Chapter VIII.

RELATION OF PLANTS TO ATMOSPHERIC NITROGEN.

IN GENERAL.

If the atmosphere varied largely in its chemical constituents, this would doubtless have an appreciable influence on vegetation. Laborious studies at Montsouris and elsewhere have shown that there is a measurable variation in the quantity of ozone, so called, of ammonia, and of carbonic acid gas, and Morley, at Cleveland, has shown an appreciable, but very slight, systematic variation in the proportions of nitrogen and oxygen. But all these variations are so small as compared with the variations in the quantity of air brought to the plants by the wind, that their influence on vegetation, if any, can not be separated from that of the wind, and is probably entirely inappreciable as compared with other influences.

On the other hand, the general fact that plants must have nitrogen in order to produce albuminous and other nitrogenous compounds has long been apparent. The question how to furnish this nitrogen to the plants in such a chemical form that it can be readily assimilated by the cells has undoubtedly been, consciously or unconsciously, the problem of the agriculturist for many ages. Without nitrogen, which is usually supposed to be furnished by fertilizers, manures, rich soils, or the alluvial deposits of the rivers, no nutritious seeds are formed, and the more molecules of nitrogen that we can force the plant to take up into its tissues the more and better seed we may expect to obtain in the harvest.

THE AMOUNT OF NITROGEN BROUGHT DOWN BY THE RAIN TO THE SOIL.

According to Marié-Davy, nitrogen is added to the soil by the natural meteorological process of rainfall. Nitrogen can exist in water either as a dissolved salt of ammonia or as pure ammonia, or in the state of a nitrate or a nitrite of soda or other alkali, or as compounded with carbon, hydrogen, and oxygen, as in the case of organic bodies floating in the water. The nitrogen brought down by the rain water is washed out of the atmosphere where it had existed in some one of these forms, and, although the percentage is small, yet the abso

lute quantity has an appreciable value as a fertilizer. The methods of determining the quantities of nitrogen need not here be given, but the following results of observations in Europe give at least an approximate idea of the probable effect of rains in the United States. (See Annuaire de Montsouris, 1889, p. 254.) Similar data for our own territory have not been measured, so far as I can find.

Quantity of nitrogenous compounds in the rainfall of 1888 at Montsouris.

Quantity of nitrogenous compounds in the rainfall during successive years at

1875 (Sept.)-1876 (Feb.). 1876 (Mar.)-1876 (Aug.) 1876 (Sept.)-1877 (Feb.) 1877 (Mar.)-1877 (Aug.). 1877 (Sept.)-1878 (Feb.). 1878 (Mar.)-1878 (Aug.). 1878 (Sept.)-1879 (Feb.) 1879 (Mar.)-1879 (Aug.) 1879 (Sept.)-1880 (Feb.). 1880 (Mar.)-1880 (Aug.). 1880 (Sept.)-1881 (Feb.) 1881 (Mar.)-1881 (Aug.). 1881 (Sept.)-1882 (Feb.) 1882 (Mar.)-1882 (Aug.). 1882 (Sept.)-1883 (Feb.).

It is evident that there is no appreciable difference between the warm and cold seasons. A slight addition is to be made to the above table, in order to include the quantities of nitrogen contained in the water of fogs and dew. The quantities under the column "Nitric acid " includes such nitrites as become converted into nitrates in the laboratory analysis. The great variations in the successive seasons depend upon the variations in rainfall quite as much as upon the variations in the quantity of nitrogen per liter, or the variations in the atmospheric constituents.

The variations in the quantity of nitrogen brought to the soil by the rainfall in different parts of the world is shown in the following table, as quoted by Marié-Davy from the memoir of Messrs. Lawes, Gilbert, and Warington, on the composition of the rainfall at Rothamsted. This table shows that the richness of the rain in nitrogenous compounds varies geographically quite as much as the quantity of rain does, so that in general the ground in Germany, Italy, and France receives decidedly more nitrogen per acre than does the ground in England. A further study of the subject also shows that the rain caught in cities contains vastly more nitrogen, especially ammonia, than that caught in the open country.

Quantity of nitrogen annually brought to the soil by rain.

.

The appreciable quantities of nitrogen shown in the above table must be diminished in agricultural computations in proportion as the rainfall carries it off into the rivers, since only that which remains in the soil can be supposed to have an appreciable influence on the growth of crops.

The quantity of nitrates in rain water may be expected to vary with the character of the climate and may be greatest in those regions where lightning is most frequent. Observations on this subject were made by A. Muntz and V. Marcano (Agr. Sci., Vol. III, p. 273), who showed that at Caracas, Venezuela, where thunder storms are fre

quent and violent, there is a very large amount of nitric acid, either free or combined, in the rain water. The relative values in different climates are as follows:

NITROGEN DIRECTLY ABSORBED BY THE SOIL.

Schloesing has shown that the atmospheric ammonia has its influence upon the plant greatly multiplied by the direct absorption of this ammonia from the air into the soil. The absorption is greatest when the difference between the tension of the ammonia in the soil and that in the atmosphere is at a maximum; it is therefore greatest when the soil is moist and when nitrification converts the ammonia into nitrates as fast as it is absorbed. When the earth is dry nitrification is suspended, and the ammonia accumulates in the soil up to a certain point, beyond which the rate of absorption gradually diminishes. (Agr. Sci., Vol. IV, p. 292.)

FIXATION OF NITROGEN BY PLANTS.

Experiments as to the source whence the grains (Gramineæ) and the beans and peas (Leguminosa) derive their nitrogen have been made both in Germany and France by independent methods. Thus Hellriegel and Wilfarth from 1883 to 1887 experimented upon samples of these plants, each of which was placed in a pot of sterilized quartz sand to which was added a nutrient solution, and the plants were watered with distilled water so as to keep the conditions favorable to growth. The results were that oats and barley behaved alike; when they are not furnished with nitrates there is no development beyond the reserve in the seed, and when they are fed with nitrates the harvest of dry matter is directly proportioned to the quantity of nitrate. For every milligram of nitrogen the increase of dry matter is 93 milligrams for barley and 96 for oats, respectively. Sterilization of the soil and of the pots on the one hand, and the addition of the microbes contained in the washings of cultivated soil on the other hand, cause no variation in the above results.

Peas behave quite differently from the preceding. Some plants languish if they have no nitrates, but others suddenly acquire new

life and yield a crop comparable with that obtained with a good supply of nitrate. The amount of nitrogen in the crop is sometimes a very large gain over that contained in the soil; this latter also occurs when the air is deprived of all ammonia, etc., and the nitrogen must be obtained from the free nitrogen of the atmosphere. But when the soil is sterilized by heat and the pots and seeds are sterilized as to their surfaces by washing with very dilute mercuric chloride, then peas behave like oats and barley; there is no gain of nitrogen from the air, the crops are proportional to the quantity of nitrate in the soil, and no tubercles are formed on the roots.

In all cases where the peas had gained nitrogen when planted in unsterilized soil, tubercles are formed on the roots, and, on the other hand, when they are planted in sterilized soil no tubercles are formed unless we add to the soil the washings of a small quantity of arable soil, in which case tubercles are generally formed. Such washings may themselves be sterilized by boiling or possibly by lower tempera

tures.

The authors infer that the assimilation of nitrogen from the air by peas, lupines, and other leguminous plants is not within the power of the plant as such; nor can it take place when the plant grows within a sterilized medium, but is connected with the presence of microbes and with the development of tubercles on the roots. (Agr. Sci., Vol. III, p. 215.)

The fixation of nitrogen by Leguminosa has been studied by E. Bréal, who succeeded in inoculating Spanish beans with bacteria from tubercles on the roots of Cystisa. At first the growth was vigorous, then the plant languished, but eventually recovered, flourished, and matured. Again, lucerne, growing in a pot in sandy soil, was inoculated by laying a fragment of tuberculous root of lucerne on the soil and watering the plant with drainage water. In both these cases not only did the plants gain in nitrogen, but the soils also, so that this experiment confirms the ordinary experience as to the behavior of the Leguminosæ as soil improvers. (Agr. Sci., Vol. IV, p. 79.)

Lawes and Gilbert, in a memoir published in the Philosophical Transactions of the Royal Society of London for 1889, state their conclusions as to the sources of the nitrogen in the plant as follows:

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In our earlier papers we had concluded that, excepting the small amount of combined nitrogen coming down in rain and the minor aqueous deposits from the atmosphere, the nitrogen source of crops was the stores within the soil and subsoil, whether from previous accumulations or from recent manuring. With the Graminew it was concluded that most, if not all, of their nitrogen was taken up as nitric acid. In leguminous crops, in some cases, the whole is taken up as nitric acid, but in other cases the source seemed to be inadequate. It is admitted that existing evidence is insufficient to explain the source of all the nitrogen of the Leguminosæ.

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