Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions

by Eugene W. Hilgard

Magnesia seems to be specially concerned in the transfer of phosphoric acid through the plant tissues, in the form of dimagnesic-hydric phosphate, which is rather soluble in the acid juices of plants. It is probable that, apart from the relations just referred to, such excess of lime as is known to produce chlorosis in plants interferes with the transfer of the magnesic phosphate. Some plants, as already stated, dispose of an excess of lime by depositing it in the form of oxalate, while others (such as the stone crops) excrete it on the surface of leaves and stems in the form of carbonate. But others seem to possess this power to a limited extent only.

In the case of soils containing much magnesia the proper proportion between it and lime may easily be disturbed by the greater ease with which lime carbonate is carried away by carbonated water into the subsoil, thus leaving the magnesia in undesirable excess in the surface soil. Hence the great advantage of having in a soil, from the outset, an ample proportion of lime. From this point of view alone, then, the analytical determination of lime and magnesia in soils is of high practical value.

Aso, Furuta and Katayama (Bull. Coll. Agr. Tokyo, Vol. 4 No. 5; Ibid. Vol. 6), have by direct experiment determined the most advantageous ratio of lime to magnesia in several crop plants. They find for rice and oats 1:1, for cabbage 2:1, for buckwheat 3:1; there being apparently a connection between the extent of leaf-surface and lime requirement, since leaves contain predominantly lime, while in the fruit, magnesia predominates.

Manganese.—A decided difference in the manganese content of the arid as against the humid soils appears in the table, the ratio being about 11:13 in favor of the humid soils. Manganese has not been regarded as being of special importance to plant growth in general, although, as already stated, some plants contain a relatively large proportion of manganese in their ashes; thus, e. g., the leaves of the long-leaved pine of the cotton states.[131] But no definite data showing the importance of this element to crops were available until Loew and his co-workers at Tokyo[132] established its stimulating action in a number of cases, in which crop production was materially increased by the use of protoxid salts of manganese. Aso[133] applied manganous chlorid to an experimental plot of thirty square meters, at the rate of twenty-five kilos of Mn₃O₄ per acre, and thus obtained a yield of rice one-third greater than on the control plot, at a cost of about $2.00, while the value of the increase of the product was nearly $68.00. More experimental evidence on this subject is required to establish the general value of the large-scale use of the salts of manganese; which are obtained in large quantities as a comparatively valueless by-product of the bleaching industries.

The “Insoluble Residue.”

Remembering, in discussing the facts shown by the table, that the fundamental difference between the regime of the humid and arid regions is the presence in the latter of an almost continuous leaching process, in which the carbonated water of the soil is the solvent; remembering, also, that the least soluble portion of rocks and soils is quartz or silica (sand, as usually understood), it would be predicable that this ingredient should in the humid region be found to be more abundant in soils than in the arid. This portion is represented by the “insoluble residue” of the table.

Inspection shows that both in the averages of the single states, and in both of the general averages, this difference between the soils of the humid and the arid regions of the United States is strongly pronounced; the ratio being substantially as 69% in the arid region to 84% in the humid.

We must then conclude that the leaching process must have influenced materially other soil ingredients than lime, which have remained behind in such amounts as to depress the percentage of insoluble residue in the soils. It remains to be shown what are the substances so retained.

Insoluble and Soluble Silica and Alumina.

The ingredient most nearly correlated with the insoluble residue is the free silica which remains behind with it when the acid with which the soil has been treated is evaporated to dryness. The silica is separated from the practically insoluble, undecomposed minerals by boiling with a strong solution of sodic carbonate. The amount of this “soluble silica” is obviously the measure of the extent to which the soil-silicates have been decomposed in the treatment with acid.