Several interesting experiments have been carried out with the object of affording data for estimating the rate at which the process may go on in our soils under certain conditions. An old experiment, carried out by Boussingault, illustrates, in a general way, how rapid the process is under favourable circumstances. A small portion of rich soil was placed on a slab protected by a glass roof, and was moistened from time to time with water. The amount of nitrate of potash formed under these circumstances was estimated from time to time during a period of two months. During the first month (August) the percentage was increased from .01 to .18 (equal to about 5 cwt. of nitrate of potash per acre). The increase during the second month (September) was very much less,--indeed only about a seventh of the amount.[120] The soil experimented with was an extremely rich garden soil, and all the conditions for nitrification were most favourable. Of recent experiments on the rate of nitrification, the most striking, perhaps, are those by Schloesing. He mixed sulphate of ammonia with a quantity of soil fairly rich in organic matter, and containing 19 per cent of water. During the twelve days of active nitrification no less than 56 parts of nitrogen per million of soil were nitrified per day. Taking the soil to a depth of 9 inches, this would be equal to more than 1 cwt. per acre--an amount of nitrogen equal to that contained in 6 cwt. of commercial nitrate of soda. These experiments are interesting as showing what is probably the maximum rate of nitrification under the most favourable circumstances, and where there is an abundant supply of easily nitrifiable nitrogen. That nitrification ever takes place in our soils to this extent is not to be for a moment supposed. Warington, in his Rothamsted experiments, has found that the greatest rate, working with ordinary arable soil (first 9 inches) from the Rothamsted farm, was .588 parts per million of air-dried soil per day--_i.e._, 1.3 lb. per acre (equal to about 8 lb. of nitrate of soda). Similar soil, when supplied with ammonia salts, showed nearly double this quantity. Higher results were obtained by Lawes and Gilbert with rich Manitoba soils, the average rate being .7 parts per million per day.