By Sean R. Jordan, T.Ag., Agronomist
My first job in turfgrass management was intended to be summer work
between semesters at university to help pay for the next year; but,
after one summer on the green, I never went back to that university.
It wasn’t long after I started that particular job that I realized turfgrass management is a very diverse field where, in any given day, a person could be dealing with many different subjects, including soil science, diseases, insects, equipment, fertility, irrigation and, most of all, human resources (specifically dealing with people). With managing all of the aforementioned aspects, it is not difficult to get lost in the many numbers that we constantly encounter in turfgrass management. 
This article, the first in a series intended to shed light on the different numbers associated with fertility and fertilizers, specifically focuses on particle sizing and how it can affect nutrient distribution.
First, some basic terminology to make sure everyone reading this article is on the same page (no pun intended):
Size Guide Number (SGN) – The diameter, expressed as millimeters x 100, of the fertilizer granules based on the median (or mid-point) within the batch. It means that half of the fertilizer granules are larger than the set SGN and half are smaller. This is determined by passing the fertilizer through various sieves and using the amounts retained by each to calculate the SGN. For example, a fertilizer of SGN 250 will have 50 per cent of its particles retained on or around a sieve with a 2.5-millimetre opening.
Uniformity index – A unit of measurement that indicates the consistency of particle sizing within a fertilizer material. The higher the number, the greater number of particles that are close in size to the given SGN. Fortunately, many fertilizers that are relatively high in uniformity index are available, ensuring that the majority of the particles in the product are at or very close to the labeled SGN.
To select the appropriate particle size for a turf area, there are several factors to consider, the most important being height of cut (HOC). The smaller the particle size, the more particles that will be distributed per unit area and this will then affect the efficacy of the distribution of nutrients.
For example, a fertilizer with an SGN of 100 could have 15 times more particles in a given area than a SGN 300 product. When applying nutrient sources that will produce a colour response such as nitrogen, iron and magnesium to an area with a lower height of cut (e.g. golf course greens, tees and fairways and lawn bowling greens) it is necessary to use a fertilizer with a lower SGN to ensure a better distribution of nutrients.
Using a larger fertilizer granule than what the height of cut dictates could result in a speckled appearance, especially if the turf is “hungry” or the soil is cold. Additionally, if a slow-release fertilizer uses a coating as its control mechanism, matching the particle size to the height of cut is especially important to limit the number of particles damaged by reel mowers. If the coating is compromised, the nutrient will be released all at once, negating the effect of the slow-release mechanism.
Considering the number of particles applied per unit area, you may wonder what the best way to apply minor nutrients might be to ensure uniform distribution, considering you are dealing with such small quantities of each nutrient. The two options for mixed fertilizers are blended and homogeneous. With blended fertilizers, there are a variety of prills, each supplying one or more nutrients, and, assuming they are well mixed prior to application, there should be a relatively uniform distribution of each per unit area.
With homogeneous fertilizers, they are produced by taking all of the nutrients in their necessary proportions, grinding them down into a powder and then regranulating them with some type of binding agent that will break down when wet. Both blends and homogeneous products are subject to the same physical limitations of particles per-unit-area, so the same rules apply with regard to selecting a particle size.
Choosing whether to use a blend or homogeneous product really depends on the budget allowed for the management of a particular area. Spraying or applying homogeneous fertilizers would certainly ensure that the nutrients are applied more efficiently and the colour response would be even, but this comes at a higher cost-per-unit nutrient price and additional labour.
Many times, these sprayed or homogeneous applications can be used to supplement granular-based nutrient programs. Larger areas such as roughs rarely justify the higher cost of homogeneous fertilizer, so we have to rely on multiple applications of blended fertilizer to ensure that nutrients are distributed across the whole of the turfed area over time.
In addition to basing your choice of particle size on height of cut, the immediate goal of the fertilizer should be considered as well. When establishing, overseeding or correcting for certain deficiencies such as phosphorous, using a smaller particle size can improve results. Phosphorous, unlike nitrogen and potassium, does not move very far in the soil from where it is applied.
New grass plants have a limited root system and, once they have taken up all of the phosphorous that they can intercept, the phosphorous becomes limiting. By using a lower SGN product, there is a more even distribution of the phosphorous, and this means that more phosphorous is likely available to each individual plant.
In the end, there are so many options for fertilizers out there with respect to particle size, release rate, etc., it is often difficult to sort out what makes sense for your turf site and what will yield the best results over time. There is a lot more to developing a fertilizer program than just choosing the appropriate SGN, but it is certainly an important consideration when choosing your products.
The results of choosing an incompatible particle size is not pretty and, worse than that, it is wasted money for an end product that is less than desirable. Hopefully this article has helped to clarify at least one piece of the complex puzzle that is fertility and fertilizer use and, the next time you go out and choose a fertilizer for your site, you will keep in mind where you are applying the product, the management regime involved and the immediate as well as long-term goals of the application when you choose your SGN.
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