The application quality of dry granular fertilizer depends on several variables. In general, the performance of a fertilizer applicator can be attributed to 1/3 operator, 1/3 applicator, and 1/3 fertilizer characteristics. In this context, operator refers to the person operating the equipment, and applicator refers to the equipment. Today, accurate placement and rate, or rates in the case of variable-rate application (VRA), during field operation are important for profitability as well as minimizing environmental risks associated with nutrient management. The 4Rs of nutrient stewardship include right source, right rate, right time, and right place, with a focus on enhancing fertilizer management.
Figure 1 presents variables that can impact the field application of granular fertilizers, thereby influencing rate and placement. Fertilizer source is crucial because nutrient concentration is among the physical properties of granular material(s) that may vary, which impacts the delivery and field deposition of granular fertilizers. All the variables listed in Figure 1 should be considered by operators before and during application. Although all are important, this publication focuses on the physical properties of various fertilizers and provides information useful for the setup of the applicator and rate-control technology to ensure accurate delivery and deposition of granular fertilizers.
Spreader operators or managers must understand the physical properties of fertilizers because operators control the ballistic nature and particle trajectories of different fertilizers. Improper setup or handling of fertilizer can lead to uneven spread patterns that may greatly affect crop growth and yield. Often, it is difficult to observe distribution or other deposition issues with the naked eye, so careful setup and calibration are required for different individual fertilizers or blends. Three main points are covered in this publication:
- definition and description of various physical properties of granular fertilizers
- how each physical property influences spread
- typical values for common granular fertilizers used within cropping systems
When applying granular fertilizer, applicators should address five considerations:
- Understand how differences in the physical properties of granular materials relate to application accuracy.
- Know the bulk density (lb/ft3) of the granular fertilizer or blend.
- Understand particle size variation. If a high variation in particle size exists (fine to nominal particle size), swath width should be reduced to limit particle segregation.
- Adjust the swath width properly for the material being spread.
- Adjust the flow divider, spinner disc speed, and fin angle in accordance with the specific material(s) being spread.
The physical properties that directly affect the quality of how granular fertilizers spread are prioritized and listed below. It is commonly understood that particle size followed by particle density are the most impactful factors influencing the deposition of granular fertilizers.
- particle size (referred to as granule size in the fertilizer industry)
- particle density
- bulk density
- particle shape
- crushing strength
- flowability
- coefficient of friction
Physical Property Descriptions
- Particle Size
Particle size is a measure of the average granule size reported by a single but nominal diameter measurement for an entire fertilizer load or sample. Since particle size varies within fertilizer particle size distribution, indicating the variability in size is normally reported for fertilizers. Particle size and size distribution have a direct influence on spread width and uniformity. Therefore, two important points need to be understood about particle size in relation to spreading fertilizers:
- Particle size significantly impacts spread width.
- Granule fertilizer presents a risk of product segregation when spread (larger particles are generally thrown further by a spreader than smaller particles).
Therefore, a wider spread width can be used for larger particles. For example, a study reported that urea with a particle diameter of 4.7 millimeters can have a spread width of 65 feet, whereas urea with a particle diameter of 1.7 mm has a spread width of only 33 ft (Broder & Balay, 1983). Secondly, the more variation in particle size within a fertilizer load, the greater the risk of uneven distribution and/or segregation. This is especially true in the case of fertilizer blends. Fertilizer with a wide range of particle sizes, including very small particles, will be difficult to evenly spread because small particles (e.g., dust and fines) may land behind the spreader (assuming no wind).
If a blended fertilizer is used, the particle diameters of different products should be within 10% of each other to avoid segregation. Alternatively stated, if the particles size of a product to be blended is within 10% of the average screen size analysis of the other product(s) in the blend, the blend will remain mixed when handled and applied to the field. Remember, particle size and size variation can vary for an individual fertilizer (Figure 2) based on its source, handling, and transportation methods. Hoftsee and Huisman (1990) and Miserque and Pirard (2004) note the influence of particle size on spreading and, most importantly, the segregation of blended fertilizers.
The particle size of fertilizers can be impacted by many factors, including transportation, conveyance, handling, and metering. These processes can reduce the size of some particles, which can increase the particle size variability within a load. A variety of metrics are used to describe particle size and particle-size distribution. Commonly, particle size is reported as the median particle size (d50) or size guide number (SGN) of a sample. Particle size distribution can be expressed using the granulometric spread index (GSI) or the uniformity index (UI) with companies possibly reporting one or both on fertilizer specification sheets. Particle size (d50), GSI, UI, or SGN are determined with the use of either sieves or a particle size analyzer. Both processes provide a measurement of diameter size distribution as presented in Figure 3. The following information and equations are used to compute these various metrics.
- d50 is the median particle size for a fertilizer sample or load and is the most common metric to report particle size. Units are usually reported in millimeters by most fertilizer manufacturers.
- SGN (size guide number) values report on the average (not median) particle size multiplied by 100. For example, a fertilizer with an average particle size of 1.5 mm equates to an SGN of 150. Using sieves, one can interpret an SGN of 150 as 50% of particles were retained on a sieve with a 1.5 mm opening.
One scenario that may require SGN measurements is the creation of a bulk-blend. The SGN can indicate the compatibility of blending individual fertilizer products. Preferred fertilizer blends have an SGN with no more than a difference of 10. This difference of 10 or less allows fertilizers to be mixed, which then allows the blend to be spread as evenly as possible while minimizing the risk of segregation. As the difference in SGN increases, incompatibility increases, leading to a high risk of product segregation during spreading. Table 1 provides some typical SGN guidelines.
| Difference in Size Numbers (SGN) | Expected Compatibility |
| 0–10 | Good compatibility |
| 11–20 | Moderate compatibility (special handling precautions may reduce segregation tendencies) |
| >20 | Incompatible |
- GSI (granulometric spread index) is a variable used to quantify particle size distribution or the variability of a fertilizer. The lower the GSI value, the more uniform the fertilizer’s particle size, which is a desirable feature for spreading. Ideally, one would prefer the computed GSI to be under 15 to ensure a uniform spread. The equation to compute GSI is:
Where:
d84 and d16 = the diameter of mass fraction at the 84% and 16% percentile level, respectively, for a sample
d50 = the median diameter for a sample
- UI (uniformity index) is another computed variable that expresses relative particle size variation. In practice, UI values within the range of 40–60 indicate that the fertilizer particles are uniform in size. The larger the UI value within this range, the more uniform the product’s particle size variation. Values outside the 40–60 range indicate large variability in particle size distribution. UI is the ratio of larger (d95) to smaller (d10) granules for a specific fertilizer multiplied by 100:
Where:
d95 = size of sieve opening that retains 95% of the sample
d10 = size of sieve opening that retains 10% of the sample
or more simply,
d95 = 95% of the amount of particles at or below this specific diameter
d10 = 10% of the amount of particles at or below this specific diameter
- Particle Density
Particle density indicates the mass-to-volume ratio of particles and is reported as lb/ft3 or kg/m3. Unlike bulk density, particle density does not include the space between individual particles but rather a measurement of the particle density itself. Beyond particle size, particle density of a fertilizer must be taken into consideration for spreader setup and evaluation of the risk of segregation for blended products.
Particle density directly affects the ballistic properties and has a direct impact on the spread width of a fertilizer. Higher-density particles can be spread wider and with higher spinner disc speeds. Lower-density particles cannot be spread as wide and have the potential to break up under higher spinner disc speeds, creating fines and dust. Therefore, as variation in particle density within a sample or load increases, so does the potential for segregation. When considering segregation and trajectory width, particle density has more impact than bulk density. It is best to select materials for granular fertilizer blends that are consistent in particle density to prevent the segregation of fertilizer products during spreading.
- Bulk Density
Bulk density represents the mass-to-volume ratio of a bulk sample, including the space between individual particles. It is reported as lb/ft3 or kg/m3. Bulk density is measured by weighing a container of known volume filled with a fertilizer sample. However, bulk density measurements can be reported in a variety of ways so it is important to have awareness of their definitions and which one to use when spreading fertilizer. Bulk density metrics can include loose (also referred to as loose pour) or packed (also called tapped). Other density metrics include true, apparent, and bulk density (see definitions below). For spreading, loose pour or bulk density is used within rate controllers or for computing setup.
- bulk density. The mass-per-unit volume of a material, including voids between particles.
- loose bulk density. The mass-per-unit volume of a material after it has been poured freely into a container. ISO 7837:1992 outlines a standard protocol for measurement.
- packed bulk density. The mass-per-unit volume of a material poured into a container followed by mechanically tapping on the container until no further volume change occurs. ISO 7837:1992 outlines standard protocol for measurement.
- apparent density. The mass-per-unit volume of a material, excluding voids between particles.
- true density. The mass-per-unit volume of a material, excluding voids between particles and all porous spaces.
Similar to particle density, if bulk density lacks uniformity, an uneven spread will result. If a rate controller is used when changing products, it is critical that bulk density is entered into the spreader setup to ensure accurate metering of the second fertilizer. Bulk density is directly related to product metering since the density is converted into the application rate (e.g., lb/ac or tons/ac).
Therefore, knowing and adjusting for changes in bulk density are important since it is tied directly to the accuracy of product metering and, thereby, the rate applied and the agronomic return.
- Particle Shape
Particle shape can vary among fertilizers. Shapes can be classified as round (spherical or egg-shaped), cubic, rectangular, and irregular. Urea and DAP are examples of spherically shaped fertilizers, whereas potash is irregularly shaped. The shape can influence the behavior of material during conveyance and distribution. Round particles generally roll along and then travel off the spinner vanes. Round particles also tend to bounce more when being metered, impacting the spinning disc/vanes. Irregularly shaped particles tend to slide along the vanes, encountering a coefficient of friction that is more influential on particle dynamics, such as exit velocity, than is encountered by spherical particles. Also, irregular particles are more prone to segregation than spherical particles. The blending of fertilizers with different shapes increases the potential for segregation. However, particle size difference has much more impact on segregation than particle shape.
- Crushing Strength
Crushing strength is defined by the International Fertilizer Development Center as the resistance of granules to deform or fracture under pressure (International Fertilizer Development Center, 1986). Crushing strength is especially helpful in gauging the handling and storage properties of a granular material and determining the pressure limits applied during bag and bulk storage. Crushing strength is expressed in kg/granule for this reason. Particle hardness or strength can govern the reaction of fertilizers to handling, transportation, storage, and application. Particle hardness measured as pounds of force (lbf) or newtons (N) refers to the amount of force that particles can withstand before rupturing and may be reported as crushing strength or particle hardness.
Storing, transporting, and spreading can all affect a granular fertilizer’s particle density and particle size. Crushing-induced variability in the granules’ physical properties may increase more in blended than single fertilizers, and problems with fines and dust can occur. Therefore, it is important to factor crushing strength and/or particle hardness so that an even spread can be attained on the field. For some fertilizer spreaders, crushing strength or particle hardness can be very influential. The flow of fertilizer particles onto the spinner disc can be harsh on some spreaders, thereby increasing the importance of disc speed and particle hardness to minimize the possibility of particles exploding.
Hardness directly influences the spread width and operating disc speed. Harder products can be spread wider and used with high spinner-disc speeds (>800 rpm). Soft fertilizers need to be spread at slower disc speeds, resulting in reduced spread widths. Soft products should be spread at disc speeds below 800 rpm. The specific speed is determined as the maximum disc speed at which no particle fracturing or shattering is observed. A quick way to measure crushing strength in the field is to apply pressure to individual granules. A simple finger test can be used to evaluate hardness or strength at the time of spreading:
- Granule crushed between thumb and forefinger is soft. Spinner disc speed is usually <700 rpm.
- Granule crushed between the forefinger and a hard surface is medium hard. Spinner disc speed is usually 700–800 rpm.
- Granule not crushed between forefinger and hard surface is hard. Spinner disc speed is usually > 800 rpm.
- Flowability
Flowability refers to a material’s ability to flow under humid conditions. It is an important property to consider during handling, metering, and deposition of fertilizers. Flowability can affect the accuracy of metering and placement. More flowable materials can be metered at higher flow rates, and their particles will tend not to stick together or bridge during conveyance. As humidity increases, less flowable materials will stick together, making them difficult to meter and evenly apply. Poor flow increases particle segregation and reduces spread width.
Flowability in blended fertilizers can impact the product segregation and spread width.
- Coefficient of Friction
The coefficient of friction is the degree of friction experienced between a material and another component, such as a spinning disc, ground surface, air, etc. A higher degree of friction will result in longer contact with the spinner discs, resulting in a larger departure angle and a more uneven spread. The coefficient of friction and particle shape are directly related to how and when a granular fertilizer particle will exit the spreader.
Summary
The application of granular fertilizers in terms of metering, deposition, and distribution are impacted by the physical properties of the product. Accuracy of product delivery is critical to ensure that the right rate and place are achieved to maximize crop yield and farm profitability. In order of importance, the physical properties that impact the quality of fertilizer granular spread are particle size, particle density, bulk density, particle shape, crushing strength, flowability, and coefficient of friction. A spreader operator and manager should understand how these properties influence spread quality to properly set up and operate equipment for individual fertilizers while also maintaining acceptable delivery accuracy.
Acknowledgments
This fact sheet was reviewed by Dena Wilson and Katrina Cornish, PhD, Professor, Department of Food, Agricultural and Biological Engineering, The Ohio State University; Benjamin Boelter, Highway Equipment; Eric Richer, Extension Educator (Agricultural and Natural Resources), The Ohio State University; Curt Woolfolk, Senior Agronomist, The Mosaic Company; and Miles Grafton, PhD, Professor, Massey University.
Common Values for Different Physical Properties
| Product | Grade |
Loose bulk density5 |
Particle density |
d50 (in.) |
Crushing strength5 |
Coefficient of friction |
| Prilled Urea | 46-0-0 | 45–51 | 139 | 0.09 | 0.8–1.2 | 0.3 |
| Granular Urea | 46-0-0 | 45–51 | 76 | 0.09 | 1.5–3.5 | 0.3 |
| Prilled Ammonium Nitrate | 34-0-0 | 53–61 | 104 | 0.09 | 1.2–1.7 | 0.7 |
| Crystalline Ammonium Sulfate | 21-0-0 | 62–69 | 82–102 | 0.06 | 1.5–2.5 | 0.5 |
| Ammonium Sulfate | 21-0-0 | 49–65 | 82–102 | 0.06 | 1.5–2.5 | 0.5 |
| Diammonium Phosphate (DAP) | 18-46-0 | 54–66 | 100 | 0.11–0.13 | 3.0–5.0 | 0.5 |
| Granular Monoammonium Phosphate (MAP) | 11-52-0 | 56–66 | 97 | 0.09 | 2.0–3.0 | - |
| Powdered Monoammonium Phosphate (MAP) | 10-50-0 | 53–62 | - | - | - | - |
| Granular Triple Superphosphate (TSP) | 0-46-0 | 59–75 | 124 | 0.10 | 1.5–3.5 | - |
| Ammonium Phosphate | 16-20-0 | 56–75 | - | - | - | - |
| Muriate of Potash (KCl) | 0-0-60 | 64–75 | 100 | 0.09 | 2.43 | - |
| Product | Grade |
Loose Bulk Density |
Particle Density |
d50 (mm) | Crushing Strength (kg/granule) |
Coefficient of Friction |
| Prilled Urea | 46-0-0 | 720–820 | 1200–1300 | 2.2 | 0.8 | 0.3 |
| Granular Urea | 46-0-0 | 720–820 | 1200–1300 | 2.2 | 1.5–3.5 | 0.3 |
| Prilled Ammonium Nitrate | 34-0-0 | 850–975 | 1800 | 2.2 | 1.2–1.7 | 0.7 |
| Crystalline Ammonium Sulfate | 21-0-0 | 1000–1100 | 1315–1640 | 1.5 | 1.5–2.5 | 0.5 |
| Ammonium Sulfate | 21-0-0 | 785–1040 | 1315–1640 | 1.5 | 1.5–2.5 | 0.5 |
| Diammonium Phosphate (DAP) | 18-46-0 | 880–1050 | 1600 | 3.0–3.2 | 3.0–5.0 | 0.5 |
| Monoammonium Phosphate (MAP) | 11-55-0 | 900–1050 | 1550 | 2.4 | 2.0–3.0 | - |
| Powdered Monoammonium Phosphate (MAP) | 10-50-0 | 850–1000 | - | - | - | - |
| Granular Triple Superphosphate (TSP) | 0-46-0 | 950–1200 | 2000 | 2.7 | 10–38 | - |
| Ammonium Phosphate | 16-20-0 | 900–1200 | - | - | - | - |
| Muriate of Potash (KCl) | 0-0-60 | 1030–1200 | 1600 | 2.3 | 48 | - |
Additional Resources
Case study: Distribution uniformity of a blended fertilizer applied using a variable-rate spinner disc spreader
(elibrary.asabe.org/abstract.asp?aid=44059)
Effect of vane number on distribution uniformity in single-disc rotary fertilizer spreaders
(elibrary.asabe.org/abstract.asp?aid=21998)
Evaluating of broadcasting uniformity of centrifugal and oscillating granular broadcasters
Fertilizer consistency, bulk blending of fertilizer material: effect of size, shape, and density on segregation
(pubs.acs.org/doi/10.1021/jf60131a020)
Fertilizer manual (3rd ed.)
(search.worldcat.org/title/37001658?oclcNum=37001658)
Granular fertilizer particle dynamics on and off a spinner spreader
(doi.org/10.1016/S1537-5110(03)00062-X)
Uniformity of granular fertilizer applications with a spinner truck
(DOI:10.13031/2013.16062)
References
Broder, M. F., & Balay, H. L. (1983). Effect of granule size on application [ Conference presentation]. Fertilizer Industry Round Table, Washington D.C., United States.
Hoftsee J. W., & Huisman, W. (1990). Handling and spreading of fertilizers part 1: Physical properties of fertilizer in relation to particle motion. Journal of Agricultural Engineering Research, 47, 213–234.
doi.org/10.1016/0021-8634(90)80043-T
International Fertilizer Development Center (IFDC). (1986). Manual for determining physical properties of fertilizer. International Fertilizer Development Center, Muscle Shoals, Alabama.
ureaknowhow.com/wp-content/uploads/2015/08/1986-IFDC-Manual-for-Determining-Physical-Properties-of-Fertilizer.pdf
Miserque, O., & Pirard, E. (2004). Segregation of the bulk blend fertilizers. Chemometrics and Intelligent Laboratory Systems, 74(1), 215–224.
doi.org/10.1016/j.chemolab.2004.03.017
Originally written July 28, 2016, by John Fulton, PhD, The Ohio State University; and Kaylee Port, Project Coordinator, The Ohio State University.