Calciphile, Calcifuge and Silicophile plants.
The subject has been somewhat exhaustively discussed by Contejean[199] who enumerates and has classified under the three general heads of calciphile, calcifuge and indifferent, over 1700 species of European plants. Unfortunately he had but few soil analyses at his disposal, and was inclined to consider as non-calcareous, most soils that gave no effervescence with acids. But notwithstanding this disadvantage so far as his contention of the efficacy of chemical soil-composition, and especially of lime is concerned, he disproves very effectually the physical theory of Thurmann, by numerous examples from France and elsewhere in Europe; and also disposes very definitely of the claim that there is a special class of “silicophile” plants. He concludes that silica (and sand) is merely a neutral and inert medium which offers refuge to the plants “expelled” by lime; and that clay similarly exerts no chemical but only a purely physical action. That potash, phosphoric acid and nitrogen, while most essential as plant-foods, exert otherwise little if any effect on general plant distribution. He alludes similarly to magnesia; and his final conclusion is that “chemical are in general more potent than physical influences,” and that the most widely active influences are carbonate of lime and chlorid of sodium. He does not, of course, deny the potent influence of moisture upon plant distribution.
Since these publications were made, many observers have investigated the subject, and the broad distinction between lime-loving or calciphile and lime-repelled or calcifuge plants has been very generally recognized and discussed: but the cause of this discrimination by plants is still more or less the subject of controversy. Some still claim that the calcifuge plants (such as the chestnut, the huckleberries and whortle-berries, the heather and many other Ericaceæ, most sedges, etc.) are repelled by calcareous lands because they need a large supply of silica, which they suppose cannot well be assimilated in presence of much lime; hence they also designate the calcifuge plants as “silicophile”; while others attribute the preference of calciphile plants to the physical effects produced upon the soil by lime, as outlined above ([chapter 20, page 379]).
The contention that the presence of much lime in soils renders silica insoluble and hence unassimilable by plants, is at once negatived by the fact that waters exceptionally rich in silica, partly simply dissolved by carbonic acid, partly in the form of water-soluble alkali-silicates, are very abundantly found in the arid region. This is especially the case in California, where moreover a number of species of very rough-surfaced horsetail rushes and grasses prove the ready absorption of silica when wanted, even in strongly calcareous soils. But the question is whether the supposed class of silicophile plants is a reality or merely a theoretical fiction, based upon the habit of speaking of “siliceous” soils as a class apart from other and especially heavier or clay soils. As a matter of fact, the siliceous soils usually so called are simply those poor in clay and lime—in other words, “light” lands, the outcome of the weathering of quartzose rocks into sandy soils, which in the humid region are always poor in lime because thoroughly leached. In the arid region, on the contrary, sandy lands are quite commonly just as calcareous as the heavier soils, and show no “silicophile” flora.
According to the writer’s observations and views, it being obvious that some plants are practically indifferent to the presence or absence of lime in the soil except in so far as it influences favorably the physical conditions, moisture must always stand first as the condition of maximum crop production, and as a conditio sine qua non of the best development of plants on all kinds of soils; its best measure being a matter of special adaptation to each species. But this being understood, he agrees with Contejean as to the commanding influence of lime in determining the adaptation of soils to plants, both cultivated and wild. At the same time, it is obvious that the absence of the opportunity to observe really native vegetation, adapted to the soils through ages, has created for European observers difficulties which are readily solved where original native floras are available.
Schimper[200] says pointedly that observations prove that the differences between the location of plants on calcareous and siliceous soils are not constant, but vary from province to province; that e. g., the list of indifferent (bodensteter) plants for the Alps do not hold good in the Dauphiné, still less between the Carpathians and Skandinavia. According to Wahlenberg the following species are calciphile in the Carpathians, and according to Christ indifferent in Switzerland: Dryas octopetala, Saxifraga oppositifolia, most of the leguminous species, Gentiana nivalis, G. tenella, G. verna, Erica carnea, Chamæorchis alpina, Carex capillaris. Geum reptans is reported by Bonnier to be exclusively calciphile on Mont Blanc, exclusively silicophile in the Dauphiné; indifferent in Switzerland. A great number of similar contradictions are reported by others as well, and the entire subject thus becomes rather vague; so that Schimper and others suggest that climatic conditions may in part be responsible for these discrepancies.
In all, or nearly all these cases, it is tacitly assumed that the underlying geological formation has essentially been the source of the soil, and that its character is determined accordingly. But this assumption is wholly arbitrary unless confirmed by actual direct examination. A soil-formation overlying limestone on the slopes of a range may be wholly derived from non-calcareous formations lying at a higher elevation, or may have been leached of its original lime-content by abundant rains. The feldspars constituting rocks designated as granite, may or may not be partially or wholly of the soda-lime instead of the potash series; the mica may or may not be partially replaced by hornblende, in which cases the soil would be calcareous to the extent of determining the character of the flora as calcifuge or calciphile, without its being at all evident in the physical character of the soil, which would still be “granitic” or “siliceous.” Such observations in order to be critically decisive, clearly require that the observer should be, not merely a systematic botanist, nor a mere geologist or chemist, but all these combined. There is good reason to believe that most or all of these supposed contradictions would disappear before a critical physical and chemical examination of both the soils and the rocks from which they are supposed to have been derived. Contejean himself, in placing so many of his long catalogue of plants into the doubtful groups, suggests many cases in which the above considerations may explain the apparent discrepancies.
What is a calcareous soil? The definition adopted for this volume has been given in a previous chapter ([chapter 19, page 367]); viz, that a soil must be considered calcareous so soon as it naturally supports a calciphile flora—the “lime vegetation” so often referred to above and named in detail. Upon this basis it has been seen that some (sandy) soils containing only a little over one-tenth of one per cent of lime show all the characters and advantages of calcareous soils; while in the case of heavy clay soils, as has been shown, the lime-percentage must rise to over one-half per cent to produce native lime growth. While in the United States observations of the contrasts between calciphile and calcifuge floras are easily made in the field, and the facts must attract the attention of any fairly qualified observer, in Europe they would have to be made the subject of special cultural investigation based upon soil analysis; a procedure not yet fully accredited abroad, any more than in the United States. In a general way it has however been recognized by Maercker, as shown at the end of the preceding chapter. How far this estimate was based upon American precedents, can now be only conjectured. Certain it is that the European definition of calcareous soils remains to the present day a wholly different one from that stated above; and from this have arisen the greater part of the doubts and differences of opinions among European botanists as to the classification of plants in relation to calcareous soils. Two per cent of lime (equivalent to nearly double the amount of carbonate) is the prevailing European postulate for a calcareous soil. Some go so far as to postulate effervescence with acids, requiring about 5% of the carbonate.
Predominance of Calcareous Formations in Europe.—It is not generally recognized even among geologists how abnormally predominant are limestone formations in Europe. In all works on European agriculture we find the “lime sand” mentioned as a normal ingredient of soils, specially provided for (or against) in the operations of soil examination. Its presence is the rule, its absence the exception. Soils as poor in lime as are those of the long-leaf and short-leaf pine regions of the United States, are there very exceptional and (like the “Haideböden” of northern Germany) have long remained almost uncultivated. Calcareous soils being the rule in the regions of intense culture, the ideas of both agriculturists and agricultural chemists have in Europe, in the main, been based upon them as normal soils; so that instead of comparing calcareous, and non-calcareous soils properly speaking—i. e., such as would not bear native lime-vegetation—the majority of comparisons has actually been made between soils which, in the American sense, were all or chiefly within the calcareous class. It is characteristic of this state of things that the injuriousness of an excess of lime is among the foremost themes of European (especially French and English) agricultural writers, as against the beneficent effects prominently assigned to lime in America. No such popular saying as that “a lime country is a rich country” exists in Europe; on the contrary, we constantly hear, and see in books, the mention of “poor chalk lands,” and in France especially the deleterious effects of excess of lime upon crops is the theme of remark. Excess of lime in their marly lands has been the despair of French vintners, and Viala was specially sent to America to find some vine to serve as a grafting stock which would resist the tendency to chlorosis which renders many of the American phylloxera-resistant vines useless to the viticulturists of France. Viala did not find such grapevines until he reached the cretaceous (chalk) area of Texas, where the native vines had long ago adapted themselves to marly soils; and these vines have solved the problem for French viticulture.
And England, France, Belgium and most of western Europe are rich countries, largely owing to their abundant limestone formations; and it may be questioned whether, had this been otherwise, Europe would so long have remained the center of civilization; for starving populations are not a good substratum for high mental culture and progress. It may equally be asked whether the invariably calcareous character of arid soils, as heretofore shown, has not, together with their general high quality, been largely a determining factor in the location and persistence of so many ancient civilizations in arid lands; as outlined in [chapter 21, page 417]. In this connection, the proper distinction between calcareous and non-calcareous soils passes from the domain of natural science to that of the history of human civilization.