The Romance of Modern Chemistry

CHEMISTRY AND AGRICULTURE which are carried out in the wonderful little laboratories of the plant. All very well, the reader will say, the plant may take in the bulk of its nitrogen in the form of nitrates from the soil, but how do the nitrates come to be there at all ? To understand this it is necessary to remember that the atmosphere contains small quantities of nitrogen in the combined form, namely, as ammonia, a compound of nitrogen and hydrogen, and as nitric acid, which, as already stated, is a compound of nitrogen, hydrogen, and oxygen. The ammonia in the atmosphere has been given oft' from decaying organic matter, and the nitric acid is due to the power of an electric discharge, such as light- ning is, to induce the nitrogen and oxygen of the air to combine to some small extent. Now these two nitrogenous substances, ammonia and nitric acid, the one an alkali and the other an acid, dissolve easily in water, and are either absorbed by the soil direct, or are washed down into it by the rain. Quite a large amount of combined nitrogen gets into the soil in this fashion, in addition to what is already there as the remains of earlier vegetation. Experiments carried out at Rothamsted have shown that the total quantity of nitrogen carried to the soil by rain in one year is between four and five pounds per acre. When ammonia compounds get into the soil their latter end is near, for there they are tackled by micro-organisms whose object in life it is to convert all other nitrogenous bodies into nitrates. Since, from the point of view of the plant, a nitrate is a much more digestible form of nitrogen than any ammonia compound, these nitrifying bacteria are valuable agents in the nourishment of the plant. Apart from the carbon, nitrogen, hydrogen, and oxygen, 222