Incubation tests were run to compare nitrification rates of soil from different depths of a Piano silt loam (Typic Arguidoll). Nitrates accumulated most rapidly during the first week of incubation (5 ppm NO3‐N/day) in samples taken from 15 cm of the soil surface. A reduction in rate to 2 to 3 ppm NO3‐N/day after the first week could be due to reduced NH4substrate. Nitrates did not accumulate during the first three weeks of incubation for subsoil samples taken below 30 cm. After three weeks the accumulation rate was 1.2 ppm No2‐N/day or less. It is concluded that nitrification is largely confined to the Ap horizon in the Piano silt loam.