Best way to keep container soils moist? (Willem)

Already published at my desertification blog on March 9, 2007

Best way to keep container soils moist?

March 9, 2007

Posted by willem van cotthem in fertilizer – nutrients, success stories – best practices, soil, water, technologies. trackback , edit post

Working for more than 20 years already with water absorbing polymers (also called “crystals” in gardening circles !) and having developed the soil conditioning method TerraCottem (see, I was very much intrigued when I encountered on the internet a discussion forum on “the best way to keep container soils moist“.

Let me take you through some nice and sometimes amusing contributions about several topics related to moist soils (!):


It is obvious that some plants need more water than others. So, no need to say that there is no general rule for watering container plants (flower pots, plastic or concrete tanks, etc). Each species should be treated according to its nature.

I couldn’t agree more.


Someone felt the need to underline the existence of different methods: watering can, drip irrigation, hose with shower wand, self-watering containers, wicks, battery timers, lawn irrigation controls, etc.

I kept looking for the answer on the question “Best way to keep container soils moist?“.


One of the contributors to the debate suggested “to decrease intervals between irrigations (water more often). It promotes gas exchange at the roots and implies that your soil is more open (better aeration) than soils that offer extended irrigation intervals“.

It seems obvious to me that shorter intervals between irrigations help to keep our containers moist. The more often you give water, the smaller the chance that the soil will be dry. It is less clear to me how this promotes gas exchange at the roots and creates a better aerated soil.


A number of participants in the discussion came up with problems and solutions for the use of a water hose to keep the container soil moist. Here are some of the “bright” ideas :

* I have seen connectors to attach garden hoses to standard kitchen/bathroom faucets. Perhaps this will ease the strain on the back.

* I had such a gadget and it was a life saver! There are many such devices on the market, much improved than what I used back then.

* A couple years ago I got a hose that came with a plastic adapter, tried it on my all of a 3 thread faucet in the kitchen! Right: didn’t work! Have been meaning to see if I can find a metal adapter.

* The longest one I saw had a reach of 60 feet. However, it would be very easy to make quite a bit longer with a few connectors.

* I bought an indoor watering hose to get water to my plant room from the bathroom across the hall. It wouldn’t tighten enough and sprayed water out the sides. After I took it off, the sink has never been right – still sprays a bit. So now I fill a 2 gallon watering can in the bathtub and run back and forth. Maybe it’s time to try another adapter, if I can find a different style.

* Since watering at frequent intervals (as needed) really IS the best method of maintaining soil moisture in the summer, tools such as this would certainly help make that task easier.

* If you are having a problem making an attachment to a kitchen or bathroom faucet check with the appliance parts distributors in your town. Portable dishwashers are routinely connected to an amazing extensive assortment of faucets.

* There are fertilizing systems which reside where the hose meets the bib. One might be able to use these systems to amend the town water.

Do I need to tell you that I was still looking for the best way to keep my container soil moistened ?


One observation looked rather peculiar : “If watering techniques are easily achieved, then well-draining soiless mixes would likely be more widely adopted“.

Sorry, but I am not working with soiless mixes. I only want to know which is the best way to keep my CONTAINER SOIL moist !


Someone said : “I use self-watering containers, a well-draining soiless mix, and hand carry water from the kitchen. It’s a lot of work, but it does the job“.

Big deal, but what was the question again? Yes, BEST WAY to keep CONTAINER SOIL moist !


Now we come closer ! A couple of contributions gave me a hint :

* “Lately I prefer using water absorbing mats in the bottom of the planters. Then I realized the fabric was very similar or the same as capillary matting, which I had a roll of, so I cut that into a piece the size of the bottom of the container and put it in the bottom before I add the soil“.

* “Additionally, with your prior comments on PWT, how does that affect a container with capillary mats around the inner circumference of the container but not used on the bottom?“.

Here I decided to surf later on the internet again, looking for information on water absorbing mats. Maybe a solution ? I will keep you informed.


One participant simply recommended : “To add : water gel crystals/polymers or perlite or ? “.

I will not continue my research in that direction, because I know the advantages of perlite, vermiculite and the like, but last and maybe not least some contributors to the debate came up with ideas on :


Here we are ! The water absorbing (water stocking) hydrogels, belonging to many different groups of chemical (organic) substances, known since the sixties, but only largely applied to grow plants since the eighties. Let us first have a quick look at the sometimes remarkable contributions:

* I used to use the potting soil with the polymers.
* With my situation, not able to water often, I used the crystals. I was very happy to have them. For me it made a big difference.
* I’d add more perlite or crystals.
* Some of the “extra-absorbent” characteristics mentioned by manufacturers of polymers are exaggerated, and as biodegradation occurs these polymers actually reverse their effect and hold moisture so tightly it is unavailable to plants.
* Soils can usually be designed, so forest products (bark), peat and other organic media components that adequately hold moisture can be used with no ill effects. These products, even in containers, provide the plant(s) some nutrient value and fodder for the micro-organisms that polymers inhibit. Some degraded polymer components even have some of the same effects on mammals as female hormones, which can affect mammalian fertility and potency. Additionally, the polyacrylamides in some garden-grade moisture holding polymers are made from (and contain) the monomer acrylamide, a known carcinogen.
* I learned on here a few years back that baby diapers, good soil and fertilizer work very good. If I want my plants to live, I use the diapers.
* How do you use the diapers, exactly? I happen to have an abundance of those this year, and less free time to water.
* I have read about using unused diapers to help with reducing watering and have tried a similar product and = ho-hum and a yawn.

The debate was closed with :

* OK – no more polymers!


Being a biologist (botanist) myself , there is some chance that I know something about plant growth and growing plants. Well, having studied the characteristics of those extremely different “water crystals” (like the laymen call them), I decided in 1983 to start research work on application possibilities of some of these polymers in agriculture, horticulture and forestry. In fact I was looking for a simple method to grow food for rural people in the drylands, where regularly harvests fail and hunger shows up.

I’ll be brief now ! With my team at the University of Ghent, Belgium, we developed a compound of granules and powders, called TerraCottem (see This granular mixture can easily be mixed with potting soil (5 g per kg) or with the top layers of the soil (30 cm / 1 foot), where it achieves the following tasks:

Comparative test of plant growth with a minimal volume of water: Left, fern growing in 1 kg of potting soil with 5 g of TerraCottem (TC) – Center, the control plant : fern only in potting soil (no TC) – Right, fern in potting soil with mineral fertilizer. All plants were only watered with the same quantity of water when the control plant started wiltening. Remarkable difference, don’t you think ?

* It stocks water in the selected hydrogels (not toxic at all, I even eat them in small quantities of course already for more than 20 years now, and I am still in good condition for a 73 year old guy without “the same effects on mammals as female hormones, which can affect mammalian fertility and potency“!). It keeps the soil (also the one in containers) moist for a longer time (GOT IT ?).

* It fertilizes the soil with organic and mineral nutrients (macro- and micro-nutrients).

* It enhances microbiological activities in the soil.

* It stimulates root growth.

* It aerates the soil.

Sorry guys (and girls!), but if you are still looking for the best way to keep container soils moist, why don’t you try TerraCottem, just once?. It has many application fields and we use it all over the world on our humanitarian projects (see that nice example of the UNICEF project in Algeria elsewhere on this blog). I wish you a lot of success and less pain in the back.


Published by

Willem Van Cotthem

Honorary Professor of Botany, University of Ghent (Belgium). Scientific Consultant for Desertification and Sustainable Development.

3 thoughts on “Best way to keep container soils moist? (Willem)”

  1. So I am trying to compare TerraCottem to other polymers.

    The estrogen like compounds in the environment are a concern.

    Can you please point me to where exactly you explain the difference in yours and others?

  2. Where could i buy some Terra Cottem/ I look at the original website, but there’s no information where to purchase this product.
    Thanks a lot

  3. Pingback: Aeroponic System

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