The nitrification bacteria belong, it would seem, to the aerobic[116] class of ferment--_i.e._, they cannot develop without a free supply of oxygen. Exclusion of the air is sufficient to kill them, and in those portions of the soil where access of air is not freely permitted, nitrification will be found to be correspondingly feeble. Thus it has been found in experiments with different portions of soils, that but little signs of nitrification occur in the lower soil layers. According to experiments by Schloesing on a moist soil, in atmospheres respectively containing no oxygen and varying quantities of it, the action of oxygen in promoting nitrification was strikingly demonstrated. In an atmosphere of pure nitrogen, entirely devoid of oxygen, the process no longer took place, but the nitrates already present in the soil were reduced and free nitrogen was evolved. In an atmosphere, on the other hand, containing 1.5 per cent of oxygen, a considerable amount of nitrification took place; while in the presence of 6 per cent, nitrification took place to double the extent. An addition of 10 to 15 per cent again doubled the quantity. When the amount of moisture added was increased, the effect of larger percentages of oxygen was found to be less marked. The reason of this is that the oxygen probably acts as dissolved oxygen; the addition of water meaning at the same time an addition of available oxygen. This condition exemplifies the value of tillage operations. The more thoroughly a soil is tilled the more thoroughly will the aeration of its particles take place, and consequently the more favourable will this necessary condition of nitrification be rendered. The benefits conferred on clayey soils by tillage will in this respect be especially great. _Temperature._