As there is a certain proportion of nitrates that finds its way even below the first 27 inches of soil, the above results do not show their total production. To accurately estimate this amount we must ascertain the quantity escaping in drainage-water. Here, again, the Rothamsted experiments furnish us with valuable data. The amount found in drainage-waters of course naturally varies very much, and depends largely on the rainfall; but taking an average of twelve years, this has been found to amount to between 30 and 40 lb. per acre--an amount not so very far short of that found in the first 27 inches of the soil itself. This was from comparatively poor soil, it must be remembered, and a much larger quantity would undoubtedly be produced in the case of richer soils. Adding then the results together, we find that in soils like those at Rothamsted, when in bare fallow, between 80 and 90 lb. of nitrogen are converted into nitrates in some fourteen months' time--an amount equal to about 5 cwt. of nitrate of soda. It is a fact of no little practical significance that nearly one-half of this large quantity is found in the drainage-water. _Amount produced at Different Times of the Year._ Some indication of the rate at which nitrification takes place during the different months of the year is obtained from a study of the results of the analyses of drainage-waters which we have just referred to. This, however, it must be remembered, only furnishes us with a very approximate indication. The month showing the greatest amount of nitrates in the drainage-water must not necessarily be regarded as that during which nitrification has been most active, for the amount chiefly depends on the rainfall. In illustration of this it will be found that the drainage-water during the autumn and early winter months contains most nitrates, not because nitrification is most active then, but because the rainfall is greatest, and a large proportion of the nitrates formed during the drier summer months is being only then washed from the soil. The amount of nitrates in drainage-waters steadily diminishes from autumn through the winter months, and is least in spring. The total amount of nitrates found in the drainage-water is, therefore, not a safe guide. What, however, does furnish us with a more reliable indication is the _percentage_ of nitrates in the drainage-water. Regarding the results of the analyses of drainage-water (see Appendix) from this point of view, it will be seen that this is greatest during the month of September, and least during April.[122] _Nitrification of Manures._ A subject which has not yet been specially referred to, but which is of great practical importance, is the nitrification of manurial substances. It is unfortunate that the amount of research hitherto devoted to this important question has been slight, and that the knowledge we possess is therefore very limited. _Ammonia Salts most easily Nitrifiable._