23-01-2013, 10:54 AM
Slow- and Controlled-Release and Stabilized Fertilizers: An Option for Enhancing Nutrient Use Efficiency in Agriculture
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Introduction
The fertilizer industry faces a continuing challenge to improve its products to increase
the efficiency of their use, particularly of nitrogenous fertilizers, and to minimize any
possible adverse environmental impact. This is done either through improvement
of fertilizers already in use, or through development of new specific fertilizer types
(Maene, 1995; Trenkel et al., 1988).
Improvement of fertilizers already in use is done through appropriate product design
(Bröckel and Hahn, 2004). The product profile is determined by its chemical and physical
properties, environmental safety and its stability against mechanical stress, hygrometry
and temperature. With solid fertilizers new product design is mostly aimed at improving
handling properties (reduction of dust formation and caking/hygroscopicity).
Increasing the efficiency of mineral nitrogen (N) fertilizers1 use is not easy, because
plants take up N normally as nitrate or ammonium ions, through their roots from
the soil solution. However, ammonium-N, unlike nitrate-N2, can be retained on soil
constituents so that soil and plants compete for ammonium-N, either already available
in the soil or applied (Amberger, 1996). This competition for nitrogen, with the
exception of nitrate-N is the main problem when it is added as mineral fertilizer to feed
plants. Only a certain proportion of the N is taken up, or can be taken up, and used by
the growing plants.
Slow- and controlled-release fertilizers
According to AAPFCO (1997): There is no official differentiation between slow-release
and controlled-release fertilizers.
According to Shaviv (2005): “The term controlled-release fertilizer (CRF) became
acceptable when applied to fertilizers in which the factors dominating the rate, pattern
and duration of release are well known and controllable during CRF preparation.” Slowrelease
fertilizers (SRFs) involve the release of the nutrient at a slower rate than is usual
but the rate, pattern and duration of release are not well controlled. Trenkel (1997)
noted that it became common practice to denote microbially decomposable nitrogen
products, such as urea-formaldehyde, as ‘SRFs’.
Nitrification inhibitors
Nitrification inhibitors delay the bacterial oxidation of the ammonium ion (NH4
+) by
depressing over a certain period of time (four to ten weeks) the activity of Nitrosomonas
bacteria in the soil. These bacteria transform ammonium ions into nitrite (NO2
–), which
is further transformed into nitrate (NO3
–) by Nitrobacter and Nitrosolobus bacteria. The
objective of using nitrification inhibitors is to control the loss of nitrate by leaching or
the production of nitrous oxide (N2O) by denitrification from the topsoil by keeping
N in the ammonium form longer and thus increasing N-use efficiency. Furthermore,
nitrification inhibitors – by delaying the conversion of ammonium to nitrate – avoid
undesirable high nitrate levels in plants used for human and animal nutrition. Inhibiting
nitrification, however, will not prevent mineral N from entering water bodies by direct
N application of fertilizers and by runoff (Edmeades, 2004).