The determination of total nitrogen in plant tissue using a Technicon BD-40 digestor provided one of the best analytical tools for Kjeldahl digestion. The method is rapid and precise and utilizes the minimum laboratory space for such operations. The ammonia found is determined using the automated continuous flow methodology of an AutoAnalyzer. Up to 300 samples/day may be analyzed with a single system. Recovery of added nitrogen was 100.9±4.1%. Comparison with AOAC method 2.049 gave an average difference of 0.02% nitrogen for 8 plant tissues.
Fourteen laboratories participated in a collaborative study of 6 homogeneous plant tissue samples to determine the elements P, K, Ca, Mg, Mn, Fe, Al, B, Cu, Zn, and Na by plasma emission spectroscopy. Samples were dry ashed using AOAC method 3.007(a) (13th Ed.). An NBS citrus leaf standard was prepared and a portion of the resulting solution was sent to each collaborator to evaluate sample preparation errors independent of instrument error. Coefficients of variation were better than those obtained in an earlier collaborative study on multielement analysis by spark emission spectroscopy. The plasma emission method has been adopted official first action for determination of P, K, Ca, Mg, Mn, B, Cu, and Zn in plant tissues.
Seven laboratories participated in a collaborative study of wet ashing of plant material with a nitric-perchloric acid mixture and dry ashing at 500°C in order to determine the elements calcium, copper, iron, magnesium, manganese, potassium, and zinc by atomic absorption spectrophotometry. Five different homogeneous leaf tissue samples were analyzed and a synthetic standard solution containing all of the elements studied was included to evaluate sample preparation errors independent of instrument errors. The results indicated that plant tissue preparation by either wet or dry ashing gave similar elemental composition results for the tissues analyzed. The wet and dry ashing techniques have been adopted as official first action.
The measured value of soil pH depends in part on the laboratory procedures used, such as the soil-solution ratio and soil solution electrolyte composition. One of the most significant factors affecting the measured value of soil pH is the electrolyte concentration of the soil solution. Since electrolyte concentration of agricultural soils can vary greatly during the year and between years, the date of sampling can result in highly variable pH values for samples with the same percentage of base saturation when soil pH is measured in deionized water. For example, we found a different relationship between extractable calcium (Ca) and pH (1:1 in deionized water) for about 18,000 soil samples from the same geographic area taken during winter of 2 years, differing in winter rainfall. On average, samples taken during the wetter year had higher pH for a given value of extractable Ca, consistent with a reduced ionic strength (more leaching) in the wet year. In a comparison of pH in water with pH in 0.01 M calcium chloride (CaCl 2 ) for 1,186 soil samples received from clients, the median difference in pH was 0.67. It is notable that 20% of the samples had a difference of .0.8 and 10% had a difference of .0.9 pH units. Some samples with differences larger than the median may not receive a lime recommendation when needed because of the erroneously high pH reading in water caused by low ionic strength. The stability of pH readings in 0.01 M CaCl 2 essentially eliminates this problem.
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