Tuesday, October 30, 2018

Dealing with hydrophobic soil


Hydrophobic is defined as "afraid of water." When the term is used to describe soil, its meaning is modified to "repels water." This is of concern to us as gardeners and landscapers, of course, because if water is not being absorbed into the soil then it is not available to the roots of our plants.

This is particularly serious for newly sown seeds, which must have even moisture supplied by the soil in order to germinate and during their initial growth period when a scarcity of water can kill newly sprouted seeds. Also, newly-planted trees, shrubs, or herbaceous specimens are already stressed and in are extreme need of water to rehydrate their leaves so that photosynthesis can take place to provide as much energy as possible during their transition into new locations.

Water Science

Okay, let's back up a bit here to look at water's chemistry. Water is made up of two hydrogen atoms and one oxygen atom giving it the familiar chemical formula of H20. The hydrogen atoms attach themselves to one side of the oxygen, covering about 1/3 of a circle, and so that the molecules look very much like Mickey Mouse ears. The side of the molecule with the hydrogen atoms has a slight positive charge and the oxygen side is slightly negative. Water molecules act like little magnets and are attracted to each other and form weak bonds, called hydrogen bonds. You notice this self-attraction, called cohesion, when water beads up into droplets. And this is important for gardeners to know when we are dealing both plants and soil. 

For further details on how water and plants interact, read my article Water science and gardening.

How does soil become hydrophobic?

Since water clings to itself, if soil is moist, there will not be a problem with water soaking in because of all the water molecules that are already clinging to the particles of soil. But if the soil has dried out so much that very little water is clinging to soil particles, then the water will not readily soak in. The dryness could be due to a drought or by fire--either way when water is applied to the soil it may only wet the top 1/8th of an inch of soil or less and the rest will run off as shown in the top photo. Or in a container, the water will run right through the inside of the pot without wetting the soil.

In general, more organic material in the soil will reduce hydrophobic problems because organic materials absorb and hold more water. Adding compost to soil, whether it is sandy or clay will improve its texture, nutrients, and ability to better handle water.

Also, undisturbed soil will hold a lot more moisture than soil that has been disturbed from plowing or from pulling weeds. This is important to know for your general landscaping areas. Minimize disturbance when pulling weeds, or better yet, just cut them off to weaken them. Undisturbed soil is also better for the climate, because disturbed soil releases more carbon into the atmosphere. For crops in the home landscapes, it's best to use raised beds, with or without hard sides, so the underlying soil remains undisturbed. (See below for some ideas on how to arrange edible beds to reduce hydrophobic conditions.)

In commercial agriculture, the farmers often use a wetting agent or surfactant, which breaks down the surface tension of the water. These products are like a soap or detergent and do facilitate the absorption of the water into dry soil. While manufacturers say they are safe for soil organisms, my feeling is for home gardeners that there are other ways to deal with hydrophobic soil that may be less risky to soil critters or plants and are less expensive.

When planting trees, build a berm around
 the planting hole so water can't escape.

Overcoming hydrophobic soil when planting trees

The best way to plant trees that have been grown in containers is to rinse all the soil from their roots and not add any enrichment materials to the planting hole--a type of tough love so the tree roots adjust to your lousy soil right away. Also, you can see and unwind or break the coiling roots so they don't have a chance to choke the tree as it grows and spread the roots out in all directions. Make sure the center of your planting hole is firm so the the root flare is above the ground level.

This harsh treatment will break off all the root hairs—the only place where water is absorbed. They will grow back quickly, but you will need to completely soak the planting hole and mud in the soil so that it just barely covers the roots that you spread out. After you have the tree situated in its planting hole, build a berm of soil or soil and mulch all the way around the hole. Then water generously again after planting. The rule of thumb on how much water to use is that for each caliper inch (diameter of trunk at 6 inches above the root flare), use 6 to 9 gallons of water depending upon how dry or sandy the soil is. (A typical watering can is 3 gallons.) Then for each caliper inch use 3 gallons of water every day for at least 2 weeks except for days where you receive 1/2 inch of rain during this time.

You may add thin layer of organic mulch over the soil in the planting hole, but no mulch should touch the trunk. You may add a somewhat thicker layer of mulch outside the planting hole. The mulch will help keep the soil moist, which is what we want when a tree is adjusting itself to your landscape. There are various mulch products you could use, but the most sustainable is chipped wood from neighborhood tree cutters. Read my article A requiem for a hickory tree for more details on this type of mulch.

In summary, what we've done by using all that water in the tree planting, building the berm, daily irrigation, and the mulch is to soak the soil and keep it moist to offset the hydrophobic tendencies. The tree will probably need extra irrigation during dry periods for the next year or so. All of this gives the new tree a good start in your landscape.

Hydrophobic soil is bad for our crops

Unlike the more natural areas in our landscapes where we work to make our landscapes more drought tolerant, the best way to grow most edibles is to provide rich soil and to keep them well-irrigated from germination to maturity. The point of this strategy is to grow crops quickly and efficiently to extract the best harvest while the plants are healthy and before bugs, blights, or other problems reduce their vitality. As I've written before, once a crop begins to fail or struggle, in most cases, it's best to pull that crop and get that plant material out of the vegetable beds and any diseased or infest plants should not be composted. This and crop rotation will reduce re-infestation during the next season. Gardeners are like "The Gambler" and know when to "fold."

Preparing soil for planting when it's dry

The best soil amendment for dry soil is compost and other organic materials because the organics retain moisture. But when the weather is dry, even your compost piles may have become too dry.  So before adding your compost into the beds, wet it with rain barrel water a few times each day over several days. The reason for rain barrel water is that tap water is treated with Chlorine and antibiotics to kill microbes so that it's safe for us to drink, but it kills the soil microbes as well, and the whole structure of healthy soil and good rich compost includes millions of microbes. If all you have is tap water, at least let it sit for a day or two to give the volatile chemicals a chance to evaporate.

Take the time needed to thoroughly wet the soil before planting

After pulling up the previous crop and weeds from this raised bed, I scraped off the top few inches of soil and added chopped marigold stems and flowers to reduce nematodes in the bed*. Then I added 3 or 4 inches of compost and topped it with the soil I'd removed. While the compost and the marigolds were moist, the soil was still quite dry. I irrigated the bed until the whole top looked wet, waited a few hours and repeated the irrigation. the next day I irrigated several times until the water was able to soak into the soil. Since I wasn't ready to plant this bed and to give the marigolds time to decompose, I covered the whole bed with a good layer of pine needles to keep it moist and while I waited, I watered the bed most days.

*For information on marigolds & nematodes see: Results: The nematode experiment

2 wide rows look wet... But just under the surface it's dry. These rows are not ready for seeding. After another day of watering several times, the soil finally became thoroughly damp. 

Wide-row and swale methods should be arranged to reduce hydrophobic soil.
Crops need a consistent source of water applied
evenly so that the beds don't dry out.

When I wrote about how I use wide-row beds in my previous post, several people had questions and wanted more details. Dealing with irrigation and how to keep soil that is above ground-level moist is certainly a big part of this discussion.

I build the wide rows so they are not flat on the top but are somewhat indented, so water can collect in the planting surface. They are like very shallow swales so when I pre-water, I add enough water to fill the swale, but not overflow, then even just a few minutes later, I fill it up again and again.

When planted, most of the crops are then planted to either side of the rows, so the water can collect between them. Also, as mentioned before, the trenches between the rows can be wider if I'll be planting larger crop plants and narrower for smaller crops. This provides flexibility that is not found in square-foot gardening and here in Florida, it's important to have the trenches so that extra water from our heavy rains will have a safe place to go without disrupting the beds or the crops.


I use swales for squash family crops, for okra, and even for tomatoes and peppers. This method provides a low spot in the middle of the planting area so that when you are watering the plants, it is concentrated in that one area and no water runs away from the planting area. For most squash family plants the swale is quite large--maybe 2 or 3 feet in diameter. Then the seeds are planted on the rim of the swales. This arrangement makes it easy to keep the soil in the swale damp while leaving the rest of the garden area un-irrigated to keep down the weeds. In the above bed, I had created a small swale (about 10 inches in diameter) under a tomato cage for my fall cucumber crop.

For more information and history on these methods, see
Wide row planting and trench composting in the vegetable garden
6 reasons to use pine needle mulch in edible gardens
Okra swales

For further reference:
USDA's Water Repellent Soils: a state-of-the-art: a thorough analysis of the problem, published in 1981, but still relevant.

A shout out to Southern Exposure Seed Exchange

This is my new favorite source of seeds. They have both common and unusual seeds including heirlooms and when you buy here, you're supporting small family-owned farms. I purchased my Seminole pumpkin seeds here last year. Read about how much fun that crop has been in my post: End of the Seminole pumpkin season.
I love that the seed packets are wrapped with pages from their catalog. My seed purchases for the cool season included mostly lettuces including, "Drunken Woman: a loose-leaf lettuce. :-)
I hope you are taking care of your soil because it sequesters 4 times more carbon than all the terrestrial plants across the globe. Or as I've been talking about this topic on my #FloweredShirtTour, "We have to stop treating our soil like dirt!"

Green Gardening Matters!
Ginny Stibolt

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