Physical properties of solid fertilizers

In the following section, most of the physical properties of solid fertilizers will be discussed. 

Fertilizer 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 rock generally increases with fine grinding down to a particle diameter of about 150 pm below that, little further benefit has been established [12]. Other materials of low solubility that require relatively fine grinding include basic slag, limestone, dolomite, dicalcium phosphate, and fused phosphates [13,141. Micronutrient or secondary 

Other fertilizers that benefit agronomically from particle size control are some of the sparsely soluble slow-release nitrogen fertilizers such as urea-formaldehyde, isobutylidene diurea, and oxamide. The rate of dissolution, and hence the rate of nitrogen availability from these materials, has been shown to be dependent on particle size the larger the particles, the slower the release. 

Compaction is a dry granulation process in which mixed raw materials are agglomerated by pressure. One of the characteristics of the feed raw materials that affects compactibility is particle size. Generally, in order to obtain optimum compaction, the feed raw materials should fall within a wide range of particle sizes, preferably in the range of 0.1-1.0 mm. Material smaller than 0.1 mm will likely contain too much air, and particles larger than 1.0 mm will require energy to crush them to a smaller size [16,171. 

Fertilizers preferably should be free from partides so smalt that they generate air-borne dust during handling and application, not only to ensure farmer acceptance but also to prevent unhealthy working conditions and environmental damage around fertilizer plants and bulk-heindling operations. 

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Fertilizers are packaged in a variety of different forms including solids, liquids (solutions and suspensions), and gas (anhydrous ammonia). The important physical properties of solid fertilizers are particle size, particle strength, caking tendency, chemical stability, and hygroscopicity. 

There are a number of other physical properties of solid fertilizers which are of importance for various reasons. These physical properties are briefly discussed below. 

Urea, a highly water-solnble white crystalline compound, has by far the highest nitrogen content among solid fertilizer compounds (46.6%) as noted, its commercial synthesis was introduced by BASF in 1922 (fig. 7.1). Its good physical properties and lack of fire and explosion hazards make it the most commonly used solid nitrogen fertilizer, while liquid urea is commonly used in nitrogen solutions, and the prilled compound is a common part of mixed fertilizers. 

Physical and Chemical Properties. Ammonium nitrate is a white, crystalline salt, dj° = 1.725, that is highly soluble in water, as shown in Table 3 (7). Although it is very hygroscopic, it does not form hydrates. This hygroscopic nature compHcates its usage in explosives, and until about 1940, was a serious impediment to its extensive use in fertilizers. The solid salt picks up water from air when the vapor pressure of water exceeds the vapor pressure of a saturated aqueous ammonium nitrate solution (see Table 4). 

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