The AOAC indirect, AOAC direct and EEC direct available P
2O
5 methods are compared by two different collaborative studies on DAP and GTSP. The collaborative studies found the AOAC direct and EEC direct available P
2O
5 methods to be lower than the AOAC indirect P
2O
5 method when the citrate insoluble P
2O
5 was greater than 0.3% P
2O
5. Results found in the Association of Florida Phosphate Chemist Check Sample Program confirmed the lower analysis.Until 1950 the primary method of analysis for available P
2O
5 was the AOAC indirect method. A faster method for available P
2O
5 was developed and adopted in the 50's by the AOAC. Diammonium phosphate from phosphate rock was not manufactured until 1959. The current fertilizer of today was not evaluated by the AOAC direct available P
2O
5 method because the fertilizer had not been manufactured.The current practice to use the empirical method for determining available P
2O
5 for plants was evaluated in the 30's, but it is still being applied to fertilizer materials of today. Several agronomic studies with current fertilizers have shown the so-called citrate insoluble P
2O
5 to be available to plants, which implies a new agronomic study should be evaluated for today's fertilizer to determine its availability.Based on the high cost of P
2O
5 production, available P
2O
5 methods must be equal when applied to the same fertilizer. Manufacturers of P
2O
5 in fertilizers cannot tolerate a 1.0% P
2O
5 difference between different available P
2O
5 methods. No one manufacturer or buyer should have an advantage over another because of the analytical method used to determine the available P
2O
5. If the so-called citrate insoluble P
2O
5 can be proved as available P
2O
5, the manufacturer should be able to sell that product as total available P
2O
5. If every regulatory agency developed their own available P
2O
5 method, the result would be confusion in determining the available P
2O
5 and inequities in the marketing of the product. The AOAC should establish methods that are equal in value for available P
2O
5.Quick, unofficial available P
2O
5 methods used to determine large volumes of fertilizer samples can be used by regulatory agencies to speed up the analyses. When deficient fertilizer samples are found by the unofficial method, the official methods should be used to determine true availability. The quick methods of questionable accuracy should be used for screening only. Regulatory agencies should operate in a posture of being fair to the consumer and manufacturer.
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