The technological, physiochemical and engineering knowledge for developing agricultural delivery systems has now reached a stage where a wide selection of routes, sites, modes and rates of administration are available. This was the theme of an international workshop on 'Controlled Release Technology in Agriculture and Veterinary Practice'' that was organised at Ooty on October 2, 2000 by the Controlled Release Society (CSR) Indian Chapter. The event was co-sponsored by the JSS College of pharmacy, Ooty and Scitech Centre, Mumbai.

Shaviv Avi, Associate Professor, Water-Soil-Environment, Technion-HT, Haifa, Israel presented three papers on the subject. His first paper was on, 'Coated fertilisers - an efficient means for controlling nutrient release and reducing environmental hazards'.

He said, "Controlled release fertilisers (CRFs) have received, in the last years, increasing recognition as efficient means for increasing nutrient use efficiency, reducing pollution hazards and improving conservation of natural resources. Coating of soluble fertilisers with a non-soluble membrane is considered the most efficient way to control nutrient release. Despite significant achievements in their production technologies, their use is limited. This is mainly due to high costs and the limited attention paid to the potential advantages offered by their use and the partial understanding of their action mechanisms."

"The review stresses the agronomic advantages expected from a single application of an efficient CRF, which supplies nutrients according to plant and demand during the season. This leads to significant reduction of nitrates and phosphates leaching to water sources, gaseous emissions (NOX, NH3) and improves conservation of natural resources. The review describes the main CRFs used in practice, emphasizing their characteristics, advantages and disadvantages. Special attention is given to proper understanding of release mechanisms of coated CRFs to provide improved tools for their characterization, performance evaluation and prediction of agronomic and environmental advantages in agricultural use. Accordingly, the difference between well performing CRFs and non-efficient fertilisers, which only slowly release the nutrients, is shown," he added.

Prof Avi also spoke on the title, 'Statistically based model for diffusion release from a population of polymer coated controlled release fertilizers'. He said, "A statistically oriented model for describing the release from a population of polymer coated controlled release fertilizer (CRF) granules by the diffusion mechanism was constructed. The model is based on a previously developed mathematical-mechanistic model describing the three stages occurring during diffusion release from a single granule of a coated CRF. The first stage is a lag period during which the granules take up water from the moist surrounding until a critical build-up it obtained."

He added, "The main factors affecting this period is membrane (coating) permeability to water, granule radius and coating thickness, or their product. In the second and third periods linear and decaying release take place, respectively. The rates in these stages increase with membrane (coating) solute permeability and with the saturation concentration of the fertiliser. They are inversely affected by the product of granule radius and coating thickness. All the factors controlling the release can be determined experimentally."

"The statistical model is based on the assumption that the cumulative release from a population of granules can be calculated by integrating over the release of the individual granules within a given range of radii of granules and coating thickness. The variation in radii and coating thickness is assumed independent and is accounted for by multiplying the release function by the product of the two respective distribution functions," he said.

His third presentation was on 'Environmental significance of using controlled release agro-chemicals'. He said, "Synthetic agro-chemicals such as fertilisers, pesticides, insecticides and herbicides are essential for the worldwide massive production of food. Fertilisers provide the nutrients for plant growth, and the organic agro-chamicals are (mostly) designed for selective killing or inhibiting of organisms that impede plant development (e.g. weeds, insects and fungi.)

"The fate of agro-chemicals in soil is dictated by physical, chemical and biological processes that are likely to remove them from the target organisms. Regardless of their mode of application, a significant share of the active ingredients used in agriculture are lost by leaching, surface runoff or volatilization. Thus, by non-controlled intrusion from the soil into water sources or the atmosphere, such essential ingredients may turn into environmental hazards (e.g. water pollution by nitrate, phosphate, pesticides or herbicides, air pollution by nitrous oxides or organic volatile compounds)."

He added, "The extent of loss is considered to be roughly proportional to the excess of the supply of the agrochemical over its demand by plants or other target organisms. Synchronizing agrochemical supply with organism demand is thus essential for minimising losses to the environment. It may also provide "optimal" growth conditions (e.g. synchronized nutrient supply), improve cost effectiveness and assure high quality food crops."

While describing the kinds and application of fertilisers, he said, "There are basic differences between the fertilisers (mostly non-organic) and the organic agro-chemicals, which to a certain extent may affect their release. Fertilisers are highly water-soluble and induce high ionic strength and/or osmotic pressure, whereas most of the herbicides and pesticides have low water solubility and thus induce low ionic strength or osmotic pressure. And yet the controlled release of both fertilisers and organic agro-chemicals is generally based on similar physical or chemical/biochemical principles."

The main mechanisms for controlling release of agro-chemicals are based on:

1. A physical barnier through which the active ingredient is released into the soil: or

2. Chemical or bio-chemical degradation of a compound in which the active component is either bound or protected. The action mechanism greatly affects the pattern (fickian, linear or sigmoidal type) and the duration of the release and its dependence on soils properties and environmental conditions."

Professor Tejraj M Aminabhavi, Department of Chemistry, Karnatak University, Dharwad spoke on 'Eco-Friendly Polymeric-Pesticides.' He said, "pesticides embedded into polymeric matrices or dispersed with a polymer are referred to as polymeric pesticides (PP). Such systems increase the efficiency by targeting, reducing the loss by leaching or preventing volatilization of the pesticide. Volatilization of pesticides has been of main environmental concern because the applied pesticides will produce hazardous effects to the environment, especially to the ozone layer."

He added, "The controlled release (CR) technology has proven its advantages, which avoids the above-mentioned problems, especially with the toxic pesticides. Polymers in various forms have been used as effective CR devices. However, the more useful ones are the stimuli - responsive (sensitive to temperature, pH, moisture and light) polymers for the release of active agents. Since the first introduction of polymers in 1940's for fumigation and mulching, agricultural applications of polymers have grown at an enormous rate. Different types of biodegradable polymers have been employed in agro-areas for the CR of pesticides, soil conditioning, plant protection, seed coating and gel planting."