Multiple regression was used to quantify the relationships between NO3 in surface soil versus several selected environmental variables. Measurements were made on summer-fallowed Wood Mountain loam in the field. The temperatures and moisture conditions measured during one growing season were simulated in the laboratory using three soils. The most important factor influencing NO3 change was wetting and drying. The latter's effect was dependent on the soil moisture content just before the change occurred and/or the temperature during the period of change. Nitrification accounted for 12% of the increases in NO3 in the top 2.5 cm of soil; 88% of the increase resulted from upward movement of NO3-salts into the surface layer of soil as a result of evaporation. Of the NO3 produced by nitrification in the Wood Mountain loam, about 17.4% was credited to temperature effects. Thus only about 2.1% of the NO3 increase in the top 2.5 cm of soil in the field was due to temperature. In the simulation study, 19 C appeared to be a critical temperature above which the rate of nitrification was sufficient to counterbalance NO3 losses resulting from leaching and/or denitrification. The effect of moisture change on NO3 change appeared to be directly proportional to the total C and total N content of the three soils.