The use of polymers in agriculture has rapidly expanded since the introduction of plastic films for greenhouse covering, chemical plant protection, and field mulching in the 1930s and 1940s. Today, all major classes of polymers, including plastics, coatings, elastomers, fibers, and water-soluble polymers, are utilized for a variety of applications such as controlled release of pesticides and nutrients, soil fertilization, seed coatings, and plant protection. Additionally, there is growing interest in the use of degradable plastics for crop mulching and agricultural plant containers. Composting is the preferred method for the biodegradation of these materials, as it allows for the combination and processing of different biodegradable materials to improve soil quality.
Agricultural Mulches
Agricultural mulches play a vital role in supporting crop growth by reducing weed growth, maintaining moisture levels, and increasing soil temperature, all of which improve the plant growth rate. The use of black polyethylene mulch has been shown to increase yields by two to threefold and result in earlier ripening at a 6 ha melon farm. Mulches also inhibit weed growth, prevent soil compaction, and reduce the need for certain cultivation practices. Transparent polyethylene films are more efficient in heat trapping than black or smoke-gray film, resulting in increased soil temperatures. However, conventional films can cause problems at harvest or during agricultural practices the next year due to the difficulty and cost of removal or disposal. Biodegradable films or those with short service lifetimes are being developed to address these issues, but only a few have been commercialized. Photodegradable materials containing light-sensitizing additives have been proposed, including systems composed of nickel and ferric dibutyldithiocarbamates and a combination of substituted benzophenones and titanium or zirconium chelates. Biodegradable films based on starch with added polyvinyl alcohol, copolymers of polyethylene, acrylic acid, and polyvinyl chloride have been developed in laboratories in the USA. Effective fumigant mulches require films with decreased porosity to reduce the potential escape of volatile chemicals.
Controlled Release of Agricultural Chemicals
The controlled release (CR) method provides active chemicals to specific plant species at predetermined rates and times, primarily through the use of polymers that regulate the delivery rates, mobilities, and periods of effectiveness of the chemicals. This method has several benefits, including reduced effects on non-target plants and decreased leaching, volatilization, and degradation of the chemicals. The macromolecular structure of polymers is key to reducing chemical loss during production.
CR polymer systems fall into two categories: in the first, the active agent is dissolved, dispersed, or encapsulated within the polymer matrix or coating, with release occurring via diffusion or through biological or chemical breakdown of the polymer. In the second category, the active agent is either a part of the macromolecular backbone or attached to it, with release resulting from cleavage of the bond between the polymer and bioactive agent.
Physical systems that incorporate agricultural chemicals include microcapsules, physical blends, and dispersions in plastics, laminates, hollow fibers, and membranes. Kinetic models have been developed for chemical release in each of these systems.
CR systems can use natural polymers such as starch, cellulose, chitin, alginic acid, and lignin, which have the advantages of availability, relatively low cost, and biodegradability. However, they are insoluble in standard solvents for formulation, encapsulation, and dispersion, which can be overcome by encapsulating the chemical in situ. For example, gelatinized starch cross-linked with a chosen pesticide by calcium chloride or boric acid traps the pesticide within granular particles.
Fertilization is one of the most important applications of CR technology in agriculture. Urea, a main nitrogen source, reacts with formaldehyde to form a polymer that subsequently releases urea, making it a simple and inexpensive CR system.
While there are few applications for biodegradable synthetics in agriculture, polycaprolactone has been used for small planting containers. Polycaprolactone planting containers have undergone substantial degradation within a reasonable period, making them suitable for automated machine planting of tree seedlings. For example, after being in soil for six months, they lost 48% of their original weight, with 95% weight loss within a year.