The relation of dissolved oxygen, soluble carbon, and redox potential to the movement of nitrate in soil columns and into submerged tile lines was studied. Columns 300 cm long were drained at depths of 180, 240, or 300 cm with the water table in all columns maintained at 175 cm. After adding ammonium nitrate and irrigating, the peak in nitrate concentration moved downward to the 160‐cm depth, but nitrate concentrations decreased at 180 cm and almost disappeared at the 240‐ and 300‐cm depths. The soil solution at 180 cm contained an average of 5.2 ppm nitrate‐nitrogen and a maximum of 16 ppm. In contrast the soil solution at 240 or 300 cm, which had passed through a submerged zone of 65 or 125 cm, averaged 0.5 ppm and had a maximum concentration of 1.1 ppm. Disappearance of nitrate was associated with decreases in redox potential, oxygen content of the soil solution and oxygen levels in the soil atmosphere, and with increases in soluble iron and manganese. Denitrification occurred without submergence if the soil redox potential was sufficiently low. Results of this experiment indicate that nitrate concentrations probably can be reduced in tile effluents by submerging the tile lines.

A field plot study was conducted to determine the effects of added alfalfa, four flooding times, and two temperatures on EH (Oxidation‐reduction potential) and dissolution of iron and manganese from a silty clay soil. With no alfalfa added, flooding caused only a small decrease in EH at either temperature. The addition of alfalfa at the higher temperature resulted in a reduction in EH to −100 millivolts. Flooding time had no significant effect on EH until the third day, when the soils subjected to shorter flooding periods began to dry out. Flooding without organic matter increased the Mn2+ and Fe2+ concentrations in solution to maxima of only 3.7 and 2.1 ppm, respectively; while the combination of flooding, organic matter and high temperature increased the levels to 46 and 30 ppm. Flooding time had no effect on the dissolution of iron and manganese. The results of this study show the importance of temperature and organic matter on dissolution of iron and manganese and the relation between EH and soluble iron and manganese.

The decrease in soil nitrate nitrogen as related to redox potential was studied in the laboratory and in the field in Imperial Valley, Calif.Partial anaerobic conditions were induced in laboratory flasks containing an atmosphere of 21% oxygen by adjusting the soil‐water ratio. When the water content of a soil having a water saturation percentage of 48% was increased to 44.5% or above, large losses of N2 gas occurred both with and without addition of organic matter. Significant denitrification coincided with a decrease in the redox potential (Eh).Measurement of redox potential and nitrate concentrations of ground water sampled at various depths in an irrigated field indicated that nitrate concentration was high near the surface but decreased at depths approaching the water table. The diminishing nitrate concentration and redox potential, with depth, indicate that some nitrates were reduced to nitrogen gas before they reached the water table.Only 1.5% of the 280 kg N/hectare applied to a cotton crop (Gossypium hirsutum L.) was discharged in the tile effluent during the growing season. Denitrification in the soil profile apparently reduced the amount of nitrate reaching the tile drainage system.

A field study of waterlogging and subsequent drying of the soil profile has shown that under irrigation culture in Imperial Valley reducing conditions became prevalent. Reducing conditions, as indicated by declining Eh values, became most favorable for dissolution of Mn and Fe near the soil surface. However, the concentrations of Mn2+ and Fe2+ were lowest near the surface, probably because of their leaching from this zone, and the shorter time of contact between soil solution and soil particles. The concentration of Fe2+ and Mn2+ were higher in the deeper horizons of the soil profile.In the plow layer, the reducing intensity increased, beginning 9 m away from the tile and reaching a maximum 18‐ to 23‐m from the tile drain. The Eh levels in the soil profile declined immediately after irrigation began and rose immediately after irrigation stopped indicating the importance of atmospheric oxygen to the oxidation‐reduction status and, therefore, to the solubility of iron and manganese compounds. Soluble organic carbon apparently was not related to the concentration of Mn2+ or Fe2+ in the soil solution even though the decomposition of soil organic matter is important in oxidation‐reduction reactions in the soil profile.

The redox potential (Eh) of irrigated desert soils was evaluated under a wide range of conditions. Factors important in controlling Eh were temperature, flooding time, soil water content, and energy source. Field heterogeneity necessitated using 10 to 20 electrodes (placed in a 30‐cm square) to characterize a treatment. The Eh varied over a short distance with variations not due to poisoning or erratic electrode readings.A 5C increase in temperature at the 15‐cm depth resulted in a 50‐mV decrease in redox potential. The length of soil saturation time correlated directly with the decrease in Eh. When the soil was not saturated during irrigation (sprinkler or drip), Eh decreased less than when the soil was flooded. The amount of energy available to microorganisms has a major effect on how low the Eh decreased in a flooded soil.