pounds are water insoluble (Sikora and Giordano, 1995). The European Economic Community, for example, set An increase in the concentrations of water-insoluble iron phosa minimum level of 93% of total available P (water ϩ phate (Fe-P) compounds in acidulated P fertilizers has raised the citrate) as water-soluble P in fully acidulated P fertilizers concern that the agronomic effectiveness of these P fertilizers will marketed in the European community (Anonymous, decrease. This study was conducted to evaluate the agronomic effectiveness of three sources of single superphosphate (SSP) varying in 1975). Several researchers have found that the water-Fe-P impurities for upland and flooded rice (Oryza sativa L.) grown insoluble fractions of acidulated P fertilizers after water for 55 d. A modal chemical analysis and X-ray diffraction were used leaching are lower in agronomic efficiency compared to characterize the SSP samples and their water-leached forms. A with the original fertilizers without leaching or MCP. greenhouse study was conducted where rates of 0, 5, 15, 30, 50, andThe P availability of the isolated water-insoluble frac-100 mg P kg Ϫ1 as total available P (water plus citrate-soluble P) were Int. Fert. Dev. Cent.

An increase in the concentrations of water‐insoluble iron phosphate (Fe‐P) compounds in acidulated P fertilizers has raised the concern that the agronomic effectiveness of these P fertilizers will decrease. This study was conducted to evaluate the agronomic effectiveness of three sources of single superphosphate (SSP) varying in Fe‐P impurities for upland and flooded rice (Oryza sativa L.) grown for 55 d. A modal chemical analysis and X‐ray diffraction were used to characterize the SSP samples and their water‐leached forms. A greenhouse study was conducted where rates of 0, 5, 15, 30, 50, and 100 mg P kg−1 as total available P (water plus citrate‐soluble P) were applied from each SSP source and monocalcium phosphate (MCP) to a Hiwassee clay loam (thermic Rhodic Kanhapludult). The water‐soluble P contents of the SSP sources were 46, 80, and 86% of the total available P (water + citrate) corresponding to an increase of Fe content (2, 4, and 7%) in the phosphate rock sources used for SSP production. The main Fe‐P impurities in the SSP samples were identified as Fe3NaH8(PO4)6·6H2O and Fe3H9(PO4)6·6H2O. Single superphosphate with only 46% of water solubility was 91% as effective as MCP in increasing dry matter yield and 76% as effective for P uptake by upland rice. The other two SSP sources were as good as MCP in effectiveness for upland rice. All of the SSP sources were equally as effective as MCP in producing dry matter yield and P uptake by flooded rice.

Two of the most common impurities found in superphosphates (single superphosphate [SSP] and triple superphosphate [TSP]) in the forms of Fe3KH8(PO4)6·6H2O and Fe3KH14(PO4)8·4H2O were synthesized (H8‐syn and H14‐syn, respectively), characterized, and agronomically evaluated to investigate cost‐effective means to optimize the utilization of phosphate rocks (PRs) containing Fe impurities. A solubility study showed that more P was released from both compounds as pH increased in the 0.01 M KCl solutions (pH 3.0–7.5) and more P was released from H14‐syn than H8‐syn. The two Fe‐K‐P compounds were mixed and compacted with monocalcium phosphate (MCP) at 0, 25, 50, 75, and 100% of total P as MCP. In a greenhouse study, rates of P were applied at 0, 10, 20, 40 and 80 mg P kg−1 from H8‐syn, H14‐syn, and MCP, while the compacted mixtures were applied only at 40 mg P kg−1 to an Ultisol (thermic Rhodic Kanhapludults, pH 5.3) cropped with upland and flooded rice (Oryza sativa L.) for 65 d. The results showed that P uptake and dry‐matter yield were greater with H14‐syn than H8‐syn for both crops and both compounds were more effective for flooded rice than upland rice. The calculated values of relative agronomic effectiveness (RAE) of H8‐syn and H14‐syn with respect to MCP were 32 and 72% in dry‐matter yield for upland rice and 55 and 102% for flooded rice, respectively. To reach 90% of maximum dry‐matter yield obtained with MCP, it required approximately 43 and 35% of total P as water‐soluble P (WSP) in the mixtures of H8‐syn and H14‐syn with MCP for upland rice and only 17 and 11% for flooded rice, respectively.

A study was made to determine the fate of fluorine in the production of wet-process phosphoric acid and to explore methods of effective pollution control. Results indicated that the quality of the acid is improved by precipitation of the metallic impurities as complex fluoride compounds, and up to 85% of the fluorine can be immobilized as an insoluble compound in the by-product gypsum solids. Twelve different fluorine compounds were identified as possible precipitates in the production of wet-process phosphoric acid. Fluorite and chukhrovite were sufficiently insoluble in aqueous solutions to suggest long-term storage in gypsum ponds without undergoing reactions that will release fluorine to the atmosphere or natural waters.

There is concern that the use of lower quality phosphate rock can result in elevated amounts of Fe–Al–P water‐insoluble compounds in fertilizers and, consequently, low agronomic effectiveness. Therefore, studies were conducted to evaluate the effect of some of these compounds on plant growth. Four commercial superphosphates varying in chemical composition (two single and two triple superphosphates) were selected for the study. Fertilizer impurities were collected as water‐insoluble residues by washing each P source with deionized water. A modal analysis, based primarily on elemental chemical analysis and x‐ray diffractometry, was used to estimate the chemical composition of each P source. Water‐soluble monocalcium phosphate (MCP) and the water‐leached fertilizer residues were prepared to give a range of fertilizers in terms of water‐soluble phosphorus (WSP) (0–100% of the available P as MCP). The water‐leached fractions, MCP, and the mixtures of MCP with water‐leached fractions were applied to supply 40 mg available P kg−1 to a thermic Rhodic Kanhapludult with pH values of 5.2 ± 0.05 (unlimed) and 6.4 ± 0.08 (limed). Wheat (Triticum aestivum L.) grown in a greenhouse for 101 d served as the test crop. The requirement for WSP was source and pH dependent. At a soil pH of 5.2, the fertilizers required 73 to 95% WSP to reach the maximum dry‐matter yield, while they required 60 to 86% WSP at pH 6.4. To reach 90% of the maximum yield, all superphosphate fertilizers required <50% WSP. These results show that it is not always necessary to have high water solubility as required by legislation in many countries.