An experiment in which the effect of increasing periods of chloroform
fumigation on nitrification in soil was determined indicated that the
active nitrifying bacteria were located at sites in the soil which wer
e readily accessible to diffusing gas. This suggestion was supported b
y the distribution of potential nitrifying activity in relation to soi
l pore size, which indicated that nitrifying bacteria were concentrate
d in soil pores in the size range 136-214 mu m.