Hydrophobic Soil

Hydrophobic Soil

I stood at my kitchen sink last Tuesday, pouring water into my pothos. The water just sat on top for a second, then rushed down the sides of the pot like it was late for an appointment. None of it actually went into the soil. I lifted the pot and sure enough, it was still light as a feather. This is hydrophobic soil, and if you are dealing with it right now, I feel your pain.

When soil gets too dry for too long, it actually starts to repel water instead of absorbing it. I know that sounds backwards. Soil is supposed to hold water, right? But organic matter in potting mix (like peat moss or coco coir) develops a waxy coating when it dries out completely. This coating makes the particles water-resistant. So instead of your plant getting a drink, the water just takes the path of least resistance and drains straight out the bottom, leaving your plant as thirsty as before.

I have killed exactly three plants this way before I figured out what was happening. I thought I was watering them. The pot got wet, water came out the drainage holes, so I assumed everything was fine. Meanwhile, the root ball stayed bone dry and my plants slowly died. Not my proudest moments as a plant parent.

Why water runs straight through

The science behind hydrophobic soil is pretty straightforward once you understand it. When organic materials like peat moss, coco coir, or decomposed bark dry out completely, their structure changes at a molecular level. The surfaces become coated with waxy organic compounds that naturally occur in these materials. Water has high surface tension, which means water molecules stick together pretty strongly. When water hits these waxy surfaces, it beads up and rolls away instead of spreading out and soaking in.

Think about how water behaves on a freshly waxed car. That is essentially what is happening in your pot. According to research from the University of California Agriculture and Natural Resources, peat-based growing media can become severely water-repellent when the moisture content drops below 20 to 30 percent by volume. Once it hits that threshold, normal watering just does not work anymore.

You can actually test if your soil is hydrophobic pretty easily. Pour a small amount of water on the surface and watch what happens. If the water beads up and sits there, or if it immediately runs to the edges and drains out without soaking in, you have got hydrophobic soil. If the water gradually sinks into the surface and darkens the soil, you are fine.

The problem gets worse the longer your soil stays dry. I went on vacation for two weeks once and came home to find that my snake plant (which I thought was indestructible) had soil so hydrophobic that water literally pooled on top for several seconds before finding a crack to escape through. The root ball was dry as a bone despite me thoroughly “watering” it when I got back.

Container plants are especially prone to this because the soil dries out from all sides, not just the top. In the ground, soil might dry at the surface but usually stays somewhat moist deeper down. In a pot, the whole thing can dry out evenly, and then the whole thing becomes water-repellent at once.

Bottom watering fix

Bottom watering is my go-to solution when I catch hydrophobic soil early. This method forces the soil to absorb water from below, which works better than trying to pour water on top of a surface that is actively repelling it.

Here is how I do it. I fill my sink, bathtub, or a large container with a few inches of room-temperature water. Then I set the pot in the water so that it comes up about halfway to two-thirds of the way up the side of the pot. You need drainage holes for this to work, obviously.

The water will slowly start to wick up through the soil from the bottom. This takes time. I am talking 30 minutes to an hour for most plants, sometimes longer if the soil is really dry. I usually check progress by touching the top of the soil. Once it feels damp, I know the water has made it all the way through.

With really stubborn cases, I have left plants sitting in water for up to two hours. I know everyone says never let your plants sit in water, and normally I agree, but this is a special circumstance. You are trying to rehydrate completely dried out soil, not maintain healthy moisture levels. The University of Maryland Extension actually recommends this soaking method specifically for rehydrating dried root balls.

One trick I learned from trial and error is to add a tiny drop of dish soap to the soaking water. I am talking one or two drops in a whole sink of water. The soap acts as a surfactant, which breaks down the surface tension of water and helps it penetrate the waxy coating on the soil particles. Do not overdo the soap though. Too much can damage roots or leave residue in your soil.

After the soil has absorbed water from the bottom, I lift the pot out and let it drain completely. Do not leave it sitting in water after the soil is rehydrated. Once the excess water has drained away, the soil should stay moist for a normal amount of time. The hydrophobic properties usually go away once the soil is properly wet again.

I use bottom watering as a quick fix about once every couple of months when I have neglected something. It works great for succulents and cacti that I forget about because I am trying to underwater them, then accidentally take it too far.

Above: A close up look at the symptoms.

Refreshing the soil

Sometimes bottom watering is not enough. If your soil has been hydrophobic for a long time, or if the potting mix is old and breaking down, you might need to actually replace it. I know repotting is annoying, especially if the plant is huge or if you have a lot of plants to deal with. But old soil does not just repel water, it also gets compacted and runs out of nutrients.

I try to refresh soil every 18 to 24 months for most of my tropical plants. Succulents can go longer because their soil does not break down as fast. But if you see your plant struggling even after you fix the hydrophobic issue, the soil might just be done.

When I repot, I shake off as much of the old soil as I can without damaging the roots. Some people rinse the roots under water to get all the old mix off, but I find that too harsh for most plants. I just gently work the old soil away with my fingers and then pot up in fresh mix.

Choosing the right new soil makes a difference. I avoid mixes that are pure peat moss because peat is the worst offender for becoming hydrophobic. Instead, I look for blends that include perlite, bark, and coco coir. These components help maintain air pockets and drainage, and they do not get quite as water-repellent when they dry out. Cornell University research has shown that adding coarse materials like bark or perlite to potting mixes helps reduce water repellency issues.

After repotting in fresh soil, I water the plant thoroughly right away. This helps settle the new soil around the roots and makes sure everything starts off properly hydrated. Fresh potting mix usually absorbs water pretty well, so you should not have any issues with that first watering.

Using wetting agents

Wetting agents are products designed specifically to help water penetrate hydrophobic soil. They are basically concentrated surfactants, similar to soap but formulated for use on plants. I was skeptical about these when I first heard about them, but they actually work.

You can buy commercial wetting agents at most garden centers. Brands like Soil Moist or Hydretain are popular. You mix them with water according to the package directions (usually a few drops per gallon) and then water your plants normally with the solution. The wetting agent breaks down the surface tension and helps water soak into the soil instead of running off.

I have used these products on a few of my larger potted plants that were too heavy to repot easily. My big fiddle leaf fig got hydrophobic soil last summer, and I was not about to wrestle that thing out of its pot. A couple of waterings with a wetting agent solution fixed the problem completely.

The cheaper alternative is the dish soap method I mentioned earlier, but using it as a top watering solution instead of just for bottom watering. Mix one or two drops of mild dish soap (I use unscented) into a gallon of water and water your plant with that. It works on the same principle as commercial wetting agents.

One warning though: do not use wetting agents or soap water every time you water. Once the soil is rehydrated and accepting water normally again, go back to plain water. Constant use of surfactants can build up in the soil and potentially cause issues with salt accumulation or affect the soil biology. I use them as a treatment, not a regular practice.

Some potting soil manufacturers now add wetting agents directly to their mixes. If you struggle with hydrophobic soil a lot, look for products that mention this on the bag. It can help prevent the issue from developing in the first place.

Above: The tools you need to fix this.

Preventing dry soil

Honestly, the best way to deal with hydrophobic soil is to never let it get that dry in the first place. This sounds obvious, but it requires actually paying attention to your plants instead of watering on a random schedule.

I check my plants every few days by sticking my finger into the soil. If the top inch or two is dry, most of my tropical plants are ready for water. For succulents and cacti, I let the soil dry out completely, but then I water them before it has been dry for weeks on end. There is a difference between “dry” and “desert dry.”

Setting reminders on my phone helped me a lot when I first started keeping plants. I am not naturally an organized person, and I would go weeks without even looking at some of my plants. Now I have a recurring reminder every Sunday to check everyone. I do not necessarily water everyone every Sunday, but I at least look at them and stick my finger in the soil.

Mulching the top of the soil can help slow down moisture loss. I use a thin layer of orchid bark or moss on top of the potting mix for some of my plants. This is especially helpful for plants in terracotta pots, which dry out faster than plastic. The mulch layer reduces evaporation from the soil surface.

Grouping plants together also helps maintain humidity and slow moisture loss. I have most of my plants clustered in one corner of my living room. They create their own little microclimate where the air stays a bit more humid than the rest of the apartment. This means the soil does not dry out quite as fast.

For plants that I know I will forget about, I use self-watering pots. I have my snake plants and ZZ plants in these because I am terrible at remembering to water them. The reservoir at the bottom keeps the soil from ever getting completely bone dry, which prevents the hydrophobic soil issue entirely.

Pay attention to your environment too. My apartment gets incredibly dry in winter when the heat is running. I have to water more frequently during those months because the soil dries out faster. In summer, with higher humidity, I can go longer between waterings. Adjust your habits with the seasons instead of sticking to a rigid schedule.

If you travel a lot or know you will be away from your plants, water them well before you leave and consider moving them away from bright windows where they will dry out faster. I also ask my neighbor to check on mine if I am gone for more than a week. She does not really know plants, but I just tell her to water anything that feels dry when she sticks her finger in the soil. Works well enough.

The bottom line is that hydrophobic soil happens when we let our plants get too dry for too long. Stay on top of your watering routine, and you will avoid the whole problem. But if it does happen, do not panic. Bottom watering, fresh soil, or wetting agents will get you back on track. I have brought plenty of plants back from the edge of hydrophobic disaster, and if I can do it, you can too.

References

University of California Agriculture and Natural Resources. “Managing Substrates for Optimal Plant Growth.” UC Davis Department of Plant Sciences.

University of Maryland Extension. “Rehydrating Dried Container Plants.” Home and Garden Information Center.

Cornell University. “Water Quality and the Home Landscape.” Department of Horticulture, Cornell Cooperative Extension.