The nature of the soil is another important condition regulating this loss. Some soils are very much opener and more porous than others; in such soils, of course, the loss by drainage will be greatest. We are apt at first sight, however, knowing the great solubility of nitrates, to overrate this source of loss. We have to remember that while nitrates are constantly being washed down to the lower layers of the soil, there is likewise an upward compensating movement of the soil-water constantly taking place. This is due to the evaporation of water from the surface of the soil, which induces an upward capillary movement of water from its lower to its higher layers.[85] This upward movement of water is very much increased, in the case of soil covered with vegetation, by the transpiration of the plants. The climate and the season of the year will affect the extent of this upward movement. Where there is a heavy rainfall it will be very much less than in dry climates. After a long period of drought the nitrates will be found to be concentrated in the top few inches of the soil; and in hot climates this sometimes takes place to such an extent that the surface of the soil has been actually covered with a saline crust, caused by the rapid evaporation of soil-water under the influence of a burning tropical sun. From this point of view it will be seen how very much less powerful a single shower of rain is—even although at the time it is heavy—in causing loss of nitrates by drainage, than a continuance of wet weather. In the former case, where the showers are separated by an interval of dry weather, the nitrates washed down into the lower layers of the soil are slowly brought up again by the capillary action caused by evaporation.
Amount of Loss by Drainage.
What the actual amount of loss is which takes place in this way it is wellnigh impossible to say. What it amounts to under certain definite circumstances has been discovered by actual experiment at Rothamsted. Taking the circumstances most favourable to extreme loss—viz., unmanured fallow land—the highest amount registered at Rothamsted for a year is 54.2 lb. per acre from soil 20 inches deep, while the smallest amount is 20.9 lb. In the former case, the drainage-water was equivalent to 21.66 inches, while in the latter, to 8.96 inches. The average for thirteen years on unmanured fallow soil has been 37.3 lb. (for 20 inches), 32.6 lb. (for 40 inches), 35.6 lb. (for 60 inches). The point of especial interest in this connection is that an annual loss of nitrogen, equal to over 2 cwt. of nitrate of soda, may take place from a comparatively poor arable soil lying fallow.
The loss on cropped soils is of course very much less—in short, should amount to very little—especially in permanent pasture, where it is reduced to a minimum. Taking an average, Mr Warington is of opinion that the loss in England may be put at 8 lb. per annum per acre.[86]
Loss in Form of Free Nitrogen.
The other chief natural source of loss of nitrogen is due to its escape from the soil in its "free" state. This source of loss is very much less important than that by drainage, and probably amounts to very little. That, however, it takes place is beyond a doubt; and that it may—as we shall see by-and-by—under certain circumstances amount to something very considerable is also proved. Where large quantities of nitrogenous organic matter decay, and where, consequently, the supply of atmospheric oxygen is insufficient to effect complete oxidation, "free" nitrogen may be evolved in considerable quantities. Similarly, it may be evolved in the case of vegetable matter decaying under water. In soils rich in organic matter the reduction of even nitrates may take place, accompanied with the evolution of free nitrogen, which is thus lost.
Total Amount of Loss of Nitrogen.
What the rate of total loss of nitrogen is from these different sources does not admit of easy calculation. Sir John Lawes, in dealing with the question of soil-fertility, estimated some years ago, by comparing the soil of old pasture at Rothamsted with that which had been under arable culture for 250 years, that during that period some 3000 lb. of nitrogen per acre had disappeared from the arable land. Examples of decrease of nitrogen in Rothamsted soils, under various conditions of culture, will be found in the Appendix.[87]
Loss of Nitrogen by Retrogression.
A source of loss of nitrogen may be here mentioned which has to do with diminution of amount of available nitrogen, rather than absolute loss of nitrogen to the soil, and which we may term loss by retrogression. Nitrogen in an available form, such as nitrates, has been found to be converted into a less available form. This retrogression may be effected, as in the case of nitrates, by reduction—i.e., by removal of the oxygen in combination with the nitrogen, which in many cases may be set free, and thus partially although not necessarily entirely lost. Such reduction is due to the action of bacteria of the denitrifying order.[88] Or, on the other hand, nitrogen may be converted into some kind of insoluble form which seems to resist decomposition and lies in an inert condition in the soil utterly unavailable for the plants' needs. A striking example of this retrogression of nitrogen seems to be afforded in the case of farmyard manure. It has been found in the Rothamsted experiments, as has been pointed out in the preceding pages, that when farmyard manure is applied, year after year, to the same land in large quantities, a very considerable percentage of its nitrogen does not (i.e., within a reasonable number of years) become available for the crop's uses. What, indeed, becomes of the nitrogen is a mystery; but it is highly probable that some such kind of retrogression as that above referred to, whereby the nitrogen is converted into some inert organic form, takes place.