Spatial and temporal variation in soil inorganic N concentration, and soil test P and K, in red raspberry fields and implications for soil sampling strategies

Fertilizer is commonly applied as a band in red raspberry (Rubus idaeus L.) fields, resulting in complex spatial and temporal variation in soil inorganic N concentration, and in soil test P and K. The objectives of this study were to determine the spatial and temporal distribution of soil inorganic N in red raspberry fields receiving different N fertility treatments, to use the data to determine the most appropriate sampling strategies for estimating the quantity of soil inorganic N at various times during the growing season, and to evaluate the same sampling strategies for soil test P and K. Treatments were a control that received no manure or fertilizer N, 55 kg N ha(-1) as urea or as Duration T60, a slow release N fertilizer, banded in mid-April, or 100 kg total N ha(-1) as solid broiler manure broadcast or banded in early March, or banded in mid-April. Soil inorganic N was sampled at 10 inter-row locations 8, 23, 38, 53, 68, 83, 98, 113, 128, and 143 cm from the crop row, and for 0-15, 15-30, and 30-60 cm depth, for four sampling dates for the control and urea treatments, and for 0-15 and 15-30 cm depth on one sampling date for the remaining treatments. Random sampling and four systematic sampling strategies were evaluated for their bias in estimating soil inorganic N concentration and soil test P and K, and with respect to the number of soil cores required to achieve a given precision and probability level combination. The random sampling strategy gave unbiased estimates of soil inorganic N and soil test P and K, however, the number of cores required to obtain a given precision at a given probability level were generally greater than for the systematic sampling strategies. The systematic sampling strategy involving sampling only in the crop row and in the centre of the inter-row, the current industry standard, gave expected values that could sometimes be substantially lower than the true value, and was therefore not recommended for use in raspberry fields. The best systematic sampling strategy used samples collected from the crop row, from the fertilizer band, from the centre of the inter-row, and from midway between the fertilizer band and the centre of the inter-row.