Only takes place in slightly Alkaline Solutions.
But while a certain slight amount of alkalinity is necessary, this must not exceed a certain strength, otherwise the process is retarded. This is the reason why strong urine solutions do not nitrify. The amount of carbonate of ammonia generated in them by putrefaction renders the development of nitrification impossible by rendering the alkalinity of the solution too great.[113] The practical importance of this fact is considerable, as it shows the importance of diluting urine very considerably before applying it as a manure. Similarly, when large quantities of lime, especially burnt lime, are applied to soils, the result will be to arrest the action of nitrification for the time. The presence of alkaline carbonates in the soil, unless in minute quantities, is apt, therefore, to seriously interfere with the process.[114]
Action of Gypsum on Nitrification.
It has been found by Pichard that the action of certain mineral sulphates is extremely favourable to the process, and among these gypsum. Warington has carried out some experiments on the action of gypsum in promoting nitrification. The reason of its favourable action is probably because it neutralises the alkalinity of nitrifying solutions. It thus permits the process to go on in unfavourable conditions. Where, therefore, too great alkalinity exists for the maximum development of nitrification, the best specific will be found to be gypsum.[115] The practical value of gypsum as an adjunct to certain manurial substances, where nitrification is desired to be promoted as rapidly as possible, such as sewage and farmyard manure, will thus at once become apparent. So far as there is a proper degree of alkalinity maintained, the presence of large quantities of saline matter does not seem to interfere with the process.
Presence of Oxygen.
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.
Another of the conditions determining the rate at which nitrification takes place, and one which is most important, is Temperature. According to Schloesing and Müntz the temperature at which maximum development takes place is 37° C.[117] (99° F.), at which temperature it is ten times as active as at 14° C. (57° F.) Below 5° C. (40° F.) the action is extremely feeble. It is clearly appreciable at 12° C. (54° F.), and from there up to 37° C. (99° F.) it rapidly increases. From 37° C. (99° F.) to 55° C. (131° F.), at which temperature no nitrification takes place, its activity decreases; at 45° C. (113° F.) it is less active than at 15° C. (59° F.), and at 50° C. (122° F.) it is very slight. These results by Schloesing and Müntz have not been exactly confirmed by Warington. He has found that a considerable amount of nitrification goes on at a temperature between 3° and 4° C. (37° and 39° F.), while the highest temperature at which he has found it to take place is considerably lower than 55° C. (131° F.) Thus he was unable to start nitrification in a solution maintained at 40° C. (104° F.) It would thus seem that the nitrifying ferments are able to develop at lower temperatures than most organisms; and although nitrification entirely ceases during frost, yet in a climate such as our own there must be a considerable proportion of the winter during which nitrification is moderately active.
Presence of a sufficient quantity of Moisture.
The presence of moisture in a soil is another of the necessary conditions of nitrification. It has been shown that it is at once arrested, and indeed destroyed, by desiccation. Other conditions being equal, and up to a certain extent, the more moisture a soil contains the more rapid is the process. Too much water, however, is unfavourable, as it is apt to exclude the free access of air, which, as we have just shown, is so necessary, as well as to lower the temperature. During a period of drought the rate at which nitrification takes place will, therefore, be apt to be seriously diminished.