The Misunderstood "Filler" Materials

Let’s start off right by understanding that “fillers” in fertilizer products are completely unavoidable and absolutely necessary for turf managers to properly make fertilizer applications. When you hear the term “filler” in fertilizer, it typically summons a negative connotation to mind. Fillers are traditionally thought of as serving one purpose, adding weight to the bag or container. Nothing could be further from the truth.

Perhaps the root cause of this misconception of “fillers” is the perceived definition of what they truly are. Most people believe that all fillers are nothing more than inert materials that add no value beyond finishing out the weight requirement of a bag because they do not directly contribute to the health of the turf or soil as they are not plant feeding substances. Rubbish, I say!

Yes, one of the main benefits that most fillers provide is adding weight and bulk to a product formula; typically calculated on a 2,000-pound base. That benefit alone is incredibly important to the safety of applying fertilizer, but depending on the filler material used, there are far more positive attributes that often go overlooked and undervalued.

What “Fillers” Actually Do

Let’s start with determining a more accurate definition of what “filler” materials are and, more importantly, what they do.

Filler: A material added to fertilizer products that provide one or more of the following benefits (in no particular order of importance):

Dilute/reduce the concentration of the nutrients in the product
Provide additional weight and/or bulk to the product formula
Ensure/enhance product performance (dust suppression, prevent caking, etc.)
Adjust soil pH

Let’s examine each of these concepts to fully understand the important role fillers play in today’s fertilizer products.

Dilution/Reduction of the Nutrients

Filler material is used to dilute and/or reduce the concentration of the product’s nutrients. While it may seem odd to want to do this, it’s actually quite necessary for a number of reasons. It’s easy to understand that in any bag, jug or container of fertilizer, all contents are not pure nutrients. In fact, it’s impossible. There are no fertilizer products with an NPK analysis of 100-0-0. All forms of granular and liquid raw material components are not 100% plant nutrient concentrated.

Urea has the highest nitrogen content (46%) than any other commonly used turfgrass raw material component. The remaining 54% of urea consists of carbon, hydrogen, and oxygen. A 50 lb bag of nothing but pure urea has an analysis of 46-0-0, which means there is only 23 pounds of actual nitrogen in that bag. Urea represents the top end of the raw material’s nutrient concentration.

Here are some other commonly used raw material component’s concentration levels.

Ammonium Sulfate consists of 21% N, 24% sulfur and the remaining 55% consists of hydrogen and oxygen.
MOP (Potassium Chloride) typically has a K content of 51% while the remaining 49% consists of chlorine.
MAP (Monoammonium Phosphate) consists of 11% N, and 22% P; the remaining 67% consists of hydrogen, oxygen, and carrier material.

Since the raw materials are already diluted from the start, it begs the simple question on why fertilizer products require further dilution. The answers are pretty simple…for the safety of the product and the ease of application.

Safety of the Product

Although raw material components are manufactured in a diluted form, that does not mean that they are safe enough to apply directly to turfgrass. Yes, many professionals can and do apply these raw components quite successfully, but the key word here is “professionals”. When a product is being designed it cannot be assumed that professionals, who know how to navigate the risks associated with raw component applications, will be the only people applying them. It’s the responsibility of the manufacturer to create an effective product that is also safe to apply for non-professionals.

Turfgrass can easily be damaged by applying too much nutrients, especially nitrogen. The delicate leaf blades, stems and roots can burn from an over-application of even safely designed products, much less products that are loaded to the maximum levels of nutrients that physics will allow. For this reason, filler helps mitigate the burn potential of fertilizer products.

Ease of Application

Unlike many agriculture crop fertilizer applications, turfgrass applications are almost always applied at much lower rates. By this very nature, granular turfgrass fertilizer products necessitate the inclusion of filler material to successfully hit these desired lower nutrient rates.

For example, if you wanted to apply straight urea (46-0-0) to one acre of turf at 0.75# of nitrogen per 1,000 square feet, you would need to apply 1.63 lbs. of urea per 1,000 square feet or 71 lbs. per acre. That’s a very small amount of product being applied over a large area. The margin of error for this application is extremely small to successfully hit the target rate.

Filler material added to a product formula increases the margin of error to help make hitting the target rate easier and safer. This is also the main reason why most fertilizer products rarely exceed 30 units of nitrogen per bag.

Another example illustrates this idea effectively. If you wanted to apply a 32-0-6 at the same 0.75# of nitrogen per 1,000 square feet, you would only need to apply 2.35 lbs. of the product per 1,000 square feet or just over 102 lbs. per acre. This is an increase of 42% more product being applied than straight urea, making it easier and safer to hit the target rate.

The “sweet spot” for most turfgrass granular applications, in terms of the amount of product applied, is around 3.0 to 3.5 lbs. per 1,000 square feet. This rate is high enough to ensure an effective, on target, application rate but low enough to not require excessive amounts of product be applied to achieve the desired nutrient application goal. These rates can and do vary with respect to the turfgrass site the product is being applied to.

The rule is quite simple…the lower the nutrient numbers on the fertilizer bag, the more it must be applied to hit any application target.

This is where fillers play a very critical role, allowing for an easier application of the product.

Provide Additional Weight and/or Bulk to the Product Formula

It’s constantly assumed that this lone filler benefit is the only reason why they are used at all, which is incorrect.

Years ago, the majority of turfgrass fertilizers were homogeneous composition rather than blended. Many people still refer to them as “pure fertilizer” products since each granule in the bag contains the various listed nutrients on the label and looks virtually identical. From the “eye test”, these products appear to be filler free, however, they aren’t…you just can’t see them. These homogeneous products also contain filler; they always have for all the other reasons explained in this section. The main reason homogeneous fertilizers contain fillers is to increase the bulk density of the product.

As the turfgrass industry shifted their preference from homogeneous to blended fertilizer products, mostly due to a fairly significant price point difference, the filler simply became more visible once the bag was opened. Nonetheless, however, the filler is still fulfilling the exact same function, increasing the bulk density of the product.

So, this begs the question as to why bulk density is so important in fertilizer product formulations.

Granular fertilizer products need to have sufficient bulk density to be properly applied since lighter weight products don’t spread very well. Most broadcast spreaders, both walk behind and tractor-mounted, use a spinning disc or discs to apply fertilizer evenly and in consistent dispersion widths. Lighter bulk density materials cannot be thrown nearly as far, or as accurately, as heavier bulk density materials. Fillers provide the extra weight needed to develop a fertilizer product that can be applied safely and effectively.

Where fillers became vilified as just “rocks in the bag” directly correlates to when the industry started demanding less expensive fertilizer bag prices. Raw material nutrient components are far more expensive than any filler components. In essence, the industry traded down from expensive nutrients being in the bag to instead utilizing considerably cheaper fillers; all for the sake of lowering overall bag price.

One of the most disturbing trends in T&O industry over the last couple of decades has been a significant drop in the actual amount of actual nutrients contained in a bag of fertilizer. This development has occurred largely because distributors, lawn care companies and end-users shifted their focus to the cost per bag or per application rather than the cost per nutrient that’s being applied. Consequently, over time, the typical N-P-K analysis of an average fertilizer product has dropped significantly.

Ideally, most N-P-K fertilizer analysis should be formulated to carry the highest possible concentration of plant nutrients, which will keep the filler content as low as possible. Very rarely will a blended fertilizer product ever be completely filler free. The challenge facing our industry comes in the form of changing the mind set of how we think about the cost of fertilizer in a holistic sense. Rather than calculating the economics of fertilizer applications on the simple price per bag basis, the time has come to think more all-encompassing by evaluating and considering other expense related factors, such as freight cost.

A trucking company doesn’t care how much filler is in a bag of fertilizer. A pallet of low N-P-K fertilizer weighs the same as high N-P-K fertilizer. However, when six bags of a low N-P-K product are now needed to deliver the same nutrients as three bags of a high N-P-K product, the freight costs just doubled. It simply makes more common sense to spend a little more per bag on higher N-P-K fertilizer than to spend more money on freight to ship extra bags.

Don’t misunderstand the message here, lower N-P-K fertilizers have a place in the turf world. However, in terms of balancing both product options, the pendulum has swayed decisively toward the low N-P-K side. As long as the industry continues to demand these products, fillers will continue to be a part of what’s going into those bags.

Ensure/Enhance the Product’s Performance

There are many “filler” materials that go into almost every fertilizer bag that have no nutritional benefit at all…but that’s by design. These materials are all designed and included in the product formula for the exact same purpose, to ensure or enhance the product’s performance.

Dust Suppression Agents

At one time or another, it’s likely that you’ve poured some fertilizer into a spreader and the moment you started to apply it, a cloud of dust spewed from the spreader. There are few things more aggravating to an applicator than dusty fertilizer products. Unfortunately, it’s a natural part of creating fertilizer products.

There are multiple reasons why fertilizer may come out of the bag dusty.

All fertilizer raw material components have some level of inherent dustiness associated with them, and some components are inherently dustier than others. Potash components, for instance, are notoriously dusty. Straight urea is moderately dusty. And methylene urea, as well as most manufactured components, are not very dusty at all. Since blended fertilizers contain a variety of these components, most finished products will also have some fundamental dustiness to contend with.

Product degradation is another reason why dust accumulates in a fertilizer product. When this happens, the dust is generated by the breakdown of the components into fines, also known as attrition. This attrition can be caused by several factors.

For homogeneous and granulated products, utilizing proper production techniques are crucial to create a product with as little attrition as possible. Proper crush strength and particle shape must be achieved to maintain product integrity and minimize attrition.

For blended products, ensuring that the components are similar in shape is the main concern. Most blended products use round or semi-round shaped components, which is why they experience less attrition than granulated products. However, when pelletized components are included in a product formula, such as lime or gypsum, their jagged edges rub together and break down all the components during the agitation process.

Improper storage and handling practices can also cause dust to accumulate due to product degradation. Every single time a bag of fertilizer is moved, the components shift and rub against one another which leads to breakage that causes the creation of dust. Transfer and drop points should be minimized to reduce the opportunity for breakage.

All these potential dust generating situations are precisely why dust suppression agents play an important role in successfully manufacturing the least dusty fertilizer products possible.

Dust suppressants are materials used to control particulate matter. By including dust suppressants in a product’s formula, they significantly help in reducing the amount of all forms of dust, both intrinsic and incidental. The inclusion of a dust suppressant simply ensures better overall product quality.

The good news is that a little amount of dust suppressant goes a long way in fertilizer products. The actual amount of dust suppressant that ultimately gets added to any product formula is generally around .1%.

Anti-Caking and Anti-Dusting Agents

Another one of the most challenging problems you may have experienced when opening a bag of fertilizer was seeing some clumps and chunks of particles stuck together. These clumps are called “agglomerates”, while this unfortunate occurrence is referred to as “caking”. Caking always occurs after the product has been produced and put into bags.

The underlying reason caking happens is the result of a physical reaction between the contact points of the individual particles in the bag, which are triggered by the environmental factors of humidity and temperature. The actual physics of caking and the formation of agglomerates boils down to being caused by: the formation of liquid and solid bridges due to capillary condensation, continuing chemical reactions, dissolution, and molecular attractions in the fertilizer.

In simpler terms…the particles simply get stuck together for one of two reasons: (1) because moisture has found its way into the bag and kickstarted chemical reactions among the particles or, (2) because the molecules of some particles are inherently attracted to the molecules of other particles.

Most fertilizers have natural tendencies to form agglomerates during storage. This natural inclination is exasperated by several factors, such as relative humidity, temperature, storage time and pressure.

Relative humidity is the primary culprit that causes caking. Each raw material component has a critical relative humidity value, which is the point that a particular component begins to absorb moisture from the air. As particles absorb moisture, they start to gel and stick to each other. Fertilizer materials that have a higher critical relative humidity can be advantageous in humid environments as they experience less caking.

Air temperature also highly influences the probability for caking to occur. Higher temperatures promote chemical reactions in the bag, while lower temperatures impede them. Plus, cooler temperatures mitigate high humidity conditions, reducing the chance of particle interaction.

Storage time directly impacts the caking process simply because the longer a fertilizer product sits in a bag after being filled, the higher the likelihood that caking will occur. Fertilizer products, like most everything, do not last forever. If you’ve ever found an old bag of fertilizer sitting around, it is very likely you saw agglomerates sitting in the bag. Given enough time, most fertilizers will turn into one solid brick of unusable nutrients.

Pressure also increases the potential for caking due to the particles being forced together. While most raw material components possess high particle integrity, increased pressure will weaken them which will lead to cracking and promote the caking process. For this reason, fertilizer bags that are stacked on the bottom of a pallet are more at risk than the ones on the top layer of the pallet.

With all these factors in mind, this is precisely why every fertilizer product label includes a section on storage and handling recommendations; to help mitigate caking and the formation of agglomerates.

One additional external aspect that deserves special call-out, due to its direct influence on caking potential, is dust; or more specifically the amount of inherent dust that ends up in the bag. The more dust that gets into a bag, the more it will promote the development of the crystal bridging from condensation between individual particles.

The best preventative deterrent to caking is the inclusion of anti-caking and anti-dusting agents that are included into every fertilizer product’s formula. These helpful additives come in many different forms, depending on how the fertilizer is manufactured.

In a nutshell, effective and suitable fertilizer anti-caking/anti-dusting agents should have the following attributes:

Provide effective, long-lasting caking/dust control
Be relatively easy to apply to dry fertilizer during manufacturing
Be made of ingredients suitable for safe use in manufacturing as well as all downstream uses and applications
Not undermine the treated fertilizer’s ability to comply with applicable regulatory requirements
Enhance the treated fertilizer’s performance and marketability
Be economical

There are numerous anti-caking/anti-dusting agents on the market that are all designed to meet these criteria. The good news about these specific “filler” materials is that they do not take up much room in a bag.

For example, a popular anti-caking agent is called Attaclay; which is basically a hydrated magnesium silicate form of clay that absorbs and locks away moisture. Inside a typical 50 lb. bag of fertilizer there is approximately 12.9 ounces of Attaclay…or 1.61% of the bag.

Another popular anti-dust control agent is soy oil, which attracts and draws away the dust particulates that lead to caking. Inside a typical 50 lb. bag of fertilizer there is approximately 1.4 ounces of soy oil…or 0.17% of the bag.

As you can clearly see, these are two types of very important “filler” materials that provide and enormous number of benefits to the products that get applied to turfgrass.

Adjusting the Soil’s pH

One of the main benefits of some filler material is that they assist in raising the pH of the soil. Most soils in the U.S. are moderately acidic. Rainfall is the greatest factor in creating this acidity because, over time, rain leaches away the alkaline elements in soil, creating a more acidic soil. Additionally, the overall number of trees play a factor in this concept because fallen leaves and pine needles slowly decay and contribute to the acidity level of the soil. However, another significant contributing factor is the use of nitrogen heavy fertilizer applications. This is where the use of limestone fillers can help undo the soil damage without counteracting the benefits with the inclusion of dolomitic and calcitic limestone.

But’s let take a step back. For those who may not fully remember soils class, the “H” in pH stands for “Hydrogen”, which is why it’s capitalized, a reference to its periodic table symbol. Strangely enough, there is some controversy within the scientific community as to what the “p” stands for. One side argues it means “power”, resulting in pH literally meaning “power of Hydrogen”. The other side contends that the “p” refers to the classic Latin origins of potentia hydrogenii (capacity of hydrogen). While yet another group of scientific scholars claims it simply means “potential”. Regardless of this rather pointless, academic debate, they all agree on what the pH scale is and what it represents.

The pH scale is a logarithmic, numeric scale from 0 to 14 that is used to specify the acidity or basicity (alkalinity) of an aqueous solution (or in our case, the soil). A solution/soil that has a pH of 0 is very acidic with a high concentration of hydrogen ions. A pH of 7 is neutral with equal concentrations of hydrogen ions and their basic counterpart hydroxy ions. A pH of 14 represents a high concentration of hydroxy, alkaline ions.

In much simpler terms, pH refers to the concentration and activity of hydrogen ions in the soil. This is incredibly important to turf managers because the pH directly affects the turf plant’s ability to absorb available nutrients in the soil.

Most turf grasses thrive at a soil pH between 6.0 and 7.0. If the soil’s pH is below 5.5 or higher than 7.0, the turf plant won’t be able to get the nutrients it needs from the soil.

More specifically, when soil pH falls below 5.5, it is too acidic. Acidic soils cause deficiencies in several key turf grass nutrients, including phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca) and molybdenum (Mo). Additionally, in acidic soils, manganese (Mn) and aluminum (Al) become more available and can become toxic to the turf.

Conversely, when the soil pH rises above 7.0, it is too basic. High alkaline soils cause deficiencies in phosphorus (P) and almost all the micronutrients become completely unavailable. Basic soils also greatly diminish microbial activity in the soil. Since most soils are inherently acidic, the most common corrective maintenance practice to neutralize the soil’s pH is applying lime or limestone.

Which brings us back to the two most frequently types of utilized filler material in blended turf fertilizer products are dolomitic limestone and calcitic limestone. Previously it was discussed that the term “filler” is often perceived negatively due to the misleading belief that the only purpose they serve is adding weight to the product; often, being referred to as “rocks in the bag.” The predominant use of these limestone products is where that misconception originates.

Dolomitic limestone is a combination of calcium carbonate and magnesium carbonate; typically comprised of 50% calcium carbonate and 40% magnesium carbonate.

Calcitic limestone is almost pure calcite; typically comprised of 90% calcium carbonate and less than 5% magnesium carbonate.

Both limestone sources look very similar and serve the same two purposes in the soil; to neutralize acids and provide some additional calcium (and magnesium in dolomite’s case) for turf plant uptake. They are screened to ensure that 90% of the particle sizes range between 2.00 – 3.00 mm (SGN 200 - 300).

While limestone includes both calcium and magnesium, which are essential nutrients for healthy plant growth, it's not an adequate substitute for fertilizer. However, it is an excellent source as a filler in blended fertilizer products. This is precisely why managing soil pH is so important and why the inclusion of dolomitic or calcitic limestone provides value.

Final Thoughts

Overall, filler materials have a bad reputation due to a lack of understanding of the benefits and values they bring to a product. Quite simply, without their inclusion in blended fertilizer products, it’s highly unlikely that the product would perform as expected. When considering the total benefits outlined above, fillers should be viewed as an integral fertilizer component that should not be easily dismissed as unnecessary.