Amending Soil with Super Absorbent Polymers

by John Berends

Super absorbent polymers (SAP) are soil amendments that are marketed under many brand names. They are sometimes referred to as hydro gels, polymer crystals, or water gels. SAP work as tiny water reservoirs in the soil that are capable of retaining hundreds of times their weight in water. When the soil is hydrated, SAP retain excess moisture and nutrients from the soil. Then they are gradually depleted through feeding plants and environmental heat. This simple process has the ability to perform several important functions in the soil.

Reduction of Watering Frequency

The primary reason for using SAP is to reduce the frequency of watering needed by both container and in-ground garden plants. When used properly, SAP can reduce plant watering needs much more effectively than traditional water retaining soil amendments like peat moss, vermiculite etc. This is because, even at conservative estimates, SAP can retain 200-400 times their weight in water. As a comparison, peat moss only retains ten times its weight in water. Part of the advantages in water saving are due to the fact that the SAP retain water evenly within the root zone where it can be accessed by plants when needed. In comparison, in un-amended soil, excess water is either drained away, taking with it valuable nutrients from the soil, or worse, sits in poorly drained soil which causes a whole host of problems. When properly used, SAP can reduce irrigation needs as much as 50-75%.  

Conservation of Water and Nutrients

When soil receives excess water, SAP can retain much of the water and soil nutrients that would be lost through run-off, allowing the water and nutrients to be dispersed back into the soil when needed. This water holding ability is especially beneficial when fertilizing because the excess fertilizer is absorbed along with water and can then be time-released back to the soil providing longer lasting benefits.

Maintaining Consistent Soil Moisture

The water holding capacity of SAP helps to keep plants from sitting in water logged soil in times of excess moisture. Then, as moisture is needed, SAP release moisture back into the root zone helping to maintain consistent soil moisture between rain, and irrigation. This is particularly important for crops such as tomatoes which do not do well with inconsistent soil moisture conditions.

Aiding Soil Aeration Drainage and Deep Root Penetration

One of the most exciting functions of SAP is the ability to continuously loosen the soil allowing it to breathe and drain properly. The polymer granules swell when hydrated; then, they shrink, as water is drawn from them. Through these expansion-and-contraction cycles, the soil is loosened and aerated. Because they are mixed deep in the growing medium rather than used as a top dressing, this aerating process takes place deep in the root zone, helping to encourage rood development and penetration.

Long Lasting, Cost Effective and Environmentally Friendly

SAPs are non toxic and safe to the environment. They are “inert”, meaning they will not be absorbed into plants or migrate into ground water. They hold nitrates, phosphates, potassium, iron, zinc, boron and other elements next to the roots of plants where they can be used. This reduces the leaching of these elements from the garden soil and into ground water. When mixed with soil, they may last up to 5-7 years, and then, over time, they decompose causing no environmental harm.  

How to Use SAP

There are generally two recommended methods for using SAP to amend garden soil. With the dry method, pre-hydrated crystals are mixed into the medium. Suggested ratios vary depending on the product selected, but are generally at a rate of 1.5-2.75 lbs. per cu. yd. With the pre-swelling method, crystals are hydrated prior to mixing with soil, but are generally 2 ounces of dry crystals to at least 1.5 gallons of water. The amazing thing is how a small amount of crystals can expand and absorb such a large amount of water in just a few hours. Pre-swelling the crystals is a graphic illustration of their water holding capacity.

Regardless of which method is used, it is recommended that the SAP are thoroughly mixed or tilled into the soil or growing medium and that the soil is watered adequately to achieve full initial hydration.

The following are various super absorbent polymer products and company websites:

Hydrosource www.hydrosource.com/prod01.htm

Water Crystals http://www.watercrystals.com/

Agrosoke http://www.agrosoke.net/

AquaRocks www.megagro.com/aquarocks.htm

Soil Moist http://www.soilmoist.com/

Water-Keep www.water-keep.com

Water-Gel Crystals www.watergelcrystals.com

John Berends a free-lance writer and can be contacted at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it