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Development and Validation of a Phosphate Rock Decision Support System

Research and Market Development Division, IFDC, P.O. Box 2040, Muscle Shoals, AL 35662

View larger version (23K): [in a new window]   Fig. 1. Important and potentially important phosphate rock deposits (Van Kauwenbergh, 2003).

View larger version (33K): [in a new window]   Fig. 2. Schematic representation of the main components in the phosphate rock decision support system (PRDSS).

View larger version (39K): [in a new window]   Fig. 3. Schematic representation of all the components, factors, and processes in phosphate rock decision support system (PRDSS).

View larger version (13K): [in a new window]   Fig. 4. Relationship between the percentage of P extracted by the first (NAC1) and second (NAC2) neutral ammonium citrate extractions of seven PR sources that includes Huila PR from Colombia, which has a high free carbonate content of 13% (data from Chien and Hammond, 1978).

View larger version (102K): [in a new window]   Fig. 5. The relative agronomic effectiveness (RAE) of phosphate rock as a function of soil pH and the solubility of phosphate rock as indicated by the % P removed in a second neutral ammonium citrate (NAC2) extraction.

View larger version (77K): [in a new window]   Fig. 6. Percentage reduction in relative agronomic effectiveness (RAE) as a function of soil P fixation and the solubility of phosphate rock indicated by % P removed in a second extraction using neutral ammonium citrate (NAC2) extraction.

View larger version (16K): [in a new window]   Fig. 7. Calculation of the factor for the effect of soil organic C on relative agronomic effectiveness (RAE) in volcanic ash soils (andisols) and all other soil orders in the PRDSS.

View larger version (22K): [in a new window]   Fig. 8. An example of how the actual percentage value for a soil with an effective Al saturation of 45% is calculated based on the solubility of phosphate rock determined by second extraction neutral ammonium citrate (NAC2) and crop species. This value is then subtracted from the basic RAE value in Fig. 2.

View larger version (24K): [in a new window]   Fig. 9. The percentage reduction in RAE based on the free carbonate (CO3) content of a phosphate rock (PR) and PR solubility as determined by second extraction neutral ammonium citrate (NAC2). This example is for a soil with an effective Al saturation (EAlS) of 45%. The solid line represents the calculated line for reduction in RAE for a soil with EAlS of 45%, maize as crop in Fig. 8, and a PR that has no free CaCO3. The high free CO3 content of a rock and PR solubility both play a role in reducing the RAE due to EAlS.

View larger version (13K): [in a new window]   Fig. 10. Relative economic effectiveness (REE) of phosphate rock (PR) as indicated by a curve that is a function of the relative agronomic effectiveness (RAE) and the price ratio of P derived from water-soluble phosphorus (WSP) and PR. Values of RAE above the REE curve indicate that PR is more agronomically effective than WSP; below the curve, WSP is more agronomically effective than PR.

View larger version (19K): [in a new window]   Fig. 11. The substitution value index (SVI) is the relative amount of P as phosphate rock (PR) compared to water soluble phosphorus (WSP) fertilizer needed to give the same relative yield. An example of this would be if a phosphate rock (PR) with 80% relative agronomic effectiveness (RAE) is used. To get a 75% of the maximum yield (75% relative yield), 52 kg P ha–1 of PR compared with 23.5 kg P ha–1 of WSP is needed. The SVI is thus 52/23.5 = 2.2.

View larger version (19K): [in a new window]   Fig. 12. Relationship of relative agronomic effectiveness (RAE) estimated by the phosphate rock decision support system (PRDSS) and observed RAE calculated using data from field and greenhouse experiments. Observed results include all factors (e.g., climate, soil pH) except liming.

View larger version (25K): [in a new window]   Fig. 13. A sensitivity analysis of the estimated relative agronomic effectiveness (RAE) as calculated by the phosphate rock decision support system (PRDSS). Factors affecting RAE include phosphate rock solubility measured by a second extraction using neutral ammonium citrate (NAC2), soil pH, organic C (%), sand (%), P fixation (%), Al saturation (%), and rainfall (mm). Values in brackets are the range of values for each factor in the PRDSS.