1.2.2. Mechanism of controlled release
It is vital to become acquainted with the mechanism of controlled
release, the direct measure of the effectiveness of a CRF. Generally, the
controlled release mechanism is difficult to conceive as it depends on
numerous factors such as the nature of the coating material, the type
of CRF, agronomic conditions and much more. Different mechanisms
are cited in the literature and these are still under development. Liu
[9] and Shaviv [5] proposed a release mechanism for coated fertilizers
called the multi-stage diffusion model. According to this model, after
applying the coated fertilizer, irrigation water penetrates the coating
to condense on the solid fertilizer core followed by partial nutrient
dissolution (Fig. 2). Subsequently, as osmotic pressure builds within
the containment, the granule consequently swells and causes two
processes. In the first, when osmotic pressure surpasses threshold membrane
resistance, the coating bursts and the entire core is spontaneously
released. This is referred to as the “failure mechanism” or “catastrophic
release”. In the second, if the membranewithstands the developing pressure,
core fertilizer is thought to be released slowly via diffusion forwhich
the driving force may be a concentration or pressure gradient, or combination
thereof called the “diffusion mechanism”. The failure mechanism
is generally observed in frail coatings (e.g. sulfur or modified sulfur),