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The anhydride is described as a gummy, semisolid mass insoluble in water, so that, if it were present in any appreciable quantity, we would expect such a concentrated mixture to be almost solid. On the contrary, such mixtures were fairly mobile even a t room temperature and to a large extent soluble in cold water. This seems to indicate that the so-called anhydride, obtained by heating lactic acid a t 130” C. in a current of dry air, is rather a concentrated solution 01 lactide in acid, and that increasing the temperature merely continues the process of lactide formation instead of causing a conversion of anhydride to lactide, or that the anhydride is an unstable intermediate product. As previously pointed out, Monin4believed the former to be the case,
All these points seem to support the assumption that the anhydride is present as a colloid. Since the amount of anhydride never reaches more than a small value, it may be of interest to note than an approximate analysis of an equilibrium mixture of lactic acid may be made by a single titration, combined with the calculated values from Table I. Thus, if we titrate 1 gram of acid with 0.1 N alkali, the titration measures the free lactic acid (neglecting anhydride), and by combining the result with Equations 7 and 8 the amounts of lactide and water may be calculated. In Fig. 2 two curves are drawn from which may be read the
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6c O / N NoOH per oram x‘od FIG. 2-(I) PER CENT LACTIDE PLOTTED AGAINST Cc 0 1 X NaOH PER GRAMACID (11) PER CENT LACTICACIDPLOTTED AGAINST Cc. 0.1 N NaOH PER GRAMACID
equilibrium values for acid and lactide calculated from the results of a single titration.
The Optimum Reaction Ranges of Crop Plants’ By E. T. Wherry BUREAUO F CHEMISTRY, WASHINGTON, D. C.
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I t is believed to be of immediate practical importance to establish Portant to grow the plants in media that are well balations on the earth the optimum reaction ranges for our cultioated plants. Attempts to ascertain these by culture solution experiments, as well as some in anced in nutrient constituupon which no Plants of ents and uniform throughkind are able to grow which soil has been used, haoe not been wry successful. This may be brought out by stating the reactions in terms of the concentration out in all respects other are sodium carbonate deof acidity and alkalinity, to which the plants really respond, rather than reaction; and the rePosits and weathering outcrops of sulfide ores. I n than in logarithmic functions. Sudden diminutions in yield near action should be made to the one case there is too the neutral point, perhaps connecfed with secondary physiological r a n g e w i d e l y by some disturbances, are then seen to haoe produced apparent, but not real. method that does not alter great an alkalinity, in the optima. I t is urged that the subject be further studied by methods the nutrient balance, nor other too much sulfuric free from such objections. When once the optimum range for any introduce anything capable acid. All Soils with crop is determined, the reaction of the soil in which it is to be grown of causing marked seconti0ns-i. e., active acidities may well be brought within this range by appropriate applications dary PhYsiological disturO r alkalinities-lying between these extremes SUPof acidifying or alkalinizing substances. bance. The quantity and port more or less vegetation. quality of the crop produced over the whole reaction For each kind of plant there must be some intermediate reaction range over which range are then to be ascertained. Moreover, the observations it grows the best, and the ranges are no doubt dissimilar for should be repeated in several media of different characters, different plants. Ascertaining the optimum reaction ranges moisture contents, etc., for it is likely that the optimum reacof the hundreds of thousands of existing species is a task that tion of a plant varies with texture, composition, and even will keep scientists busy for some time. However, as recently moisture conditions of the soil. A number of workers have attempted to ascertain the optiurged by 0. Arrhenius,Zit seems of immediate practicalimportance to establish the optima for our cultivated plants, so mum reactions of various crop plants experimentally. I n that by selecting the soils in which they are to be grown most cases culture solutions have been employed, but owing on the basis of reaction, or by adjusting the reaction by the to the use of iron compounds that become nonavailable as addition of acidifying or alkalinizing substances-instead of the acidity is diminished, chlorosis has appeared in the less adding lime, and then more lime, for all crops, as is usually acid cultures. The optima obtained have accordingly been done-the maximum yield, or, even more important, the best only apparent, merely representing the points a t which this physiological disturbance became effective in markedly lessenquality of each crop may be attained. The experimental determination of the optimum reaction ing growth, so that the positions of the optima have varied for a given plant is by no means a simple matter. It is im- widely with the manner in which iron was applied. Duggar,’ by adding iron in a form that remained even On 1 presented under the title “Adjusting Soil Acidity” before the joint the alkaline side, avoided chlorosis, but found the COmpOSimeeting of the Divisions of Agricultural and Food and Fertilizer Chemistry at the 66th Meeting of the American Chemical Society, Milwaukee, Wis.. tions of the solutions in other respects affected the positions September 10 to 14, 1923. of the optima markedly. 2 “Bodenreaktion und Pflanzenleben,” special publication, Forstliche Only two series of experiments directed towards ascertainVetsuchsanstalt. Munchen, published by Gustav pock,. Leipzig, - -. 1932: Kungl. Landtbuuks-Akad. Handl. Tid.,62; 417 (1923).
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Ann. Mo. Botan. Gardens, 7 , 1 (1920).
I X D USTRIAL A N D ENGINEERING C H E M I S T R Y
August, 1924
ing optimum reactions in soil cultures have come to the attention of the writer. I n 1920Joffe4 studied alfalfa and found the maximum yield and quality (nitrogen content) to be obtained near the neutral point. Somewhat later 0. Arrhenius2 published data on seventeen crops, and considered that in general two optimum points, rather than a range, existed. The chief optima he obtained were as follows: timothy, pH 9; alfalfa, barley, birdfoot-trefoil, and orchard grass, p H 8; alsike clover and sugar beets, pH 7.5; hop-lucerne, radishes, rutabagas, and wheat, p H 7 ; lupine, peas, and red clover, pH 6; oats, pH 5 ; and flax and turnips, pH 4. The method used was admittedly suited to give only approximate results, however, and the experiments were not replicated; and in view of the accumulated experience as to the increased yield produced by liming in several of the crops listed, it seems probable that a t least the last third of these optima are as fictitious as those derived from culture solution experiments. The chief reason for the failure on the part of most workers to recognize the unreality of the apparent optima obtained has been the use, in both tabulation and plotting, of the p H system of stating active acidity. Impressed by the mathematical simplicity of this plan, they overlook the fact that it is the quantity (concentration) of active acidity or alkalinity present to which plants respond; for plants are blissfully ignorant of the negative logarithms of these quantities, or of the infinitesimal amounts of active acidity present in markedly alkaline solutions. The difference in acidity represented by one pH unit from 3 to 4 is a hundred times as great as that from 6 to 7 , and the plant behaves correspondingly. A curve plotted with one axis laid off in equally spaced pl3 units therefore gives a distorted and misleading picture of’ the relations between the plant’s growth and the reaction of the culture medium. What may appear to be a gently arching curve with a significant optimum may actually be a rapidly increasing growth curve cut off at some random point by a secondary physiological disturbance incident to the method used. For example, in Fig. 1thedatagivenby 0. hrrheiiius for red clover are plotted by two methods-the first with the p H values, the second with the concentrations of hydrogen and hydroxyl ions on the opposite sides of neutrality, as abscissas; for convenience the concentrations are stated in “acidity units’’ and “alkalinity units” (instead of the more commonly used grams) per liter, these units representing 10-7 gram-equivalents of the respective ions. The upper (pH) curve effectually conceals whatever relations exist; but the lower (concentration) curve shows the
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methods of culture, of applying alkali, or of supplying necessary nutrients, the yield-concentration curve would probably remain more or less horizontal for some distance to the alkaline side of the neutral point, and the real optimum range be found to lie in that region (red clover as is well known growing luxuriantly in distinctly alkaline soils). hfost culture solution work has yielded data that give curves of identical character with those of Fig. 1, the physiological disturbance, as already noted, being in many cases chlorosis. Further efforts should be made to ascertain the optima of our more important crop plants by methods adapted to avoiding secondary physiological disturbances of such magnitude that sudden marked decrease in yield occurs near the neutral point. The writer is planning to carry out work along this line, and it is hoped that the presentation of this paper will encourage others to do likewise. It is therefore only a question of time before the optima for our crop plants shall be approximately established. The progressive crop producer will some day desire to use a soil which, while complete and balanced in nutrients, also has a reaction lying well within the optimum range for the particular crop to be grown. He will require, then, some simple means of determining the reactions of soils with reasonable accuracy and rapidity. This can be accomplished by the use of modern indicators. For approximate work the spot plate method may be used; the soil is mixed with about twice its volume of distilled water, shaken and allowed to settle, and the clear liquid is pipetted off. Tiny drops of each of several sulfonephthalein indicators are placed in cavities in a porcelain plate, and l or 2 cc. of the soil extract added to each, the reaction being indicated by the colors assumed. More precise determinations can be made by means of the Barnett double wedge,5 a form of which adapted to field use is now on the market. If the soil reaction of the tract available turns out not to be within the optimum range for the crop it is desired to grow, such reaction must then be modified by the addition of appropriate substances. To diminish the acidity, lime in some form is the most available material; to increase the acidity, free sulfur, ammonium sulfate, aluminium sulfate, or acid peat may be applied. There is apparently no royal road toward ascertaining the amount of any of these which will be required; it seems necessary to make an application to the soil in the field, wait long enough-if possible, several monthsto enable combination with the soil of the substance applied
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”optimum” to represent a point a t which some toxicity or deficiency inherent in the method of culture used has led to sudden falling off in the ability of the plants to thrive. Were this physiological disturbance prevented by the use of other 4
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Soil Science, 10, 301 (1920).
to take place, and then perform the determination of the soil reaction over again. By interpolation or extrapolation the amount required to bring the soil within the desired optimum range can then be calculated. 8
G. D. and C. W.Barnett, Proc. SOC.E x p l l . B i d . M e d . , 18, 127 (1921).
