How Do Moisture Control Potting Soils Work?


Picture of Seedling in Moist Soil
Moisture and Growth

Two essential factors contribute to a plant’s access to water: Available water holding capacity (AWC) and water retention (WR) within the growth medium. The latter relates to the moisture in the soil available for plant use at any given point in time. AWC relates to the total crop water availability potential between the wilting point and field capacity.

Moisture control potting soil uses a combination of materials that have 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. Having potting soil that constantly provides the plant with the exact amount of water required 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 temperatures. 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.

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 their 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 of soil, or inches of water per foot of soil.

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

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 is dependent 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? Here’s an experiment you can do 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 all the way to the tip of the leaves – capillary action in action. Red celery, anyone?

How much water do my plants needs?

Sufficient levels of soil moisture are an essential condition for proper plant formation and high crop yields. For the plant, water serves as an agent of moisture restoration and as a temperature regulator.

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

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

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

The Alpine succulents get watered occasionally, 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.

USDA

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 life-blood of living organisms, both for the plant 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 a variety of environments. There are distinct benefits, not least of which are a defense to 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 to 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 (or 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 or “add-on” or “plug-in” devices that work with a clock-based controller to help its efficiency.

SMSs must be able to reliably block or allow irrigation at a predetermined level, according to the EPA, 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 each year.

That’s equal to 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 can 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 then release the stored water slowly as the moisture equilibrium changes.

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

Usually, that would be an indicator of over-fertilization – maybe that specific mix has some nitrogen included. Too much nitrogen boost foilage but 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 that 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.

It produces sticky solutions sprayed on dirt roads for dust control when the molecular weight is just right. At deficient concentrations (1/4 – 1/2 percent), a particular procedure produces a carbomer that forms a clear gel for hair care 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

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 a versatile material for various applications.

Picture of adding potting soil to a hanging basket

Applications for Bentomite

  • Foundry: Bentonite is used to manufacture molding sand for the production of iron, steel, and non-ferrous castings as a 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: The adsorption/absorption properties of bentonite 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: The adsorption/absorption capabilities of bentonite are extremely valuable for wastewater purification. Low permeability soils, which naturally include 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 is used to remove contaminants from oils, and its adsorptive qualities are critical in the production of edible oils and fats (such as soya, palm, and canola oil). Bentonite functions as a clarifying agent in beverages like beer, wine, mineral water, and products like sugar and honey.
  • Agriculture: It is a porous ceramic carrier for various herbicides and insecticides after thermally processed.
  • Other uses include pharmaceuticals, detergents, paints, and paper, where the active ingredient in the manufacturing of 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, which forms holes on the surface of basalts 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.

Silicates

Silica gel is an amorphous and porous type of silica, made up of 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.

The question we have to ask ourselves is: “How willing will they be to surrender that bond and return to a state of moisture depletion?”

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

FAQ

Conclusion

Without water, none of the physical processes in the atmosphere and environment can take place. 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, ductility, and density of soil
  • Influences soil temperature and heat capacity
  • It prevents soil from becoming 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 that has smaller spaces yet 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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Tony O'Neill

I am Tony O'Neill, A full-time firefighter, and professional gardener. I have spent most of my life gardening. From the age of 7 until the present day at 46. My goal is to use my love and knowledge of gardening to support you and to simplify the gardening process so you are more productive

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