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 (http://www.soilmoist.com/ 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