Follow-Up Postings:

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RE: Soil repelling water

It's called hydrophobic soil. On bare soil, it's pretty common. It's caused by a water-repellent organic coating on and around soil particles at the surface. The coating can be formed in a number of natural ways - char from forest fires, decayed organic matter in the soil, and exuded by soil-borne micro-organisms just to name a few. It can also be caused by pollution, but if that were the case you would probably already know it. My SWAG would be that the soil has been bare for a few months, and the compost that had been used as mulch got to the just the right stage of decomposition to leave behind a water-repellent coating.

To fix it focus on breaking up the coating and keeping the coated particles dispersed, and preventing the surface soil from completely drying out. Cultivation of the top few inches (don't damage any plant roots) followed by an organic mulch on top will usually fix it, and adding some new compost or stable organic material (peat moss) in the top few inches of soil will help too.


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RE: Soil repelling water

Soils very often will create a barrier to aid in conserving moisture and that barrier will often cause water applied to run off, for a short time. One reason that soils should have mulch applied.


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RE: Soil repelling water

Are you on FL? The sand there is famously non-wetting.


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RE: Soil repelling water

  • Posted by ericwi Dane County WI (My Page) on
    Thu, Apr 25, 13 at 10:00

If you put down some mulch, about three inches deep, and water through the mulch, the problem will go away. I have done this with shredded tree leaves, and previously with shredded cypress bark. They both work well.


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RE: Soil repelling water

Breathe holes? anybody?
Since you said you watered the beds I'd assumed that there's already something on them which makes mulching a bit harder and in some cases impossible.


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RE: Soil repelling water

  • Posted by nil13 z21 Mt. Washington L (My Page) on
    Thu, Apr 25, 13 at 11:11

I'm guessing southern california, sandy soil with low organic matter. If so, the addition of organic matter and mulching will help with hydrophobic soil.

i am impressed that you asked here before adding stuff willy nilly. bravo.

I think you should perform a soil structure test where you put a sample in a jar of water, shake, and measure the layers. Also, I recommend sending a sample to be tested. I use UMass because they are cheap. $10 base test and $5 each for organic matter and salinity.

Here is a link that might be useful: how to determine soil structure


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RE: Soil repelling water

If beet is in SoCal it's most likely sandy soil with low OM and a very fine silt content "hiding" in the sandy profile.

Sand is often associated with "pour through"...which would lead one to believe the water would go straight through...but some sands (like the notorious "sugar sands" the poster from FL mentioned) will disperse the water laterally. There's a fine silt fraction all around the majority sand component which makes the soils act in that manner. Basically, the silt can situate itself in a semi-dense layer in the sand.

It takes a good amount of OM to counter this and the crux of it in SoCal and similar arid areas is the wind/drying of surface soils lessening the effectiveness and lasting power of added OM.

Breaking up the soil and digging/tilling/scraping in OM and mulching on top of the addition goes a long way in helping this.


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RE: Soil repelling water

In the florida sands as well a thick mulch eliminates the problem, works well around mature plants. When trying to germinate seeds with frequent waterings is the main problem.


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RE: Soil repelling water

  • Posted by TXEB 9a (My Page) on
    Thu, Apr 25, 13 at 16:57

"Sugar Sand" video - link below

Here is a link that might be useful: Sugar Sand in FL


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RE: Soil repelling water

  • Posted by nil13 z21 Mt. Washington L (My Page) on
    Thu, Apr 25, 13 at 17:10

so it's the fine silt that is the problem, nc? very interesting. makes sense. The sand itself also seems very fine.

do you happen to have any resources pertaining to that soil type.


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RE: Soil repelling water

  • Posted by TXEB 9a (My Page) on
    Thu, Apr 25, 13 at 17:10

nc-crn - a question ... any thoughts about using low density inorganic materials, such as vermiculite or perlite, to disrupt the surface continuity of the silt?


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RE: Soil repelling water

  • Posted by nil13 z21 Mt. Washington L (My Page) on
    Thu, Apr 25, 13 at 17:19

tx, the soilin that video looks awfully familiar. My experience has been that perlite doesn't help. it just floats around like the fine dry material that won't take up water. Calcined clay seems to help though.


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RE: Soil repelling water

"any thoughts about using low density inorganic materials, such as vermiculite or perlite, to disrupt the surface continuity of the silt?"

Technically, it can work to an extent, but I don't like adding things to non-container soils that can compact and don't have nutrient holding capability. As an aside, as long as there's soil moisture the porous nature of vermiculite/perlite can hold water saturated nutrients, but it's harder to control that as soil volume gets larger. Some soils can get away with it, but I avoid it as personal preference.

Another issue is the size of the verm/per and it's ability to displace/disrupt the distribution of the fine silt in the sand. It seems more of a crapshoot vs OM which can more easily integrate itself into the soil while it breaks down.

I fear the amount of verm/per needed to solve this kind of issue would be no better compared to an OM addition + mulching.


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RE: Soil repelling water

  • Posted by TXEB 9a (My Page) on
    Thu, Apr 25, 13 at 17:46

Makes sense, thanks. I never thought about the silt issue before. I've experienced the organic coating problem, but not high silt soil.

I'm a chemist, but not well versed in the vast complexities of soils. The issue of silt and sugar sand has me thinking surface tension all over again. In that sense, decomposed or decomposing OM makes far more sense as a way of potentially moderating surface tension via subtle surfactant effects.

One of my main long-standing interests in chemistry applications is in the culinary arts. Drawing a parallel, I think of silt as rock flour. If you want to see another surface that can be hard to wet look at ordinary fine milled white flour. Put it in a bowl, pour water on top and watch it sit in a pool. It is amazingly resistant to wetting.


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RE: Soil repelling water

beets_me has USDA 10 / SS 23 and on the "My Page" lists southwest gardening as of interest which kind of directs me away form Florida.


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Seems likely the OP is not in fl.

However, regarding florida sands and food crops, mulching is critical - more so than in other soils and climates. Always having the ground covered goes a long way to solving the excessive dryness. I have been able to grow crops with zero irrigation, which is fairly unheard of, at least since pioneer times. Mulch does not solve the problem of severe lack of nutrition, unfortunately.


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RE: Soil repelling water

Wow! So many interesting responses. Thanks to each of you for taking the time to pass along your knowledge and advice.

The soil structure test results indicated that the soil in question is 72% sand and only 8% clay, with 20% silt and very little organic matter. The structure test directions suggested digging a foot down into the soil, which happened to be about where the soil got really thick and moist--it was so thick that it tended to roll into balls rather than break up when I rubbed it between my hands.

Being just outside of San Diego, California in an inland river valley, perhaps all that makes sense, but I was more than a little surprised that our soil doesn’t contain more clay. When dry it cracks like clay. When wet it balls up like clay.

Anyway, if you don’t mind a couple more questions..

Since all the plants going into this bed are southwest and California natives that prefer gritty, fast draining soil that isn’t very rich, I’m going back and forth here on whether to use peat, compost, or a mix of the two. Maybe some top soil? What would you suggest?

Also, as while amending the hydrophobic soil, should I dig down into that really thick soil to blend the sand and amendments with the clay or leave that sub-layer alone?


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With native plants you're going to want to keep your OM additions to a minimum. When roots get "too rich" of an area to grow in, especially for desert natives which tend to "reach" broadly for nutrients, they tend to concentrate root growth heavily in the amended area rather than reaching out. This can cause stability issues, especially on taller growing plants, making them prone to being uprooted during heavy rain/wind storms.

As far as amendments go for natives...you're probably best asking on another board that fits your climate or a local landscaper/nursery/etc. (or someone else here that's done it). Either way, you're not going to want to use a lot of very rich amendments...topsoil might be a wiser choice (and cheaper). While compost would help the area you don't want to make it so rich that the roots don't establish themselves well.

You're going to want to dig down and amend into or otherwise disturb/break-up that existing water-stopping layer. The silt has situated itself tightly around the existing sand.


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RE: Soil repelling water

  • Posted by nil13 z21 Mt. Washington L (My Page) on
    Mon, Apr 29, 13 at 15:13

If you are talking about a native perennial bed, I just mulch fairly heavily depending on what you are trying to grow (some plants won't even tolerate that but you are likely to not be choosing those as nurseries typically carry natives that can handle garden conditions). I don't believe in adding too much to perennial landscapes, just pick plants that like what you've got. I would just contact a local arborist/tree trimmer and have them drop off their chipped trimmings. Now a vegetable or flower bed is another story altogether. You would need to add OM for that.


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