How Do Hydrogels Release Stored Water to Plant Roots?

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How do hydrogels release stored water to plant roots? A hydrogel is a polymer network that takes in water, holds it inside its structure, and gives it back over time. The short answer is that the water sits in a swollen network and leaves gradually, with release near the roots driven by a difference in osmotic pressure. The hydrogel’s chemistry and the surrounding soil also change how much water is kept and how quickly it moves. This explainer covers that mechanism using two review articles from PubMed Central as its sources.

Quick answer

Hydrogels are three-dimensional, hydrophilic polymer networks that absorb and release water.
Water is held inside the swollen network and released gradually.
Near the roots, release is driven by a difference in osmotic pressure.
Swelling depends on crosslinking, hydrophilic groups and outside factors such as pH and ions.

What a hydrogel is

The first source describes hydrogels as three-dimensional, hydrophilic, polymeric materials with the ability to absorb and release large amounts of water. Those three words describe the structure.

The first source also explains the physical process. The change in volume of a hydrogel is driven by water diffusion, and the equation used to describe the mechanism of diffusion of water into the polymeric network of hydrogel is Fick’s first law. The same source identifies the key feature as the ability to absorb and trap water into the three-dimensional structure.

Swelling is what stores the water

The water is stored by swelling. The review describes this property as the swelling capacity, and it calls swelling capacity one of the most important metrics used to ascertain how well a hydrogel will perform in retaining water in the soil matrix.

The second source explains what controls swelling. The swelling behavior of a hydrogel is contingent upon the degree of cross-linking, the number of hydrophilic groups, and the available space within the network structure. That space is what accommodates, absorbs and retains water molecules because of the network’s hydrophilic nature.

The same source also points out that the swelling must happen in time. It is necessary for hydrogel to achieve sufficient swelling within this temporal window.

The release step: osmotic pressure at the roots

The second source gives the release mechanism directly. Polymer hydrogels gradually release previously captured water in the root zone due to the difference in osmotic pressure. The water moves out of the network because the osmotic conditions around the roots differ from those inside the gel.

The source adds that another important property of hydrogels is their capacity to release water gradually.

The same source frames the practical value in agriculture. Polymer hydrogels have become a promising material for enhancing agricultural productivity due to their ability to retain and then release water, which can help alleviate the need for frequent irrigation in dryland environments. The claim is about reducing how often irrigation is needed, and it is stated in the source for dryland settings.

Factors that change the release

Release is not fixed. The second source notes that external factors, including pH, temperature, light, electric and magnetic fields, or the presence of specific ions, can also influence the hydrogel swelling process. Because swelling and release are linked, the same factors can change how much water the gel holds and how it lets go.

The structure of the hydrogel also sets a trade-off. The second source states that in the field of agriculture, the high swelling capacity of hydrogels results in a reduction in the mechanical strength of the hydrogel. A gel that holds a lot of water can be weaker in structure,

Water in soil with hydrogels

The first source covers both bio-based and synthetic hydrogels, and its findings apply to both groups. The second source also notes that hydrogels based on chitosan exhibit comparable levels of water retention capacity. reviews the effect of hydrogels in soil. Both synthetic and bio-based hydrogels increased water retention in soil compared to unamended soil with decreasing soil water pressure head.

The review covers several soil properties, including water retention, soil hydraulic conductivity, soil water infiltration, and evaporation. The same source lists the uses of hydrogels as a soil amendment. Hydrogels are a type of soil amendment that have been applied in agriculture to increase water use efficiency, and to trap water that would have otherwise leached out of the root zone.

Soil layers matter too. Water and dissolved nutrients tend to migrate into deeper layers of soil, which have a low water retention capacity. For a related look at how roots get water from the soil, see how roots find water.

Delivery beyond water

The second source also describes hydrogels as delivery agents. A promising strategy involves encapsulating active ingredients for growth promotion, namely fertilizers and micronutrients, within slow-release smart polymer matrices. This article covers water release only.

For the soil side of that question, see how clay holds water in soil.

How we chose

This explainer uses two open-access review articles from PubMed Central. Both pages were checked directly and returned article text before any quote was used. One review covers bio-based and synthetic hydrogels and their effect on soil hydraulic properties. The other covers polymer hydrogels as delivery agents in agriculture. Together they cover the storage, swelling and release steps.

Each factual sentence in the body is tied to a verbatim quote recorded in the source notes. Sentences that could not be tied to a quote were removed. The article adds no product claims, no prices and no percentages that the sources do not state in the quoted passages.

What we could not verify

The two reviews describe hydrogels in general terms and across several materials. They do not settle every detail, and the points below are left open rather than filled in with guesses.

What the sources do not say

The sources do not give one release rate that applies to every hydrogel or every soil. They do not provide a dosing rule for a garden bed, and this article does not offer one. The second source also describes uses beyond water, and this article does not evaluate those uses.

Frequently asked questions

Where is the water stored inside a hydrogel?

The water is stored within the three-dimensional structure of the polymer network. The first source describes the ability of hydrogels to absorb and trap water into that structure.

What makes a hydrogel swell more?

The second source says swelling depends on the degree of cross-linking, the number of hydrophilic groups, and the available space within the network.

What pushes the water out to the roots?

The second source states that polymer hydrogels gradually release previously captured water in the root zone due to the difference in osmotic pressure.

Can soil conditions change how a hydrogel releases water?

The second source notes that external factors, including pH, temperature, light, electric and magnetic fields, or the presence of specific ions, can influence the swelling process. The first source reports that hydrogels increased water retention in soil compared to unamended soil.

Do hydrogels reduce the need for watering?

The second source says hydrogels can help alleviate the need for frequent irrigation in dryland environments. That claim is stated for dryland settings, and this article does not extend it to every garden.

Sources

The Impacts of Bio-Based and Synthetic Hydrogels on Soil Hydraulic Properties: A Review, Polymers, 2022: https://pmc.ncbi.nlm.nih.gov/articles/PMC9656743/
Agriculture 4.0: Polymer Hydrogels as Delivery Agents of Active Ingredients, Gels, 2024: https://pmc.ncbi.nlm.nih.gov/articles/PMC11203096/