The presence of a sufficient quantity of a base in the soil with which the nitric acid may combine, when it is formed, is another necessary condition.[112] The process only goes on in a slightly alkaline solution. The substance which acts as this salifiable base is _lime_. The presence of a sufficient quantity of carbonate of lime in the soil will thus be seen to be of first-rate importance. This furnishes an explanation of one of the many benefits conferred by lime on soils. The activity of nitrification in many soils may be hindered by the absence of a sufficiency of lime salts, and in such cases most striking results may follow the application of moderate dressings of chalk. The absence of the nitrifying organisms in certain soils, such as peaty and forest soils, may be thus accounted for. In such soils humic acids are present and the requisite alkalinity is thus awanting. _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._