SAMPLE-SIZE CONSIDERATION IN THE DETERMINATION OF SOIL NITRATE

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).