How Does Moisture Control Potting Soils Work?

The amount of water accessible to a plant largely depends on two key elements: the Available Water Holding Capacity (AWC) and Water Retention (WR) of the selected growing medium. WR indicates the quantity of moisture in the soil that a plant can utilize at any given moment. In contrast, AWC denotes the total potential for water availability to the crop, varying between the wilting point and the field capacity.

Moisture control potting soil combines increased surface area and a sponge effect for improved water retention. By maximizing these potentials, one can maintain optimal moisture levels.

The world is facing a growing demand for improved resource management. The factors that most tangibly affect us are temperatures and available potable water. Glen Frey’s “The heat is on” comes to mind. Potting soil that constantly provides the plant with the required water would be fabulous. Let’s see how we can do that.

The Role of Water in Gardening

Water is a remarkable substance. Water is the base of the metric system – length, weight, and temperature. A hundred cubic centimeters (10cm x 10cm) of water weighs one kilogram; Water freezes at zero degrees Celcius and boils at 100 degrees Celcius, etcetera.

Picture of Seedling in Moist Soil
Moisture and Growth

Every living organism requires water to survive. In live cells, it serves as a solvent, a temperature buffer, and a metabolite. Plants and microbes, unlike animals, rely on their immediate surroundings for water.

Microbes that live in or on land plants get water from the plant and water vapor from the atmosphere, whereas land plants get their moisture primarily from the soil.

Key Parameters Of Soil Moisture

The difference between the sample wet weight and dry soil weight is the amount of water. In 2010, the World Meteorological Organization added Soil Moisture to 50 Essential Climatic Variables recommended for systematic observation.

Soil moisture content measures how much water is in a given amount of soil; it can be expressed as a percentage, water by weight or volume, or inches of water per foot of soil.

Soil moisture potential or tension is the degree to which water clings to the soil and is measured and expressed in bars, which are pressure units. The more water a soil can absorb, the drier it is generally.

The amount of water in the soil available to the plant at any one time is known as plant-available water (PAW). The difference between the maximum quantity of water that the ground can contain and the wilting point when the plant can no longer absorb moisture from the soil is known as available water, measured in inches of water available per foot of soil.

The relationship between content and potential is not universal, as it depends on local soil qualities like density and texture. The gardener can devise an irrigation plan based on the PAW value.

Capillary Action

Have you ever wondered how water affects a plant? You can experiment with your kids to explain capillary action. Take a stick of partially wilted celery – it’s wilting because it’s thirsty.

Take a jug of water and add some red food coloring. Pop the celery stick in the colored water and take a photo every 2 minutes.

The product will be a sequence of pictures that show the migrating red dye to the tip of the leaves – capillary action in action. Red celery, anyone?

How much water do my plants need?

Sufficient soil moisture levels are essential for proper plant formation and crop yields. Water is an agent of moisture restoration and a temperature regulator for the plant.

The plant evaporates up to 99% of all water obtained in thermoregulation. Plants only use 0.2% to 0.5% for the composition of vegetative mass.

Plants have differing moisture needs depending on the weather conditions, requirements, and growth stages.

Cacti or many succulent plants don’t need as much water as tomatoes. There are two types of succulents; those from arid regions and those native to the Alps.

The Alpine succulents get watered occasionally and are on rocky soil where water drains off fast, so their requirements differ. And so it is with each plant. Their individual needs are unique.

Picture of close up of watering seedling by water running off the hand

Getting moisture levels right

The amount of moisture in the soil has an impact on nitrogen cycling. In the root zone, dry soil does not stimulate root expansion. Plants are unable to acquire appropriate nutrition because water delivers nutrients.

Water-saturated soils have an impact on nutrient transformation. In wet environments, phosphorus becomes more mobile and less bound to minerals.

Denitrification occurs when nitrate nitrogen is converted from a liquid to a gas and released into the atmosphere. Roots both ingest and exhale carbon dioxide.

Some gases, like carbon dioxide, can build in the soil and be poisonous to roots because gases are carried significantly more slowly via water (approximately one ten-thousandth slower than air).

A one percent increase in soil organic matter can help the soil retain an additional 20,000 gallons of water per acre that can be banked and become available when plants need it most.


The Benefits of Fluctuating Moisture Levels

An abundance of water availability does not always benefit plants. Research has shown that water availability is also crucial for pathogenesis – the development of diseases. Often, the effect of high rainfall or humidity is the rapid spread of disease above the ground and in root systems.

Water is the lifeblood of living organisms, both for plants and pathogens. Pathogens, however, need water and time. Nature has helped plants evolve to adapt to fluctuating water availability.

Plants have developed a reliable technique for maintaining water homeostasis in various environments. There are distinct benefits, not least of which are a defense against diseases, to the occasional absence of abundant water.

The Use of Technology in Moisture Control

Several commercially available devices measure soil hydration levels and, coupled with electronic systems, can trigger various watering mechanisms.

According to the United States Environmental Protection Agency (EPA), more than 28 million houses in the United States have in-ground sprinkler systems usually controlled by a clock. Irrigation schedules set to water during the peak of the growing season are seldom adjusted to account for seasonal changes or changes in plant watering requirements.

Soil moisture-based irrigation controllers (SMSs) are an alternative to clock-based controllers that efficiently modify irrigation schedules to suit landscape water needs based on direct moisture measurements in the soil, reducing water waste and promoting plant health.

SMSs can be standalone controllers, “add-on” or “plug-in” devices that work with a clock-based controller to help its efficiency.

According to the EPA, SMSs must be able to reliably block or allow irrigation at a predetermined level to receive the WaterSense label. WaterSense-labeled SMSs, like all other WaterSense-marked goods, are independently certified to guarantee that they fulfill the EPA’s efficiency and performance criteria.

According to the EPA, “installing a WaterSense labeled SMS can save an average home with an automatic landscape irrigation system more than 15,000 gallons of water annually.

Replacing clock-based controllers in all residential irrigation systems across the United States with WaterSense-labeled SMSs could save more than 390 billion gallons of water yearly.

That equals the annual household water needs of 5 million average American homes. Some utilities offer rebates to help you water smarter outdoors.”

Moisture Control Potting Soils

Potting soils that claim to have the ability to provide an extended supply of moisture to your plants, like over a vacation, typically include superabsorbent materials. We will take a look at the available options below. These materials absorb water (or most liquids) far above their weight.

Their efficacy is still under review. Forums discussing them have those that sing their praise and others that despise them vehemently. The jury is still out, in my opinion.

The argument for their inclusion in potting soil is logical and based on their ability to absorb and store water. In theory, this super absorbent material will slowly release the stored water as the moisture equilibrium changes.

Sounds logical. On one of the forums, the claims were that plants pardoned inattention, remained green until bloom time, and then failed to bloom.

Usually, that would indicate over-fertilization – maybe that mix includes some nitrogen. Too much nitrogen boost foliage and can cause blooming failure. (No pun intended)

Not all the manufacturers have listed their ingredients, but no wonder ingredient is out there that hasn’t attracted the attention of large-scale farmers, of which we can be sure. Superabsorbent materials have too many applications (other than hydrating underwatered plants). Here are some of the superabsorbent materials that we know are used:

Cross-linked Polyacrylate

Sodium polyacrylate is a versatile polymer with a wide range of uses. It’s utilized in laundry detergents as a sequestering agent to bind components in hard water like calcium and magnesium, allowing the surfactants to function more effectively and the garments to get cleaner.

When the molecular weight is right, it produces sticky solutions sprayed on dirt roads for dust control. At low concentrations (1/4 – 1/2 percent), a particular procedure has a carbomer that forms a clear gel for hair and personal care products.

Applications for Polymers:

  • Slow-release of insecticides in agricultural applications
  • Preventing water seepage into underground wires and cables
  • Stabilization of wastewater and accident spillage
  • Metal ions bonding in detergents and ion exchange resins
  • Rapid response sandless sandbags to manage floods
  • Micro-encapsulation for drug administration systems
  • Hygienic disposable baby diapers and sanitary pads
  • Ice gel packs and deep mining cooling wear
  • Instant fake snow for stage & film productions and special events
  • And, apparently, in moisture-controlled potting soils.


Bentonite is a clay formed by altering volcanic ash composed chiefly of smectite minerals, most commonly montmorillonite. Bentonite has significant colloidal qualities, and when it comes into contact with water, its volume swells many times, resulting in a gelatinous and viscous fluid.

Bentonite’s unique attributes (hydration, swelling, water absorption, viscosity, and thixotropy) make it versatile for various applications.

Picture of adding potting soil to a hanging basket

Applications for Bentomite

  • Foundry: Bentonite is used to manufacture molding sand to produce iron, steel, and non-ferrous castings as bonding material.
  • Cat Litter: Bentonite is utilized in cat litter because it absorbs waste by producing clumps, keeping the rest of the product intact for future usage.
  • Environmental Markets: Bentonite’s adsorption/absorption properties are ideal for wastewater treatment. According to standard ecological regulations, low permeability soils, which naturally include bentonite, are recommended as a sealing material in the building and restoration of landfills to protect groundwater from contaminants. The active protective layer of geosynthetic clay liners is bentonite.
  • Drilling: Bentonite’s adsorption/absorption capabilities are precious for wastewater purification. Low permeability soils, naturally including bentonite, are recommended as a sealing material in the building and rehabilitation of landfills to ensure groundwater protection from contaminants, according to standard environmental guidelines. Geosynthetic clay liners’ active protective layer is bentonite.
  • Oils/Food Markets: Bentonite removes contaminants from oils, and its adsorptive qualities are critical in producing edible oils and fats (such as soya, palm, and canola oil). Bentonite is a clarifying agent in beverages like beer, wine, mineral water, sugar and honey.
  • Agriculture: It is a porous ceramic carrier for various herbicides and insecticides after thermal processing.
  • Other uses include pharmaceuticals, detergents, paints, and paper, where the active ingredient in carbonless copy paper is acid-activated bentonite.

Lava Rock

Basalt lava rocks are a form of igneous rock made up of various mineral and chemical components. Lava rocks are porous, which means they have many open pores on their surface where liquids or gases can pass through. The existence of gas bubbles in the lava or magma flow causes porosity, forming holes on the basalt’s surface during cooling.

Vesicles are pores or cavities in the body. Basalts have a relatively low density due to their porosity. Lava rocks have a sponge-like look due to their vesicular nature, making them popular landscaping and rock garden materials.


Silica gel is an amorphous and porous type of silica comprising an uneven tridimensional framework with nanometer-scale spaces and pores. Because of its hugely hygroscopic (readily taking up and retaining moisture) property has commonly been used to ship moisture-sensitive equipment. You know, that little pouch you get with your camera or phone?

Silicates Uses

  • Preventing oxidization
  • Drying moisture-sensitive environments
  • Maintaining dry conditions in libraries for book preservation
  • Flower drying
  • Seed preservation (though recent research has established that the durability of seeds is proportional to temperatures and moisture content)
  • There are hundreds of applications where moisture needs to be drawn from an environment and concentrated in silicates.

Moisture Controlling Substances Conclusion

The common denominator of all the above materials is their infatuation with anything liquid. They are parched and in desperate need of a fluid companion.

We must ask ourselves: “How willing will they be to surrender that bond and return to a state of moisture depletion?”

Yes, they will absorb as much as possible when moisture (or fluids) is available, and they will relinquish some of that through the soil and roots’ capillary action. But only to a point. Your soil must be substantially dryer than the polymer to release water to the growing medium after the first bit.

FAQs on How Does Moisture Control Potting Soils Work?

Are there other ways to control soil moisture?

Yes. At the bottom of this post, we suggest possible alternatives – mainly increasing organic matter. Both decomposed and partially decomposed matters have water-holding capacities.

Are moisture-controlling potting soils good?

The world is continually seeking ways to improve resource usage. Some innovations explore creative solutions. Confidence comes with experience, so why not try it out?

Should I allow my plants to dry out from time to time?

Dry is a relative term. Allowing your soil to dry out completely is not a good idea. You can reduce the need for water by not tilling the surface and adding mulch to minimize evaporation.

Will cooling an environment reduces the need for water?

The plant uses much of its water intake to regulate temperatures. Some hydroponic installations regulate humidity (to control pathogens) and reduce temperatures (to limit the need for water).

Conclusion on How Does Moisture Control Potting Soils Work?

No physical process s in the atmosphere and environment can occur without water. The amount of precipitation, the intensity of plant water use, and air temperature are all elements that affect soil moisture.

Because appropriate soil moisture is critical for yields, plants will not grow and develop if the soil moisture is insufficient. Other uses for water include the following:

  • Air content, salinity, and the presence of hazardous chemicals are all affected by soil moisture levels
  • Controls the structure, flexibility, and density of soil
  • Influences soil temperature and heat capacity
  • It prevents soil from becoming g weathered

But too much water has risks too.  Also, partial drought helps plants manage pathogens that may colonize the plant and cause diseases. Your best solution is to get a medium that:

  • It has a large surface area – think of a stiff sponge vs. a sponge with big holes. You want a medium with smaller spaces that is not compact – like coconut coir.
  • It has a high content of quality organic matter
  • Could have some vermiculite included
  • Added vermicompost

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