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The Movement of Water and Nitrate Around Bands of Sodium Nitrate in Soils and Glass Beads¹

ABSTRACT

The movement of water and nitrate (NO₃^-) around bands of NaNO₃ was studied in soils and glass beads in the laboratory. A hypothesis was developed and tested concerning the predominantly downward movement of NO₃^- from a band of NaNO₃ in soils with moisture contents greater than field capacity. The hypothesis of events is as follows: Soil water moves, under the osmotic force generated, to the band of NaNO₃; then the NaNO₃ dissolves and diffuses away from the band. However, the effect of the diffusion is reduced when sufficient mobile water is present in the surrounding soil to maintain the influx of water. As a result, the NaNO₃ solution accumulates in the vicinity of the band. Once the accumulating solution exceeds that which can be held by the capillary forces of the soil, the excess NaNO₃ solution literally drops out of the band under the force of gravity. The drop-out of NaNO₃ solution continues until essentially all of the NaNO₃ has been dissolved.

This drop-out phenomenon occurred in systems in which the water was present initially in quantities greater than field capacity, was moved through the system by leaching, or was moved into the system by capillarity from a shallow water table. Evidence supporting the details of the hypothesis resulted when various shields were used to alter the NO₃^- movement around the fertilizer bands under the leaching and "static" moisture conditions.

¹ Contribution from the U. S. Soils Laboratory, Soil and Water Conservation Research Division, ARS, USDA, Beltsville, Md. Presented before Div. S-2, Soil Sci. Soc. Am., Nov. 20, 1963.

² Research Soil Scientists, Beltsville, Md.

Received for publication December 26, 1963. Accepted for publication March 20, 1964.