uantity of nitrogen as nitrates very much
greater in fallow soils than in those covered with a crop.

_Position of Nitric Nitrogen in Soil._

As we shall have occasion to see more fully in the following chapter on
Nitrification, the formation of nitrates is chiefly limited to the
surface-soil, the largest proportion being formed within the first 9 or
12 inches. For this reason we find the largest quantity of nitrates in
the surface-soil. But inasmuch as they are easily washed into the lower
layers of the soil after formation, we often find a considerable
proportion beyond the first 9 inches. The position of nitrates in the
soil thus depends very considerably on the season of the year and the
weather. In dry weather, where the evaporation of the soil-water takes
place at a considerable rate, the tendency will be to concentrate the
nitrates in the superficial portion of the soil. In wet weather, on the
other hand, the tendency will be to wash the nitrates into the lower
layers.

_Amount of Nitrates in the Soil._

The determination of the amount of nitrates in a soil is not of very
great economic importance; as this varies so much, and depends on such a
number of different conditions, such as the season, the condition of the
land, and prevailing weather. A point of very much greater economic
importance is the total amount formed in the year, and the rate at which
nitrification takes place. These questions will be discussed elsewhere,
and therefore need not here be referred to. Some interesting analyses
made at Rothamsted, however, of the amount of nitrates in soils at
different depths, merit careful consideration.

_Nitrates in Fallow Soils._

In the Appendix to the chapter on Nitrification,[76] will be found a
table containing the amounts of nitrates found in the first 27 inches of
fallow soils. The amounts vary from 33.7 lb. to 59.9 lb. per acre. The
analyses were made in September or October. In four out of the six
analyses, it will be found that by far the largest proportion is found
in the first 9 inches. In these cases the preceding summer had been dry,
and thus the nitrates had not been washed down to any depth. In the
other two cases the largest amount is found in the second 9 inches of
soil, and a considerable amount is also found in the third 9 inches.

_Nitrates in Cropped Soils._

In the case of cropped soils we find the amount of nitrates very much
less. A table containing an elaborate series