Flow ability is a crucial characteristic of powder fertilizer blends that can significantly affect their handling, transportation, and overall performance. The article emphasizes the common approaches to the evaluation and control flow ability of NPK fertilizer blends to ensure their quality and performance.


Besides the caking propensity of fertilizer blends, their flow ability is also a critical characteristic for proper handling and processing of fertilizer powders. In contrast to caking ability, the flow ability of solid fertilizer blends refers to their ability to flow freely. Flow ability is a measure of how easily a solid fertilizer blend can flow through various equipment, such as hoppers, conveyor belts, and spreading machinery. When a fertilizer blend has good flow ability, it can be handled efficiently without clogging or causing disruptions in the production or application processes. Poor flow ability can lead to issues such as bridging, arching, or rat-holing, where the fertilizer particles stick together or form blockages, impeding the flow and causing inconsistent distribution. These problems can result in uneven application of fertilizers, affecting crop growth and yield.

To ensure good flow ability, several factors need to be considered during the production and formulation of solid fertilizer blends. These factors include the particle size and shape of the fertilizer particles, the moisture content, the presence of additives or anti-caking agents, and the overall formulation of the blend. By optimizing these parameters, fertilizer manufacturers can improve the flow ability of their products, making them easier to handle, transport, and apply in the field.

The following section provides information on how the flow ability of fertilizer powder can be assessed and described, allowing for more effective management and handling of these materials




    The angle of repose serves as a useful gauge for the flow ability of bulk solids like powders or granules. This swift technique provides an approximate understanding of the flow ability of powder substances. However, it's crucial to note that the angle of repose can differ based on the methodology employed to measure it. One of the simple conventional approaches is to freely pour the material through a funnel, allowing it to heap up and form an angle.

    Funnel method for measuring the angle of repose of powders
    Figure 1. Funnel method for measuring the angle of repose of powders

    The lower the angle of repose, the more flow able is a material. How to calculate the angle of repose?
    The angles on both sides of the heap should be measured, and an average value should be taken. It should be noted that even for this method, there are variations in methodology, such as the funnel being fixed or slowly moved up. If the angle of repose method is used in a laboratory to characterize the flow ability of different components, it is important to choose a single protocol and apply it consistently across all measurements.

    The formula to calculate the angle of repose is the following:
    Angle of repose = arctan(h/r), with h : height of the heap (mm), r : radius of the heap (mm)

    Measurement of the angle of repose
    Figure 2: measurement of the angle of repose

    Interpretation and relation to flow ability of angle of repose
    The primary use of the angle of repose is to evaluate the flow characteristics of granular materials such as powders and pellets. The flow ability of bulk solids is evaluated by initially measuring the angle of repose and then referring to the angle of repose table provided below to determine the powder's classification.
    Angle of repose and expected flow properties
    Table 3: Angle of repose and expected flow properties

    It should be noted that while the angle of repose is a useful method for estimating the flow ability of powders, it is an approximate method that provides only general trends. Therefore, it is recommended to complement this method with other techniques to obtain a more accurate evaluation of powder flow ability, especially if the material will be used in a specific industrial process.


    The Carr Compressibility Index and the Hausner Ratio are two measures used to indicate the flow ability of bulk solids. The Carr Index is calculated by dividing the difference between the tap and bulk density divided by the tap density, and then multiplying the result by 100. The Hausner Ratio is calculated by dividing the tap density by the bulk density of the material. Both measures are based on the assumption that the compressibility of a solid is related to its flow ability.

    Carr Compressibility Index CI = [(ρtapbulk)/ρtap] x100

    Hausner ratio formula: HR=ρtapbulk

    with ρtap : the tap density of the material (kg/m3); ρbulk : the bulk density of the material (kg/m3)
    It is possible to relate the Carr's Index and the Hausner Ratio with the following formula:

    HR = 100/(100-CI)

    A lower Carr Index or Hausner Ratio indicates that the material is more flow able. The Carr Index can be used to categorize a material's flow ability into different classes, ranging from excellent/very free flow to approximately no flow. The corresponding Hausner Ratio ranges for each class are also provided.

    These measures are useful for engineers who need to design or troubleshoot processes involving the handling or transportation of bulk solids. By using these measures, they can gain a better understanding of a material's flow properties and take appropriate measures to optimize their processes.

    Flowability expected Hausner Ratio Carr's Index
    Excellent / Very Free Flow 1.00 - 1.11 <10
    Good / Free Flow 1.12 - 1.18 11-15
    Fair 1.19 - 1.25 16-20
    Passable 1.26 - 1.34 21-25
    Poor Flow / Cohesive 1.35 - 1.45 26-31
    Very Poor Flow / Very Cohesive 1.46 - 1.59 32-37
    Approximatively no flow > 1.60 >38

    Table 4: Interpretation and relation to flow ability of Carr index and Hausner Ratio


    • The evaluation of flow ability of a powder fertilizer blend can be done using several parameters, including the repose angle, Carr compressibility index, and Hausner ratio. These parameters provide insights into the flow characteristics of the powder and can help assess its ability to flow smoothly.
    • Repose Angle: The repose angle is the maximum angle at which a pile of powder can maintain stability without flowing. The larger the repose angle, the poorer the flow ability. A larger angle indicates a higher tendency for the powder to form an unstable pile and flow unpredictably.
    • Carr Compressibility Index: The Carr compressibility index is a measure of the compressibility of a powder and provides an indication of its ability to be properly packed and flow uniformly. A lower compressibility index value indicates better flow ability. Typically, values below 15% suggest good flow ability, while values above 25% indicate poor flow ability.
    • Hausner Ratio: The Hausner ratio is a measure of the powder's bulk density and its ability to pack efficiently. A lower Hausner ratio indicates better flow ability. Values below 1.25 are generally considered indicative of good flow ability, while values above 1.4 suggest poor flow ability.
    • By evaluating these parameters, one can gain insights into the flow behavior and flow ability of a powder fertilizer blend. It should be remembered that these parameters are not the only factors to consider, and other characteristics, such as particle size distribution and moisture content, should also be taken into account for a comprehensive assessment of powder flow ability.