Basic 14 Physical Properties of Fertilizers with Measuring

Physical Properties of Fertilizers

Physical properties of fertilizers: Fertilizer materials of very low water solubility generally must be ground to small particle size to ensure sufficiently rapid dissolution in the soil and utilization by plants. For example, the effectiveness of raw phosphate rocks generally increases with fine grinding.

Particle size control for improving agronomic properties is connected with some sparsely soluble slow-release nitrogen fertilizers such as urea-formaldehyde, isobutyldiurea, oxamide. The rate of dissolution and hence the rate of nitrogen availability from these materials are developed on particle size; the larger the particles, the slower the release.

Particle size control of fertilizers is essential to ensure good storage and handling properties. Before about 1950, almost all fertilizer materials were produced as relatively fine powders or small crystals. As a result, fertilizers usually were dusty in handling and very susceptible to hard caking during storage in piles or bags.

Granules, especially large (> about 4 mm), are desirable for aerial application to forests. The large size minimizes wind drift and reduces the lodging of granules in tree branches.

Particle size distribution is usually measured by conducting a “screen analysis” on a representative sample. However, obtaining a representative sample for screen analysis is particularly difficult because particles of different sizes segregate during any movement of the fertilizer.

Physical Properties of Fertilizers:

For testing granular fertilizers, a series of sievers suggested is U.S. standard 4-, 6-, 8-, 10-, 14-, 18- and 30- mesh, or the approximate equivalent in other sieve series.

1. Segregation properties:

Bulk handling of fertilizer may induce nonuniformity of composition throughout the bulk; this phenomenon is referred to as “segregation.” Segregation is undesirable because the resultant nonuniformity can affect agronomic response.

Segregation is a significant problem in the handling of bulk-blended fertilizers. Fertilizers are subject to vibration during transportation by trucks, rail, or other means. However, various studies have shown that such vibration is not a significant cause of segregation in most fertilizers, including typical bulk blends. Segregation due to vibration becomes essential only when the size differences between blend ingredients are extreme to the extent that particles of one ingredient are sufficiently small to “shift” downward through the void spaces between particles of another, larger size ingredient.

This type of segregation is most often encountered in handling fertilizers due to material flaws over itself. This usually is referred to as segregation due to “coning” because, in the buildup of conical storage pilers, there is a maximum opportunity for this type of segregation to occur when the material is dropped on the apex of the pile flows downward over the pile surface. The larger particles continue to travel further down the pile surface before coming to rest.

When fertilizers are propelled through the air, such as by the action of fan-type bulk spreaders, particles of different physical properties follow different paths because of varying effects of gravity and air drag. The result is segregation due to ballistic action.

2. Granules hardiness:

Fertilizers granules should have sufficient mechanical stability to withstand fracturing and without excessive sloughing to form dust. Three distinct types of mechanical strength are recognized as desirable and are discussed below.

  • Granule crushing strength
  • Resistance to abrasion
  • Impact resistance

Granule hardiness tests developed and used for fertilizers usually measure only one of these types of strength; however, in most cases, good resistance to one mechanical action is a reasonable indication of good overall acceptability.

The simplest and most widely used tests of granule strength involve determining the crushing strength of individual granules. The degree of refinement varies from the simple application of finger pressure to calibrated crushing test mechanics. The use of a simple finger test by one laboratory has been described as follows.

“A granule which could be crushed between the thumb and forefinger was classified as “soft.” If it could be crushed on a hard surface, it was regarded as being of “medium hardness.” If it remained intact when subjected to pressure by forefinger against a hard surface, it was classified as “hard.”

As fertilizers are handled, abrasion between granules can cause degradation and dust formation. Impact resistance is of interest chiefly in connection with the impact imparted by fan-type fertilizer spreaders.

4. Bulk density:

Bulk density is defined as the weight per unit volume of bulk fertilizer. Value for this property is required for bag sizing to determine the capacity of storage bins and transport vehicles and sometimes for the calibration of volumetric feeders. For routine determination of bulk density in units of pounds per cubic foot, it is convenient to provide an open-top metal or plywood box having exactly 1x1x1 ft. internal dimension.

“Loose pour density” is determined by simply pouring fertilizer into the box from a height of about 6 inches above the top of the box, leveling the top with a straightedge, and weighing on appropriate scales. It is desirable to move the fill point to avoid coning and resultant segregation. The value obtained in this manner represents the minimum weight likely to be found in a unit volume of the material.

5. The angle of repose

The angle of repose of fertilizer is the angle with the horizontal at which the fertilizer will stand when poured or dropped into a pile from a fixed overhead point. This property affects the capacity of storage areas and bins.

Reliable measurement can be made with 10kg or more of fertilizer into the farm of a small conical file while maintaining the pour point only a few centimeters above the apex of the forming pile. The angle between the pile and the horizontal can be measured directly or, by using a suitable light source, can be projected as a shadow onto a vertical screen and measured.

The angle of repose of most granular fertilizer materials is in the range of 30-40 degrees.

6. Apparent specific gravity

The apparent specific gravity of a granule may be defined as the ratio of the weight of a single granule to that of an equal volume of water (at 4C). The granule volume is that enclosed by the surface of the granule and includes any internal pores. It is numerically equal to density in g/ml.

For a given product, variations in apparent specific gravity of the granules can result in variations in hundreds, moisture-holding capacity, and storage properties.

7. Critical humidity

Critical humidity of a fertilizer material can be defined simply as that humidity of the atmosphere above which the material will spontaneously absorb moisture and below which it will not. All soluble salts, including fertilizer salts, have characteristics of critical humidifiers. Chemically speaking, the critical humidity of a salt is that humility of air at which the partial vapor pressure of water in the air exactly equals the equilibrium water vapor pressure above a saturated solution of the salt at any given temperature (30C, for example).

Critical humidity usually is expressed as “critical relative humidity (CRH),” which simply stated, is the water content of air expressed as a percentage of that required to saturate that air at the specified temperature.

8. The rate and effect of moisture absorption:

The critical humidity value alone gives no information on how rapidly moisture will be absorbed or what effect the moisture absorption will have on the physical condition of the fertilizer. Fertilizers vary considerably in their ability to tolerate absorbed moisture; thus, a method to evaluate the degree of moisture tolerance is interesting. The surfaces of bulk storage piles frequently are exposed to the humid atmosphere; therefore, it is interesting to know how rapidly moisture will be absorbed and how rapidly and what depth wetting and physical deterioration will progress.

The tests are the laboratory absorption-penetration test and the small pile bulk storage test. Two tests to evaluate flowability under humid conditions are also described: a drillability test and a laboratory flowability test.

9. Caking of fertilizer:

Fertilizers between the lime of their production and final application to the soil must be stored either the fertilizer remains completely free-flowering or that “caking” be so minimal that a small amount of normal handling will restore that original free-flowering condition.

Intergrowth of such crystals between granules or between particles of nongranular product results in caking. Contrary to a frequent misconception, caking in storage usually is not a result of moisture absorption during storage; the cause is much more likely to be the presence of excessive moisture left in the package during manufacturing or any of several other factors.

For most caking mechanisms, the presence of moisture in the form of solution-phase is essential. The higher the degree of drying at the time of manufacture, the less active the caking mechanism will be. The extent of drying required to effectively inactive caking mechanism varies rather widely with the composition and physical makeup of the fertilizer.

Mechanical weakness of granules usually promotes excessive caking. Under the pressure of storage piles, weak granules tend to deform and form relatively large contact areas between the granules. Also, the fracturing of granules can develop fines that promote caking.

High storage temperature increases the caking tendency of at least some fertilizers. If a fertilizer has a significant tendency to cake in storage. The degree of caking frequently is quite dependent on storage pressure. Pressure can be limited by limiting the height of bag stacks or bulk storage piles.

10. Chemical compatibility in blends:

The first step toward making a bulk blended fertilizer of good quality is the choice of chemically compatible ingredients. Fortunately, there are only a few combinations of major fertilizer materials that may cause trouble in this respect. Chemical incompatibility of materials may take heat development in the blend, development of wetness, gas evaluation, or caking. 

another type of incompatibility occasionally encountered is the excessive lowering of critical humidity. As pointed out in a preceding section (“critical humidity”), the critical humidity of a blend of two materials is usually lower than the critical humidity of either material alone. If this lowering is excessive, the blend may be too hygroscopic for handling, even in only a moderately humid atmosphere. In this case with mixtures of ammonium nitrate

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