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By Jerry Spencer in Consultancy on 12th Oct 2011 20:45
One of the perpetual debates heard frequently in horticultural circles is whether or not granulated homogenous fertilizers assure a more accurate application of a prescribed rate of nutrients compared to blends of similar analysis.
The science involved in blending fertilizers is little understood and has only previously been touched on. There are however a number of factors to consider in getting the product manufactured to a certain quality and analysis.
From both an agronomic and handling perspective the physical form of a fertilizer is very important. Commonly issues such as 'caking', dustiness and poor particle distribution must be overcome in order to produce a product, which is free flowing, not dusty, and can withstand varying levels of humidity.
Particle size
The importance of particle size is important when one considers a fertilizer as this directly influences where it is to be used. However, there are other issues that are raised when one considers particle size.
Materials of low water solubility must be present in a small particle size to ensure that plants can fully utilize them. Good examples of this are limestone, dolomite and Sulphur.
A mesh size of 5-15 is about the size range of bulk blended fertilizers and it can be seen that an S particle of this size is oxidized to SO4 very slowly. In order for S to be oxidized to the plant available SO4 form even at moderate rates it must be of a very fine particle size. In the case of lime research has shown that coarse lime is relatively ineffective, demonstrating the fallacy of a commonly held belief that the larger lime particles impart a longer benefit because they dissolve more slowly. 
From a blending perspective particle size is also important to ensure a degree of uniformity. It has been found that materials for blending should agree in not only their upper and lower size limits but also should be reasonably similar in particle size distribution between these limits.
Segregation Properties
When handling bulk fertilizers non-uniformity referred to as segregation may occur. This is undesirable because this can affect the agronomic performance of the product and also make it impossible to obtain proper samples and meet analytical guarantees.
Segregation occurs when individual granules differ in their physical properties to such an extent that they respond differently to the mechanical disturbances caused by handling. Particles of similar physical properties tend to congregate and so the homogeneity of the mix is lost.
Contrary to what is sometimes stated research has shown that it is the particle size that has the most influence on the likelihood of this occurring and that neither the shape nor density affects segregation to an important degree. If all the particles in a fertilizer are of identical chemical composition but differing sizes, handling induces physical segregation but does not affect chemical uniformity and this is basically the situation that exists in a compound fertilizer such as Nitrophoska.
In contrast where granulation is imperfect smaller particles can differ in composition from larger ones and lead to segregation developing. In order to avoid this solutions are: better granulation or screening to narrower size ranges.
Granule hardness
Fertilizer granules should be hard enough to withstand normal handling without fracturing.
An assay of homogenous granules will almost always show little deviation among particles - but they will not be perfectly identical. Nor do they have to be for an applicator to be certain he's laid down the correct units of each element. Good granulation requires a degree of sophistication, engineering, and (usually) temperature control which only a few manufacturers have the volume and facilities to accomplish. Often, granulated homogenous fertilizers are more expensive than their blended counterparts.
Today, formulators of blended mixtures to our industry have the capability of providing uniformity of ingredients in both bagged and bulk shipments. The reason is that they are able to purchase raw materials of similar size and density - or they have the means to grind and screen products to meet prescribed specifications. In a study conducted just a few years ago by Dr. Keith Karnock, Professor of Turf Science at University of Georgia, there was very little difference in the rate of applied nutrients between quality blended turfgrass fertilizers and homogenous products.
Let your eye be the judge! When you look into a bag or bulk container of a blended fertilizer, if it appears to have good particle distribution, it will provide the same in application. If segregation is obvious, there's cause for concern.
References
Particle Size Determines the Efficiency of Calcitic Limestone in Amending Acid Soil by B.J. Scott, M.K. Conyers, R. Fisher and W. Lill: Australian Journal of Agricultural Research, Vol. 43 1992.
Comparing Sizes in lime, Journal of Agriculture vol. 42, October 2001,Department of Agriculture - Western Australia
http://www.agric.wa.gov.au/agency/pubns/journalofag/v42/lime.htm
http://www.back-to-basics.net/agrifacts/pdf/b2b29b.pdf
Jerry works for Endeavour Turf Products Pty Ltd
e-mail: jerry.spencer@etpturf.com.au
web: www.etpturf.com.au
Read more articles in Consultancy, by Jerry Spencer or from October 2011.
