King’s observations show strikingly both the continuous solubility of the soil, and the differences between the solutions derived from soils of low and high productiveness; wholly negativing the contention of Whitney that the solutions from different soils are of practically the same composition.[114] King also calls attention to the fact, shown in other experiments made in the extraction of soils without intermediate dryings, that the amounts extracted were very much less in subsequent than in the first extraction; doubtless because the evaporation from the soil particles had carried a large proportion of soluble matters to the surface, whence it was readily abstracted by the first touch of the solvent water. At each drying not only are the soluble matters again drawn to the surface, but heating a soil even to 100° renders additional amounts of soil ingredients soluble both in water and in acids. It can scarcely be doubted that the intense heating which desert soils undergo during the warm season is similarly effective; and thus the great productiveness of these soils under irrigation, and the marvelously rapid development of the native vegetation when rains moisten the parched soil, is in part at least accounted for by this immediate availability of a large supply of plant-food.
Composition of Janesville loam.—In connection with the above data given by King, it is interesting to note the composition of the soil in the above table yielding the highest proportions of soluble matter, when analyzed according to the method practiced by the writer ([see chap. 19, p. 343]). This analysis was made under the supervision of Professor Jaffa in the laboratory of the California Experiment Station by Assistant Charles A. Triebel.
Loam Soil from Janesville, Wisconsin; sample sent by Prof. F. H. King, Madison, Wis.
This soil is a light friable loam, resembling the northern Loess in color and texture; it is highly productive. It is underlaid at 5 feet by the drift gravel of that region, enclosing much calcareous material, which evidently has had a large share in the formation of this soil, just as is the case in southern Michigan.
The soil, when dried at 110° C, consisted of
CHEMICAL ANALYSIS OF FINE EARTH.
| Insoluble matter | 69.35 |
| Soluble silica | 10.89 |
| Potash (KO₂) | .59 |
| Soda (Na₂O) | .04 |
| Lime (CaO) | .83 |
| Magnesia (MgO) | .51 |
| Br. ox. of Manganese (Mn₃O₄) | .08 |
| Peroxid of Iron (Fe₂O₃) | 3.60 |
| Alumina (Al₂O₃) | 5.26 |
| Phosphoric acid (P₂O₃) | .06 |
| Sulfuric acid (SO₃) | .10 |
| Water and organic matter | 8.72 |
| Total | 100.03 |
It will be noted that in accordance with the interpretation of analyses of soils as given in the next chapter, this is a high-class soil in every respect, except that its content of phosphoric acid is only just above the lower limit of sufficiency. But as is also shown below, in presence of a large supply of lime even lower percentages of phosphoric acid are adequate for long-continued production (see chap. 19, pp. [354], [365]) by rendering the substance more freely available; and that this is true in this case is shown by the result of King’s leachings, in which this soil yields a maximum of 419 parts per million as against 80 and 64 parts in the poor soils, which at the same time yield only one fourth as much of lime. Unfortunately we have no full analyses of these other soils for comparison; although they have served as a basis of comparison for years in the Washington Bureau of Soils.
Solubility of Soil Phosphates in Water.—The solubility of the phosphate contents of soils has been elaborately investigated by Th. Schloesing fils.[115] He found in the case of a number of soils investigated by him that the amount of phosphoric acid P₂O₅ in the soil-solution ranged from less than one millionth (or one milligram per liter of water) in a poor soil, to over three milligrams in a rich one. He also found that for one and the same soil the amount so found was constant, if about a week’s time were allowed for saturation. He calculates that while in general the amount of phosphoric acid capable of being supplied to the crop during a growing season of twenty-eight to thirty weeks would suffice for but few crops, the supply so afforded is in no case a negligible quantity, frequently amounting to more than half of the crop-requirements. Experiments with various crops prove that these dilute solutions are utilized by all of them, sometimes to the extent of completely consuming the content of the solution. The much smaller content of phosphoric acid in drain waters is accounted for by the lack of time for full saturation during the time that the flow lasts. Whitney, (Bureau of Soils, Bulletin 22) has extracted the soil-solution by means of the centrifuge from several soils; the contents of phosphoric acid thus found are in general of the same order as those shown in the preceding table by King, but much in excess of Schloesing’s figures; notwithstanding the fact that Whitney’s soils had been in contact with water for only twenty-four hours. The cause of this wide discrepancy is not clear.
Practical Conclusions from Water Extraction.—As regards the practically useful conclusions to be drawn from the extraction of soils with pure water, the data given above, and especially the results obtained by King, seem to prove that there is a more or less definite correlation between the immediate productiveness of soils and the amount and kinds of ingredients dissolved; especially in the case of phosphoric acid, the adequacy of the supply of which for immediate production is assumed to be thus demonstrable by many French chemists. Moreover, a number of King’s results, tabulated in curves, exhibit a remarkable general parallelism of the curves showing totals of plant-food extracted by water, and actual crop production. This is the more remarkable since it is known to be, not pure water, but such as is more or less impregnated with carbonic acid at least, that is actually active in soil-solution and plant-nutrition. The farther development of this method may, it would seem, lead to definite conclusions at least in respect to the immediate productive capacity of cultivated, and perhaps also of virgin soils. But it is not likely to give any definite clew as to the durability of such lands.