4. Advantages and disadvantages of slow- and controlled-release fertilizers
4.2. Disadvantages
4.2.1. Slow- and controlled-release fertilizers
As yet there are no standardized methods for reliably determining of the nutrient release pattern from such fertilizers. There appears to be a lack of correlation between the data from laboratory testing – which are made available to the consumer – and the actual functioning of the nutrient release pattern in field conditions (Shaviv, 2005).
Furthermore, when reporting the advantages of slow- and controlled-release fertilizers, in comparison to conventional mineral fertilizers, controlled-release fertilizers have not always been compared to the best existing fertilizer management practices (Hall, 1996;
Kloth, 1996; Lammel, 2005; Raban, 1995).
When testing the nutrient release pattern of (mainly) polymer-coated/encapsulated fertilizers, several manufacturers determine the duration of the release of 80% of the nutrient at 25oC (Shoji and Gandeza, 1992). However, tests for 75% or 80% release indicate that the user can reasonably expect that about three-quarters of the nutrients will be released during the growth period. It ignores the possible existence of a ‘burst’
(Shaviv, 1996). This might have severe agronomic and environmental implications, when slow-release fertilizers with a large initial ‘burst’ (such as SCU or UF), or polymer-coated fertilizers, having a large proportion of damaged granules, are compared to controlled-release fertilizers that perform well (Shaviv, 2003b, 2005). With some chemical reaction products, such as UF fertilizers, it appears that a proportion of the N may be released in plant available forms extremely slowly (or not at all). The release of nutrients can be too slow if they are too thickly coated. Sulphur-coated fertilizers, with a parabolic nutrient release pattern (with or without ‘burst’), may initially release nutrients too quickly, causing damage to the crop, and the final release of N too slow for it to be available to the plant (Shaviv, 2005). The cost of a sulphur-coated fertilizer with a rapid initial nutrient release, even if it does not cause damage to the developing plant, is more expensive than the equivalent amount of conventional soluble fertilizer.
Application of a coated fertilizer may increase the acidity of the soil. This can be the case if large amounts of sulphur-coated urea are applied, because both sulphur and urea contribute to increase soil acidity. This may, posibly, improve the uptake of phosphorus and iron.
Polymer-coated or encapsulated controlled-release fertilizers may leave undesired residues of synthetic material in the soil. Some types of polymers used in the coating of conventional fertilizers used currently decompose extremely slowly or not at all in soil. Their use may lead to an undesirable accumulation of plastic residues, up to 50 kg/
ha/yr (Hähndel, 1997). According to Shaviv (2005), polyolefin coatings have lower degradation rates than alkyd resins and polyurethane-like resins, the three main types used in practice.
Though within ten years, a 500 kg/ha maximum accumulation would be only 200 ppm of dry soil, research should be intensified to develop degradable polymeric coating materials. It is, however, obvious that coating substances that regulate the release of nutrients for several months or even longer, will not decompose immediately afterwards,
but will need a relatively long time for total decomposition; fragments smaller than sand particle size, may become part of the soil.
In modern intensive agriculture application of the optimum amount of mineral fertilizer N follows continuous monitoring of growing conditions and farmers prefer to adapt the N dressings to crop development and yield objectives. This is incompatible with the practice of early basal/depot fertilization with coated or encapsulated N fertilizers applied in one single dressing which, if applied in excess, cannot be corrected later.
The manufacturing cost of most coated or encapsulated controlled-release fertilizers is still considerably greater than that of conventional mineral fertilizers. This has prevented their wide use in mainstream agriculture (Detrick, 1995; Fujita, 1996a;
Goertz, 1993, 1995; Gordonov, 1995; Hähndel, 1997; Hall, 1996; Kloth, 1996; Van Peer, 1996). This cost differential appears to be changing with the development of Agrium’s large-scale production of ESN (Environmentally Smart Nitrogen), launched in 2000, which is available for a moderate premium.
The higher production costs are generally due to:
Some fertilizers have to go through complicated production processes;
In trying to achieve a perfect coating, producers usually employ size separation of raw granular materials, this also makes the product more expensive;
The coating material is several times greater in price than the fertilizer material;
The usually relatively small capacities of manufacturing plants, with the exception of Kingenta’s capacity in China for slow- and controlled-release fertilizers, which reached about 850,000 tonnes at the end of 2007 and which will be further increased, of Agrium’s ESN® production capacity in Canada of some 150,000 tonnes, and of Hanfeng’s capacity in China at 150,000 tonnes;
Coated/encapsulated controlled-release fertilizers require improved marketing through specialized advisory services and sales expertise compared to conventional fertilizers, except where manufacturers or extension services have developed software for the exact application of slow- or controlled-release fertilizers, and this has considerably promoted their use.
4.2.2. Nitrification inhibitors
Ammonia-containing fertilizers amended with a nitrification inhibitor can favour an increase in ammonia volatilization, if they are not incorporated into the soil immediately or soon after application. However, Linzmeier et al. (1999) found that ASN plus DMPP did not cause increased ammonia volatilization, and this was confirmed in laboratory investigations (Wissemeier and Weigelt, 1999).
Depending on the type of nitrification inhibitor, the activity of soil bacteria may be interrupted for a certain period of time, and some soil bacteria may actually be killed by bactericidal action. This could be considered an undesirable interference in natural soil – even if localized to the area where the nitrification inhibitor is applied.
DCD has been tested extensively in field experiments on agricultural and horticultural crops in the United States. Frye et al. (1989) and Frye (2005) concluded that a yield response to DCD only occurs if N is prone to losses by leaching or denitrification and then
only if those losses result in N deficiency sufficient to reduce crop yields. If nitrification inhibitors are used with N application rates only slightly above optimum, yield increases are rarely observed. However, nitrification inhibition may have environmental benefits in crop production even when there is no increase in yield.
Ammonium ions stabilized with inhibitors and not taken up by the plants may be stored in the soil and be available to the following crops, thus decreasing the amount of N required (Gutser, 1999a).
If nitrification and urease inhibitors were available in different formulations, e.g. in liquid, suspension and granular forms, particularly for grassland/pastures and dairy farming, it might stimulate farmers to use them.
4.2.3. Urease inhibitors
The increase in yield is small when urea combined with a urease inhibitor is applied to soils that are very rich in N (Edmeades, 2004). There is also some evidence that this combination may have phytotoxic effects, e.g. leaf-tip scorch (Bremner and Krogmeier, 1990; Watson, 2005; Watson and Miller, 1996). It is not clear, whether this is a direct or indirect effect of urea; whether it is transitory, and whether it only occurs in situations where large amounts of urea and inhibitor are used. However, the benefits of NBPT – reducing ammonia volatilization and increasing yield – would appear to far outweigh any observed short-term leaf-tip necrosis (Watson and Miller, 1996).