Polyacrylamide in Agriculture and Environmental Land Management
Section snippets
EARLY USES OF SOIL CONDITIONERS
Understanding the current success of and growing attention to polyacrylamide (PAM) and related synthetic and biopolymers for land care uses and environmental protection is easier if seen in the context of soil conditioner technology development. Animal and green manures, peat, crop residues, organic composts, lime, and various other materials have been used as soil conditioners for thousands of years. Conditioner identification, technology, and use have been largely a marriage of convenience
MAJOR CONDITIONER TYPES
There are three major classes of soil conditioners: natural organic materials, inorganic or mineral materials, and synthetic materials consisting primarily of chemical polymers and surfactants. Organic conditioners have typically been used to increase infiltration and retention, promote aggregation, provide substrate for micro‐ and mesobiological activity, improve aeration, reduce soil strength, and resist compaction, crusting, and surface sealing. Effects, such as increased infiltration and
SYNTHETIC CONDITIONER USES AND APPLICATION STRATEGIES
Soil conditioner use is limited by economics, often related more to transportation and application costs of bulky materials than to material price. Thus, organic and mineral soil conditioner use in production agriculture has been largely limited to a few highly efficacious materials such as lime, gypsum, and manure. High‐value nursery operations, cash crops, turf, and landscape applications are less constrained by costs. Organic and mineral conditioners are used more often if available gratis
OVERVIEW OF CURRENT PAM USE
Many field trials from the 1950s to the present investigated polymer amendment effects on crop response and soil structural and hydraulic properties; these have been summarized in several reviews and monographs (Bouranis, 1998, Bouranis et al., 1995, De Boodt 1990, De Boodt 1992, De Boodt 1993, Levy and Ben‐Hur, 1998, Polyakova, 1976, Polyakova, 1978, Seybold, 1994, Stewart, 1975, Terry and Nelson, 1986, Wallace, 1998a, Wallace, 1998b, Wallace and Terry, 1998, Wallace and Wallace, 1986a,
PAM DEFINED AND DESCRIBED
The word polyacrylamide and the acronym “PAM” are generic chemistry terms, referring to a broad class of compounds. There are hundreds of specific PAM formulations. They vary in polymer chain length and number and kinds of functional group substitutions as well as molecular conformation, the most important conformation variation being linear or cross‐linked conformation. Cross‐linked PAMs are water absorbent but are not water soluble. Water‐soluble PAMs have little if any cross‐linking and the
PAM PROPERTIES AFFECTING EFFICACY
Both molecular properties and product formulation or preparation can influence how easily PAMs are handled and applied, as well as their behavior during or following application (Callebaut et al., 1979). Barvenik (1994) noted that the most common commercial forms of PAM are as aqueous solutions, emulsions, and dry forms. Pure aqueous solutions of PAM become highly viscous at concentrations largely dependent on molecular weight. Lower molecular weight PAMs (30,000 g mol−1) remain fluid even at 50%
EARLY CONTRIBUTIONS
The reduction of erosion and management of infiltration through improved uses of synthetic‐ and bio‐polymers has been identified by Natural Resource Conservation Service (NRCS) as one of the most dramatically effective, agriculturally significant, and environmentally important advances in irrigated soil conservation management (Thomas Spofford, NRCS National Irrigation Engineer, personal communication). Practical application of the same technology in rainfed agriculture has been more difficult
SURFACE IRRIGATION
Water‐soluble anionic PAM was identified in the 1990s as a highly effective erosion‐preventing and infiltration‐enhancing polymer, when applied in furrow irrigation water at concentrations of 1–10 ppm for applications of 1–2 kg ha−1 per treated irrigation (Lentz and Sojka, 1994, Lentz et al., 1992, McCutchan et al., 1993, Sojka and Lentz, 1997, Sojka et al., 1998a, Sojka et al., 1998b, Trout et al., 1995, Yonts et al., 2000). PAM achieves these results when applied to soil via the irrigation
SPRINKLER IRRIGATION
Although the interest in PAM use for furrow irrigation has been phenomenal and dominated the initial technology development and practice adoption, there is as much and perhaps more interest in developing the technology for use in sprinkler irrigation. Farmers who irrigate with sprinklers are familiar with concepts of chemigation and generally have greater expectations for precision application of inputs, including water application. Because PAM reduces structural degradation caused by droplet
INFILTRATION
As noted in the prior section on erosion, many papers reporting erosion control with PAM application also reported increases in infiltration. We focus here on papers that were seminal in this area, that produced specific new insights on the infiltration process as affected by PAM application, or that were from field scale studies, which may provide better insight to real world deployment of the technology.
The papers that first investigated infiltration effects in which PAM application was
PAM SAFETY, FIELD RETENTION, AND ENVIRONMENTAL IMPACTS
At this time PAM is not regulated under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), however, it is regarded as a macropollutant with low toxicity and side effects. As noted earlier, PAM has been used for many decades ubiquitously in a number of food, environmental, and other sensitive applications, often involving significant disposal or release to the environment. Some safety and environmental cautions are warranted, but the low toxicity of PAMs in general, especially
PAM EFFECT ON ORGANISMS IN RUNOFF AND SOIL
As human population and the numbers of animals reared to feed the human population continue to increase, land disposal of animal and human wastes is becoming more widespread. This trend is also encouraged by the interest in some sectors for organically produced crops. When runoff occurs from fields amended with animal manures or municipal waste there is a significantly increased risk of pathogen contamination of receiving waters. Even in the general context of agricultural production, the
PAM DEGRADATION
Very few experiments have been conducted to quantify PAM degradation, especially for PAM applied to soil. Significant problems surround the ability to remove PAM from soil once applied. The use of radiolabeled PAM would provide the best option for studying PAM decomposition; however, the difficulty and cost of labeling the appropriate C bond to follow chain fragmentation are significant. These difficulties increase when attempting to label the large molecules in current use. Thus what is known
PAM AND Ca
Anionic PAMs bond to mineral surfaces only if there is sufficient electrolyte present to overcome the repulsion of the polymer anionic sites and mineral anionic sites to allow weaker van der Waals forces, H bonding, or dipole attractions to be effective; this effect is enhanced if polycations such as Ca2+ are present to “bridge” between the negative charge sites of the polymer and mineral surfaces (Laird, 1997, O'Gorman and Kitchener, 1974, Orts et al., 2001, Pefferkorn, 1999, Theng, 1982,
PAM FOR CONSTRUCTION SITES AND OTHER DISTURBED LANDS
Ironically, interest in the use of polymer soil amendments was first prompted by their use in road and runway construction during World War II (Wilson and Crisp, 1975). Interest spread rapidly to the agricultural sector from whence emanated the majority of soil‐conditioning research on polymers and soil applications for most of the years since then. Polymers have, however, been used in construction for a variety of applications, including grouting, drilling muds, dust suppression, roadway
CANAL AND POND SEALING
Water conservation and efficient transfer of precious water resources in arid zones from water source to point of use via unlined canals is becoming increasingly important. Seepage losses from unlined canals can be significant, typically 20–30% of the volume conveyed (Tanji and Kielen, 2002). In a world with ever increasing water shortages and water demands, prevention of unwanted seepage loss could be of staggering importance. In many arid areas, seepage also results in the mobilization of Se
BIOPOLYMERS
Farmers, environmentalists, the polymer industry, and other industries producing recalcitrant organic waste streams have shown interest in the possible development of biopolymer surrogates of PAM for a variety of reasons. PAM is inexpensive because the raw material currently used most commonly to synthesize the molecular building blocks of PAM is natural gas. Natural gas prices have risen greatly in recent years, resulting in about a 30% increase in PAM wholesale costs since 2000. Because so
CONCLUSIONS
The advancement of PAM‐based agricultural and environmental management technologies since the early 1990s has been rapid, dramatic, and expansive. PAM is an extraordinarily versatile polymer. The variety of its effects on the properties of water itself and the surface interactions of solids it sorbs with allow a wide range of potential management scenarios for the protection of the environment and the improved productivity of managed lands, especially in irrigated agriculture. The compound is
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