_Artificial Sources of Loss of Nitrogen._ So far, the sources of loss of nitrogen considered have been what we may term _natural_ sources. By this is meant that the loss of nitrogen from the above sources takes place in a state of nature, and not merely under conditions of cultivation. No doubt the loss due to drainage is very much greater under arable farming than would be the case where artificial drainage does not obtain; still, under any conditions, this loss must be reckoned with. On the other hand, by _artificial_ sources of loss are meant those entirely dependent on our modern system of agriculture and our modern system of sewage disposal, whereby the nitrogen contained in that portion of the produce of the farm which goes to supply our food is not returned to the soil, but is totally lost. _Amount of Nitrogen removed in Crops._ The modern tendency towards centralisation in large towns has rendered this loss--despite all that has been said to the contrary--a necessity. It is extremely difficult, however, to form any estimate of its amount. We know, of course, the amount of nitrogen removed from the soil by different crops. We cannot, however, estimate how much of this may find its way back again to the soil. The amount of nitrogen contained in the different crops will be fully dealt with in the chapter on the manuring of different crops. It may be, however, not without interest to give here some approximate indication of the amount of this loss, in order to render the view of the subject as comprehensive as possible. Recent agricultural returns for Great Britain give the total produce of _wheat_ at over 76 million bushels, that of _barley_ at over 69 million, and that of _oats_ at over 150 million. Calculating the amount of nitrogen, these quantities of wheat, barley, and oats respectively and collectively contain, and calculating also how much _sulphate of ammonia_ and _nitrate of soda_ these amounts of nitrogen represent, the following are the results:-- Nitrogen. Sulphate of Nitrate of Ammonia. Soda. Bushels. Tons. Tons. Tons. Wheat 76,224,940 37,432 176,465 227,266 Barley 69,948,266 27,324 128,813 165,896 Oats 150,789,416 56,835 267,936 345,068 ----------- ------- ------- ------- Total 296,962,622 121,591 573,214 738,230 =========== ======= ======= =======