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

Abstract

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 -1 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 NO 3 − -N and NH 4 + -N. There was a period of up to 50 days between planting and rapid crop N uptake during which loss of NO 3 − -N from the root zone could occur. Split fertilizer application reduced the risk of NO 3 − -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. Key words: Solanum tuberosum, nitrification, nitrate leaching, mineralization, sampling strategies