Some Agronomic Implications on Use of Low-Grade Phosphate Rock

Thumbnail Image
Achorn, Frank P.
Greenhill, Jeffery L.
Journal Title
Journal ISSN
Volume Title
This document explores the agronomic implications of utilizing low-grade phosphate rock in various agricultural applications. With reports of impending shortages of high-grade phosphate rock in Central Florida, many regional companies have turned to low-grade alternatives containing 62 to 68 BfL (bone phosphate of lime) for their phosphoric acid plants. However, this shift has resulted in increased phosphoric acid impurities due to the rock's nature and the different operating conditions required for its use. Consequently, meeting the tolerances needed for products such as diammonium phosphate (DAP) and triple superphosphate (TSP) has become challenging for many producers. Chemical analyses of commercial DAP samples reveal that a significant portion fails to meet the total P205 and nitrogen requirements. The reduced quality of DAP is attributed to excessive calcium sulfate levels in the phosphoric acid used during production, as well as high iron content and extended retention times in the preneutralizer. The document suggests solutions such as utilizing TVA's pipe-cross reactor (PCR) to eliminate the formation of citrate-insoluble P20s-Fe203 compounds and reduce sulfur content by optimizing reactor operation. Similarly, triple superphosphate (TSP) quality has also declined due to increased impurities in the rock and acid used during production. The impurities, including calcium, iron, sulfur, and aluminum, negatively affect the physical characteristics of TSP, making it difficult to achieve the desired screening results. Some producers reduce the acidulation ratio to address this, lowering P205 availability. Additionally, the use of ammonium sulfate solutions in TSP production raises the sulfur content of the product. While both ammonium phosphates and TSP contain significant amounts of sulfur and iron, their sulfur content is not typically sufficient to correct sulfur-deficient soils. However, it is suggested that they should still be considered as potential sources of sulfur. The document concludes that the availability of P205 will likely continue to be limited in DAP and TSP, emphasizing the importance of accounting for the unavailable P205 introduced through these products. Furthermore, it highlights the decreased water-soluble P20s in TSP and sulfur and iron's minor yet notable contribution to these fertilizers.
Micronutrients, Secondary nutrients