Representative sampling size for strip sampling and number of required samples for random sampling for soil nutrients in direct seeded fields

Abstract

Direct seeding is a common agricultural practice in the southern Canadian prairies. The associated band application of fertilizers with minimal disturbance makes conventional soil sampling problematic, as it results in considerable micro-scale variability. Sampling via the collection of strips of soil taken from across the seed and fertilizer bands has been suggested in low disturbance direct seeded fields to help account for this variability. To obtain adequate point-based random samples across entire field areas is an alternative for reliable mean soil nutrient contents. The objective of this study was to identify the representative sampling size (RSS) for strip sampling and the number of required samples (NRS) for point-based random sampling. Soil samples of 0–10 and 10–20 cm depth increments were collected from a 4 ha portion in each of two wheat fields in the Brown soil zone in south-central Saskatchewan. One field (VF field) had a history of fertilization at a variable rate while the other field (CF field) was fertilized at a constant rate. The coefficient of variation (CV) versus sampled strip length was plotted and the RSS was defined when CV ≤10 %. The central limit theorem was used to determine the NRS with relative errors of ±5 to ±20 % at a confidence level of 95 %. The results showed that the RSSs (strip lengths) were 62 and 35 cm, respectively, for assessing available nitrogen (NO3 −-N and NH4 +-N) and extractable, available P. The NRSs differed with sampled field but not with nutrient type and soil layer. With a confidence level of 95 %, about 37 and 81 random samples were needed in the VF and CF fields, respectively, to obtain mean soil nutrient contents with a relative error of ±10 %.