It is noteworthy, also, that the popular classification of the two Florida soils corresponds exactly with the differences in the percentages of plant-food; those in the “second-class” soil being uniformly lower than those in the one designated as first-class. This indicates, again, that as between soils of similar character and origin, the production and durability are sensibly proportional to the plant-food percentages when the latter fall below a certain limit; a point more fully illustrated farther on.
In the light of the above experiment and tables, it becomes pertinent to consider what are the lowest percentage limits of each of the more important plant-food ingredients compatible with profitable production.
LIMITS OF ADEQUACY OF THE SEVERAL
PLANT-FOODS IN VIRGIN SOILS.
It is obvious that the lower limits of adequacy of the critical plant-food ingredients are best ascertained in the case of virgin soils containing very small amounts of some one ingredient, while fairly or fully supplied with the rest. In such cases, which are not at all infrequent, the use of the deficient ingredient as a fertilizer should produce a very marked effect so soon as the first flush of production (always noted in fresh soil) is over. This first productiveness may, even in poor lands, range from one to three years, when there is a sudden decline.
Lime a Dominant Factor.—When we investigate the cases of such lands, it soon becomes apparent that besides the low percentage of any one ingredient, the proportions of others present require consideration. Among these, lime in the form of carbonate stands foremost. Its presence exerts a dominant and beneficial influence in many respects, as is readily apparent from the prompt change in vegetation whenever it is introduced into soils deficient in it. In discussing the results of soil analysis, its consideration is of first importance in forecasting correctly the adequacy or inadequacy of other soil ingredients ([see chapter 20, page 379]). For in general, we find that lower percentages of potash, phosphoric acid and nitrogen are adequate, when a large proportion of lime carbonate is present.—This has already been referred to in connection with the table of soils of low percentages, given above. In the interpretation of results obtained by analysis this point must always be kept in view; and in the numerical statements made below, it must be understood that they refer to virgin soils sufficiently supplied with lime to assure a constant excess of lime carbonate, maintaining the conditions of nitrification and insuring the absence of acidity. ([See chapter 9, page 146]).
Potash.—In respect to potash, the writer was led by his early investigations in the State of Mississippi to conclude that less than one-fourth of one per cent (.25) of potash constituted a deficiency likely to call for early fertilization with potash salts; while as much as .45% of the same seemed to cause the land to respond but feebly to such fertilization. He has not found it necessary to revise materially that early conclusion, whether from his own work or from that of others. Within the last decade, Prof. Liebscher of Göttingen[119] has arrived at this identical figure from analyses made of soils upon which he had conducted a seven-year series of fertilizer tests; he having found that potash fertilization produced no sensible, or at least no paying results on land giving that figure, and otherwise well provided with plant-food. The different (lower) figures given by Schloesing, Risler and other French chemists in discussing the soils of France are doubtless due to the weak acid and short period of digestion employed in the analysis; an unfortunate discrepancy of methods which precludes any direct comparison of results.
These figures apply both to the arid and the humid regions in the temperate zones. In the tropics we find very much lower percentages quoted as adequate; thus in the laterite soils of India and Samoa, according to Wohltmann, in the soils of Jamaica according to Fawcett, and in those of Madagascar according to Müntz and Rousseaux.[120] There, potash-percentages over .10% seem to be high, and in Madagascar some lands in fair production range as low as .01%. The soil-extractions have however in these cases been made with a weaker acid than above specified, so that some increase of the figures (perhaps 33 to 50%) have to be allowed for. But even then there can be no question that a far less amount of potash, as determined by acid-extraction, is found sufficient for crop production in the tropics; doubtless because of the very intense decomposing (“fallowing”) effect of the continuous heat and moisture, tending also to a rapid decomposition of organic matter and a proportionally rapid formation of carbonic and nitric acids. Such soils are of course constantly kept in a leached condition, as a result of the heavy and continuous rainfall.
Phosphoric Acid.—As regards the lower limit of adequacy of phosphoric acid, there is a remarkable agreement in the investigations made everywhere. It was placed at .05% by the writer as long ago as 1860, as the result of investigations made in the State of Mississippi; and the same figure has since been arrived at independently by agricultural chemists in France, Russia, Germany and England. The cause of this remarkable agreement is undoubtedly the readiness with which the phosphates that come under consideration at all for the nutrition of plants, are dissolved by almost any acid treatment likely to be used in soil analysis. Almost the same agreement exists in regard to the “adequacy” of .1% of P₂O₅; while all soils showing percentages between .1 and .05% are considered weak on this side, and liable to need phosphate fertilization soon. One-fourth of one per cent is an unusually high percentage in most countries; .30% and over is exceptional in non-ferruginous soils. But as stated on a previous page, a high percentage of lime carbonate may offset a smaller percentage of phosphoric acid, apparently by bringing about greater availability; and a similar effect seems to result from the presence of a large supply of humus.
On the other hand, very large percentages of finely divided ferric hydrate may, especially in the absence of lime carbonate, render even large supplies of phosphoric acid inert and useless, by the formation of the totally insoluble ferric phosphate. Aluminic hydrate probably acts in a similar manner. The following table gives examples in point, as regards ferric hydrate.
HAWAIIAN SOILS SHOWING HIGH
CONTENTS OF FERRIC OXID.