PHOSPHATES AND NITROGEN

“Phosphorus, which is a dangerous poison, as we have seen, is nevertheless found in abundance in the bodies of all animals. It occurs in the urine, whence Brandt was the first to extract it; it is found still more plentifully in the bones, and from thence it is now obtained. There is some in meat, in milk, and in cheese; also in plants, notably cereals; hence flour and bread contain it. But do not be alarmed: we shall not die of poison like the rats that have nibbled crusts smeared with grease and phosphorus.”

“But why not,” asked Emile, “if we eat it as the rats do?”

“I will try to explain,” replied his uncle. “When two or more substances are mixed together, they lose their original properties, while the new substance obtained by their combination is found to possess new properties having nothing in common with the old ones. Thus carbon, when combined with the air that we breathe, becomes an invisible gas, subtle, and unfit for breathing. In like manner lime, burning to the taste, is converted by union with carbonic acid gas into chalk, a calcareous stone void of taste. Furthermore, poisonous substances, deadly in a very [36]small dose, may become harmless and even enter into the composition of our food when they are combined with other substances. Thus it is with phosphorus. What, then, is united with phosphorus in the form in which it ceases to be poisonous and enters into the composition of meat and flour? That is what we will now consider.

“When phosphorus is burned it produces a thick white smoke, of which you can get some idea by striking a number of matches all at once. This white smoke with the slightest trace of humidity is reducible to an extraordinarily acid liquid called phosphoric acid. Since this compound results from the combustion of phosphorus, just as carbonic acid is the result of the combustion of carbon, it must and in fact does contain the air without which no combustion can take place. Phosphoric acid is no longer inflammable, however much it may be heated; being itself the product of combustion, it cannot burn again. But if there is no danger of its catching fire, phosphoric acid is nevertheless dangerous on account of its intense acidity, which makes it violently corrosive in its action on flesh. If mixed with lime, however, this formidable compound loses its injurious properties and is changed into a white substance without the least taste or the slightest poisonous effect. This substance is called phosphate of lime. Burnt phosphorus and lime, thus united, furnish the greater part of the mineral matter found in bones. Put a bone into the fire: the grease and juices that permeate its substance will be burnt up and the bone will [37]lose a part of its weight and become friable and perfectly white. Well, this bone, calcined in the fire for a long time, is composed chiefly of phosphate of lime. It contains phosphorus, the most combustible of substances, and yet is itself absolutely incombustible; it contains one of the most poisonous substances, and yet is itself quite harmless; into its composition there has entered an ingredient possessing atrocious acidity, and yet the compound itself has no taste. Similarly combined and equally harmless, phosphorus is found in meat, milk, cereals, in flour and bread.

“A cow can furnish each week about 70 liters of milk containing 460 grams of phosphate. This phosphate comes from hay, which obtains it from the soil. But as the soil contains only a moderate quantity of it, and the hay continually takes it away, the supply will at last become exhausted and the milk will become poorer and less abundant. If a kilogram of powdered bones, containing about the same quantity of phosphate as the 70 liters of milk, is spread over the pasture, it will make good the weekly loss in phosphate that the soil undergoes in the production of the cow’s milk. Hence the efficacy of powdered bones on exhausted pasture land.

“Phosphoric acid combined with other substances is found in all our agricultural products, and hence the phosphate from bones has a very marked effect on our crops. Harvests have been doubled as if by magic through the use of powdered bones. A kilogram of this powder contains enough phosphoric [38]acid for the growth of a hundred kilograms of wheat. Despite their great value as a fertilizer bones will never be thus used except to a limited extent, because they are not abundant enough and also because they are much in demand in various arts and manufactures. Fortunately in some localities phosphate of lime is found in certain coarse pebbles called nodules or coprolites. These precious stones are carefully collected and ground to powder in a mill. Then, in order to make the substance more soluble in damp soil, and thus better fitted for the nutrition of plants, it is sprayed with an extremely corrosive liquid called sulphuric acid or, more commonly, oil of vitriol. In this way is obtained the superphosphate of lime which manufacture gives to agriculture as one of the most powerful of fertilizers, especially for the raising of grain.

“We were wondering a little while ago what substances could be contained in the ashes of a burnt plant, and we have now found potash to be one of them. Moreover, since all vegetation must have phosphate in order to thrive, this also ought to be found in the ashes, phosphate being indestructible by heat. And, in fact, after the incineration of any vegetable matter whatever, as a bundle of hay or a handful of grain, the delicate processes of science can always recover this compound of phosphorus; and they further find lime, iron in the form of rust, the silicious component of pebbles, and divers other substances of less interest.[39]

“To finish this difficult but very important subject of the nutrition of plants, I must say a few words about ammonia. This word does not tell you anything since it is a new word to you. But I will make its meaning clear to you by a familiar illustration.

“You must have noticed the strong, penetrating odor prevalent in ill-kept water-closets; and you have also perceived the same odor when soiled garments are cleaned with a certain liquid that looks like clear water. Well, this odor, so pungent that it almost produces the effect of fine needles thrust up into the nostrils and brings tears to the eyes, is the odor of ammonia.

“Ammonia is an invisible gas capable of being taken up in large quantities by water, the mixture being known as aqua ammoniæ, or water of ammonia. Combined with other substances ammonia loses its pungent odor and forms compounds which are among the most effective fertilizers. These compounds furnish vegetation with one of its essential ingredients called nitrogen. By itself nitrogen is an odorless and colorless gas. In this state it forms four-fifths of the volume of ordinary air, the air we breathe. The other fifth is composed of a second gas called oxygen, also colorless and odorless. It is oxygen that our lungs demand when we breathe, and it is oxygen that is necessary when we wish to burn anything. It is this alone that plays its invaluable part in the combustion of certain substances in our blood and in the generation of natural heat; it is this that [40]in the process of combustion releases carbon, phosphorus, sulphur, and other combustibles, to combine with them and produce a compound known as carbonic acid gas in the case of burnt carbon, phosphoric acid in the case of phosphorus. In fact, to it belong the properties that we have until now attributed to the atmosphere as a whole. As for nitrogen, it has no other purpose in the atmosphere than to moderate by its presence the too violent energies of oxygen; it plays there the part of the water that we put into too strong wine.

“All vegetation requires nitrogen. Wheat, for example, must have it to develop the grain in the ear; peas, beans, lentils demand it in order to fill out their pods; the pasture and the hay-field need it if they are to furnish the nutriment that the sheep and the cow will transform into milk. But plants cannot take this nitrogen from the air, where it is so abundant; it must be served up to them after a certain necessary preparation. We ourselves need phosphorus, since it enters into the composition of our bones; we need carbon still more, the principal fuel used in maintaining the heat of the body. But are we to eat the charcoal that the charcoal-burner manufactures in his furnace, and the phosphorus used in the making of matches? Certainly not. The first would be a frightful mouthful, the second an atrocious poison. We must have them prepared in a suitable way, such as they are found in bread, milk, meat, fruits, vegetables. In the same manner plant-life requires nitrogen, not as it occurs in the [41]atmosphere, but as it exists in certain combinations, of which the most notable are the compounds of ammonia. This explains to us the highly beneficial effect of manure on our crops. Manure is composed of the bedding used in stables and the animal excrement with which it has become mixed and impregnated. Now this excrementitious matter, especially urine, yields ammonia in decomposing, as is proved by the odor arising from latrines in hot weather and so powerfully affecting the eyes and nose. Thus manure may be said to hold ammonia compounds in storage, and from them plants derive their nitrogen, as also many other ingredients.

“Let us summarize these details. In the nutrition of plants four substances are of prime importance. First, carbonic acid gas, which yields carbon, the most widely diffused of all the elements (but which we need not dwell upon here), since plants take it chiefly from the atmosphere, to which it is supplied unceasingly. After carbonic acid come potash, phosphoric acid, and nitrogen, all of which the roots extract from the soil, where it occurs in some compound or other. These are the ingredients that the soil, if it is to remain fertile, must have given back to it as fast as they are exhausted by the crops. Such is the part played by fertilizers, without which the soil becomes exhausted and ceases to produce.”