Percentage of Nitrate 1857. of Potash. = lb. per acre. August 5 .01 34 August 17 .06 222 September 2 .18 634 September 17 .22 760 October 2 .21 728 NOTE VII. (p. 188). NITROGEN AS NITRATES IN ROTHAMSTED SOILS AFTER BARE FALLOW IN LB. PER ACRE. ------------+-----------+---------------------------+-----------+---------- | Alternate | Four-course rotation. | | | Wheat +-----------+---------------+ | Depth of | and | Super- | | Claycroft | Foster's Soil. | Fallow. | phosphate | Mixed Manure. | Field. | Field. | | only. | | | +-----------+-----------+-------+-------+-----------+---------- | 1878. | 1878. | 1878. | 1882. | 1881. | 1881. ------------+-----------+-----------+-------+-------+-----------+---------- | lb. | lb. | lb. | lb. | lb. | lb. 1st 9 ins. | 28.5 | 22.3 | 30.0 | 40.1 | 16.4 | 14.6 2d 9 ins. | 5.2 | 14.0 | 18.8 | 14.3 | 26.5 | 24.6 3d 9 ins. | - | - | - | 5.5 | 15.9 | 17.3 ------------+-----------+-----------+-------+-------+-----------+---------- Total | 33.7 | 36.3 | 48.8 | 59.9 | 58.8 | 56.5 ------------+-----------+-----------+-------+-------+-----------+---------- CHAPTER V. THE POSITION OF PHOSPHORIC ACID. We now come to consider the position of phosphoric acid in agriculture. The question is, however, very much simpler in its nature than that of nitrogen, and may be consequently discussed in a much shorter space. Most soils, as we have already had occasion to point out, are better supplied with available ash-plant ingredients than available nitrogen compounds. The quantity of phosphoric acid absorbed by the plant is also less than that of nitrogen; and lastly, the different chemical compounds of phosphoric acid occurring in the soil are not nearly so numerous as those of nitrogen. Phosphoric acid, however, must be regarded as ranking next to nitrogen in its importance as a soil-constituent. _Occurrence of Phosphoric Acid in Nature._ That phosphoric acid is of universal occurrence may be assumed from the fact of the almost universal occurrence of vegetable life on the earth's surface; for plants are unable to grow without it. While thus of practically universal occurrence, its amount in most soils is very trifling. As the only source of it in the soil is from the disintegration of the different rocks, a short description of its occurrence in the mineral kingdom may first be given. _Mineral Sources of Phosphoric Acid._