Fertilizer Analytical Procedures

In the United States the analytical methods approved by most states are ones developed under the auspices of the Association of Official Analytical Chemists (AOAC) (3). Penalties for analytical deviation from guaranteed analyses vary, even from state to state within the United States (4). The legally accepted analytical procedures, in general, detect the solubiUty of nitrogen and potassium in water and the solubiUty of phosphoms in a specified citrate solution. Some very slowly soluble nutrient sources, particularly of nitrogen, are included in some specialty fertilizers such as turf fertilizers. The slow solubihty extends the period of effectiveness and reduces leaching losses. In these cases, the proportion and nature of the specialty source must be detailed on the labeling. 

It is customary to refer to a given fertilizer product by a series of numbers separated by dadies. This set of numbers is called the grade of the fertilizer product. Each of the numbers indicates the amount of a nutrient that the manufecturer guarantees is contained in the fertilizer product. This number includes only the amount of nutrient found by prescribed analytical procedures, thereby excluding any nutrient present in a form that is deemed to be unavailable for plant nutrition. The content of each nutrirat is always expressed as a percentage by... 

Detailed analytical procedures have been evolved for the estimation of phosphorus in such diverse materials as drinking water, sea water, food products, plants, concrete, ores and rocks, oils, metals, pesticides and fertilizers. Each of these materials has its own special analytical requirements. In the case of fertilizers, for example, it is usual to estimate (a) water-soluble phosphate which relates to available phosphorus and (b) citrate-insoluble phosphate which is a measure of the phosphorus which plants cannot utilise. In the case of seawater, both dissolved P and particulate P content may be estimated. 

The manifold designs are usually simple and straightforward when on-line columns are used to enhance sensitivity by retaining the interferents this procedure is often termed on-line clean-up" of the sample. Columns which selectively retain the interferent. but allow the analyte to pass freely, are connected in the sample introduction line either before of behind the injection valve (Fig. 4.3 a. b). Szpunar-Lobinska et al.[39] compared the two approaches in the determination of biuret in urea fertilizers by spectrophotometry, and strongly recommended the second one (Fig. 4.3 b) because ... 

There are three primary elements in fertilizers that require analytical determinations nitrogen (N) available phosphate (AP) for plant uptake expressed as P2O5 and water-soluble potassium expressed as K2O. Procedures are examined to determine values based on the best methods available for the primary elements, secondary elements, micronutrients in fertilizers, and other trace elements. Uniform methods can eliminate problems associated with fertilizer analyses, resulting in agreement between laboratories on a given sample. By following the exact procedures as outlined in an acceptable method, one can expect a reliable and reproducible chemical analysis. 

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