A curious instance of the effects of continued warm maceration in rock decomposition is afforded by the highly ferruginous soils derived from the black basaltic lavas of the Hawaii Islands. These lavas, like the basalt sheet of the Pacific Northwest, contain a large amount of crystallized phosphate minerals, notably apatite and vivianite. A correspondingly large proportion of phosphoric acid is found in the soils derived from these rocks, up to nearly two per cent.[148] But almost the entirety of this substance is present in the form of an insoluble, basic iron compound, difficultly soluble even in acids, and rendering it wholly unavailable to vegetation. So that actually the most pressing need of most of these soils is phosphate fertilization. The same is probably true of some of the highly ferruginous soils of California and of the Cotton States.
Sulfuric Acid.—From the absence of the leaching process in the soils of the arid region, we should expect that sulfates would be more abundant in them than in the soils of the humid. This is certainly true in the case of the alkali soils, which are characteristic of the regions of deficient rainfall. [See below, chapter 22].
Hence the showing made in the general table, indicating that sulfates are equally abundant in the soils of the humid than in those of the arid regions, is surprising in view of the efflorescences of alkali sulfates so frequently observed in the latter. This is obviously due to the fact that the majority of such alkali soils has, on account of their local nature and usually heavy lime content, been excluded from the comparison; which otherwise would have made a very different showing.
Potash and Soda.—The compounds of the alkali metals potassium and sodium, being on the whole much more soluble in water, even without the concurrence of carbonic acid, than those of calcium and magnesium, the leaching process that creates such pronounced differences in the case of the two earths must affect the alkali compounds very materially. Comparison of the soils of the two regions in this respect shows, indeed, very great differences in the average contents of potash and soda. For potash the ratio is .216 to .670 per cent on the general average, and .187 to .670 per cent, in the average by states; for soda, .140 per cent to .350 per cent on the general average, and .110 per cent to .420 per cent in the average by states. For both, therefore, the general average ratio is as one to between three and four for the humid as against the arid region.
It is curious that an approximation to the ratio of one to two, or somewhat less, is maintained in the average proportion of soda to potash in both regions; but this does not by any means hold good in detail, very high potash-percentages being often accompanied by figures for soda very much below the above ratio. This is the result of an important difference in the chemical behavior of the two alkalies, which has already been alluded to in connection with the discussion of the zeolites. ([See chapter 3, p. 38]).
The process of “kaolinization,” being that by which clays are formed out of feldspathic minerals and rocks such as granite, syenite, trachyte, etc., results in the simultaneous formation of solutions of carbonates and silicates of potash and soda. These coming in contact with the corresponding compounds of lime and magnesia, also common products of rock decomposition, are partly taken up by the latter, forming complex, insoluble, hydrous silicates (zeolites). In these, however, potash whenever present takes precedence of soda; so that when a solution of a potash compound is brought in contact with a zeolite containing much soda, the latter is partially or wholly displaced and, being soluble, tends to be washed away by the rainfall into the country drainage. Hence potash, fortunately for agriculture, is tenaciously held by soils, while soda accumulates only where the rainfall or drainage is insufficient to effect proper leaching, and in that case manifests itself in the formation of what is popularly known as “alkali soils;” namely those in which a notable amount of soluble salts exists, and is kept in circulation by the alternation of rainfall and evaporation, the latter causing the salts to accumulate at the surface and to manifest themselves in the form of saline crusts or efflorescenses. Alkali lands are a characteristic feature of all regions of scanty rainfall, and are found more or less on all the continents. The substances composing the alkali salts are retained not only in their soluble form, but by their continued presence influence profoundly, in several ways, the processes of soil formation. A more detailed discussion of this important subject is given in chapters 22 and 23.
Arid Soils are Rich in Potash.—One of the most important practical conclusions flowing from the comparison of the potash contents of the humid and arid soils respectively is that while in the former, potash is usually among the first substances to be supplied by fertilization when production languishes, in the arid regions it will as rule come last in order among the three ingredients commonly so furnished. Aside from the water-soluble potash salts always forming part of the salts of the alkali lands proper, which in many cases will alone hold out for many years under the demands of cultivation,[149] they rarely contain much less than one per cent of acid-soluble potash; occasionally rising as high as 1.8 per cent. That in such lands potash-fertilization is uncalled-for and ineffective, hardly requires discussion; while on the other hand, phosphates are commonly required for full production after ten or fifteen years of cultivation without returns. Nitrogen usually comes next in order, but sometimes is the first need.
The constant indiscriminate purchase and use of all three ingredients, so urgently recommended by fertilizer manufacturers because of their success in the humid Eastern States, is therefore very poor economy for the farmers of the arid region. Excepting cases of very intense culture, e.g. of vegetables or berries, the use of potash salts is but rarely remunerative, and therefore uncalled-for, in arid soils for a number of years.
Humus.—The figures shown in the table for the average humus-percentages in the soils of the two regions do not adequately represent the very important differences actually existing; partly because of the inadequate number of determinations made by the same method (Grandeau’s), partly because of the differences in the composition, and especially in the nitrogen-content of this substance, which render direct comparison delusive. A detailed discussion of the marked differences existing between the humus of arid and humid soils in this respect has already been given ([chapter 8, p. 135]); showing that the high nitrogen-percentage in the arid humus probably compensates largely the lower humus-percentage, while rendering nitrification more rapid, because the oxygen is not consumed by overwhelming amounts of carbon and hydrogen; which, as is already known, take precedence of nitrogen in the oxidation of humus substances. Nitrates are almost always more abundant in the soils of the arid region than in those of the humid, sometimes to the extent of influencing injuriously the quality of certain crops, such as tobacco and sugar beets. Nevertheless, nitrogen is ordinarily, in the arid region, the substance requiring replacement next to phosphoric acid. And when considered in connection with the small humus-content, so liable to burning-out, this places green-manuring with leguminous plants among the first and most vital improvements to be employed there.
The Transition (semi-humid or semi-arid) Region.—The sloping plains country lying between the Rocky Mountains and the Mississippi, quite arid at the foot of the mountains, but with rainfall increasing more or less regularly to eastward, form a transition-belt between the arid and humid region of which but a small portion has been systematically studied in respect to its soil formations. The analyses made of soils of the two adjacent states of Minnesota and North Dakota, have been placed in the general table ([p. 377]) to show how far in their general relations their soils correspond to the generalizations deduced from the comparison of the decidedly arid and humid soil areas chiefly represented in the table. Although it has not been possible, for lack of detailed data, to eliminate the soils originating from calcareous formations, it will be seen that those of semi-arid Dakota differ from those of more humid Minnesota, almost throughout, as would be anticipated from the studies of the extremes, given in this chapter.