Release characteristics of a new controlled release fertilizer

A new type of controlled release fertilizer which enables control over both release rate and release pattern was developed. A dry mixture of soluble fertilizer and thickener is contained in a non-permeable envelope. having at least one small opening. Two main processes govern the release of nutrients from the device: (a) water penetrates through the opening(s) into the dry mixture forming a distinct and sharp wetting front. The process starts with a 'burst' of water into the device; (b) nutrients leave the device through the opening either by diffusion alone or by diffusive and convective flows. Since the time pattern of the transport processes may have different forms, a simple model was used to describe the extent of wetting or release, M, as a function of time, t; M(t) = C t(alpha), where C and alpha are parameters. Several factors which affect both the water penetration and the nutrients release were investigated: fertilizer type (solubility, density), thickener type (Na-polyacrylamide and Na-carboxymethylcellulose), thickener concentration in the dry mixture, size of the device, and opening diameter. It was found that the rate of wetting and the magnitude of the 'burst' effect increase with fertilizer solubility. This is reflected in the rate of nutrients release which generally increases with fertilizer solubility. Increasing thickener concentration results in a reduced rate of wetting and a smaller burst effect. Tn some cases, the time pattern of wetting shifts from Fickian (alpha approximate to 0.5) to linear (alpha approximate to 1.0). Consequently, the release itself is affected in a similar manner. Reducing the diameter of the opening, reduces significantly the wetting rate and even more so the release of nutrients. A decrease in the opening diameter results in a linear release pattern. The ability to control both the rate and the time pattern of the release through different combinations of the main factors is discussed and demonstrated.