Spatial and temporal distribution of soil inorganic nitrogen concentration in potato hills

Zebarth, B. J. and Milburn P. H. 2003. Spatial and temporal distribution of soil inorganic nitrogen concentration in potato hills. Can. J. Soil Sci. 83: 183-195. The purpose of this study was to determine the spatial and temporal variation in soil inorganic N concentration in the potato hill, and to discuss the implications of this variation on soil sampling strategies. The experiment was conducted in 1999 and 2000 using four treatments: bare soil with no N fertilizer applied, and a potato crop with no fertilizer N added, with 180 kg N ha^-1 applied at planting, or with 120 kg N ha~^ applied at planting plus 60 kg N ha^-1 applied at hilling. Elevated (above background) soil NH+4-N concentrations were measured for 40 or more days after planting, therefore in-season sampling should be done for both soil NO3- -N and NH4+-N. There was a period of up to 50 days between planting and rapid crop N uptake during which loss of NO3--N from the root zone could occur. Split fertilizer application reduced the risk of NO3- -N loss during this time, but resulted in reduced tuber yield in 1999. Strong vertical variation in soil inorganic N concentration was measured in the potato hill as a result of fertilizer banding and soil N mineralization at shallow depths. Soil inorganic N concentrations were elevated in the hill, but not in the furrow, resulting in strong horizontal variation in soil inorganic N concentrations in the potato hill. Despite this variation, a systematic sampling strategy where soil was sampled in the centre of the hill, the centre of the furrow, and mid-way between the hill and furrow, done in combination with elevation control of soil sampling, resulted in an unbiased estimate of soil inorganic N concentration in the potato hill.