Sample Size Consideration in the Determination of Soil Nitrate
1992
Starr, J. L. | Meisinger, J. J. | Parkin, T. B.
A prerequisite to increasing N-use efficiency through the use of soil testing is the accurate determination of soil nitrate concentrations. This study was conducted to determine the influence of sample size on the magnitude and variability of NO₃-N concentrations in the surface horizon of a Beltsville silt loam soil (fine-loamy, mixed, mesic Typic Fragiudult). Five different sized soil coring tubes, 1.7 to 5.4 cm in diam. (38–366 cm³) and one block sample of 8770 cm³ were used to collect soil samples. Thirty-six of each sample size were collected twice before and twice after planting corn (Zea mays L.). Nearly all the NO₃-N values were lognormally distributed with the magnitude of skewness related more to concentration than to sample size, except for the largest sample size (8770 cm³), which was always minimally skewed. This study indicated that mean NO₃-N values could be accurately estimated using 36 of the smallest sized samples. The smallest sample size, however, was often unable to accurately estimate the spatial variability of soil NO₃-N. Further, a computer-intensive random resampling procedure indicated that 20 to 50% more samples were required with the smallest sampler to achieve satisfactory estimates of the sample mean. This study did not provide a consistent basis for choosing between samplers with diameters of 2.15 to 5.4 cm (58–366 cm³).
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