UW-Green Bay

Month: November 2014

What is Topsoil?

What is Topsoil?

Topsoil is the upper layer of soil that contains most of the available plant nutrients and in which most of the biological activity takes place. The soil’s organic matter is concentrated in the topsoil, making it noticeably darker than the subsoil. The topsoil is the primary home of the vast “soil food web”—the amazingly complex community of microorganisms, earthworms, insects, and small vertebrates that is the only proper home for the roots of crop plants.

A soil profile is the side view of soil, from the uppermost layer to the bottom layer.

  • The topmost layer of the soil is composed mainly of fresh soil and decaying organic matter. The color ranges from brown to black.
  • The second layer of the soil consists of highly decomposing organic matter. The color ranges from brown to gray.
  • The third layer of soil is composed of sand and silt. It has lost most of its nutrients. The color of this layer is light brown.
  • The fourth layer consists of clay and large rocks and bedrock. The color ranges from rust to tan.
  • The fifth layer of soil is bedrock. The color is gray.



Why is Soil So Important?

Soil is more than just the dirt under our feet. It is a home for living organisms and it provides nutrients and stability for plants to grow. Without soil, the plants necessary for people and animals to survive could not exist. By caring for our soil properly, we can ensure the longevity of both animals and people. The use of crop rotation, limiting harsh chemicals and composting will help to maintain a healthy balance of nutrients, living organisms and minerals in the soil. It is important to remember that the fresh foods on which we feast affect our health. To the question, “Why is soil so important”, the simple answer is that we are what we eat.

Understanding the Chemistry of Soil Conditioning

As previously posted, SoilMoist is a current commercially-available product used for soil conditioning.  The chemistry behind the SoilMoist product involves polymerization cross-linking of polyacrylamide (PAM) in a potassium-based salt ( forms/JRM-Form-145-Revised2009.pdf).  According to Dr. Chen’s grant proposal, the commercially available PAM products achieve water retention by “acrylamide chain segments replac(ing) an acrylic acid group”.

Diagram of cross-linked polyacrylamide network

Cross-linked Polyacrylamide (PAM) Network

Acrylic Acid Group that replaces 15-40% of Acrylamide groups:

Chemical structure of acrylic acid functional group

Acrylic Acid Functional Group

Recall from Organic Chemistry that when the Hydrogen is removed, the double-bond between the chain carbon becomes shared with both oxygens in a resonance-stabalized Carboxylate ion:

Chemical structure of Carboxylate Anion

Resonance-stabilized Carboxylate Anion Group

It is these Carboxylate ions present in cross-linked PAM network that causes the polyelectrolyte to act as a hydrogel that retains water at a vastlly greater level than it’s own molecular weight.  The anionic properties also contribute to the material’s ability to attract certain soil-based nutritional cations such as Calcium ions.

Although PAM itself is nontoxic, in industrial application, it is often contaminated with it’s starting component, acrylamide, which is highly toxic.  Because it is typically produced from fossil-based hydrocarbons, and is not readily broken down in the environment, PAM is not the most ideal substance to use in large-scale agricultural application.  Dr. Chen’s research seeks to find alternative substances that have the properties similar to PAM which can be made out of natural industrial byproduct such as cellulose from plant material, which can often be found in excess in NE Wisconsin’s paper industry.

Chemical structure of generic cellulose bio-polymer

Cellulose bio-polymer (notice ample hydroxyl groups)

Dr. Chen’s plan is to develop a process of converting the ample hydroxyl groups found in plant cellulose into either Carboxylate groups or to Sulfonate them with Sulfuric Acid.  Another method proposed is to use

SoilMoist Polymer

From the last post, SoilMoist is a soil conditioning polymer that improves the water retention of soil. When exposed to water, SoilMoist swells into a gel-like substance.

The Chemical composition of SoilMoist is cross-linked polyacrylamide

^Chemical structure of poly acrylamide.

Cross-linked polyacrylamide is highly hydrophilic, it can H-bond with water, which explains it’s usefulness as a water retention agent in soil.

Concerns have been raised that polyacrylamide used in agriculture may contaminate food with acrylamide, a known neurotoxin. While polyacrylamide itself is relatively non-toxic, it is known that commercially available polyacrylamide contains minute residual amounts of acrylamide remaining from its production, usually less than 0.05% w/w.

Information found on Wikipedia:


SoilMoist is a commercial soil additive that boosts water retention of soil. It claims to reduce the need to water plants by reducing the infiltration of water, that is to say, water will drain through the top layer of soil less quickly, improving the residence time of water in the planting layer.

The product also claims to reduce compaction of soil.

SoilMoist has products differing in the size of the polymer (granular or fine) that benefit different applications. The granular version is an all around water retention aid that can be tilled or mixed into the top layer of soil. The fine version of the polymer is advertised for use in coating plant roots prior to transplantation to prevent transplant shock.

The product is advertised for agricultural or domestic use and claims to be environmentally friendly.

SoilMoist website:


Above is a picture of a flowerbed benefiting form SoilMoist application. The picture was chosen form the SoilMoist website because the guy on the right looks like Rambo.

Introduction Group B

Investigating polymer application to improve soil retention.

Team Members:

Janelle Nehs

Reed Heintzkill

Sunghee Min

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