SCIENTIFIC.

Otago Witness, Issue 1905, 25 May 1888, Page 35

SCIENTIFIC.

CHEMISTRY AT THE FABM. ,; ' By F. A. Joseph. No. IV. Although the* atmosphere is the great storehouse of nitrogen it is, almost certain that plants', do not absorb '"nitrogen in the free or uncombined state. Elaborate experiments have proved almost conclusively that free nitrogen is not' assimilable- by plants; yet it enters into their composition 1 to "the extent' of nearly 2 per cent. It is to some 1 of the compounds of nitrogen then that we must look for the source of the element in plants. Nitric acid-is o"ne of these— a compound of hydrogen, nitrbgen, and oxygen. Nitric acid is found in the juices of plants, especially in those of a fleshy tuberose character. The acid is found in various soils, and in the drainage of soils, particularly in those near .populous districts. It is abundant in surface water near towns, and for this reason well' water containing nitrates in solution is not fit for use. Potassium nitrate, of saltpetre, is very • abundant in 6ome soils where dense' populations havelong dwelt. One great source of saltpetre is the soil of India, whence the compound is derived by washing the soil and afterwards , evaporating the liquid. Another nitrate, commonly ' called Chili > saltpetre -or sodium nitrate, is found 'abundantly in the soils of Chili. In India the potassium nitrate has found its way into the soil from the refuse of human dwellings, every midden heap being a fertile source of nitrogen and potash ; in Chili decaying seaweed has probably been the source of the sodium nitrate. Nitric acid is readily formed by the oxidation of ammonia, which is a compound of hydrogen and nitrogen; and .the addition of oxygen effects a magic change, converting the most powerful alkali into an equally powerful acid. It is probably by means o£ this reaction that plants derive their supply of nitrogen from ammonia. In the sap of the plant the compound probably exists at first as nitrates of some kind ; these again are most likely decomposed by either oxalic acid or oxalates, setting free nitric acid. The liberated nitric jacid . might, by reacting upon carbonates, produce the groundwork of albuminous substances. Ammonia is, if not directly, at least indirectly, a source of , nitrogen, in plants. It is produced by' the decomposition of both, animal- and vegetable substances/ but chiefly the former. It is" ' abundantly ' produced in farmyard manure, a considerable portion passing into the atmosphere as vanour, whence it will be washed down, by subsequent, rains tp enrich the soil in general', but not sufficiently that 9fi*the farmer • producing' it." Ammonia is best- formed from vegetable 'substances' in the soil, either by the direct union' of' 'nitro-' gen» withlt he hydrogen of the plants, or by the combination of the .hydrogen^ of the plants with 1 the free nitrogen of 'the air, or again by so acting upon water as to decompose it, using part of tlje oxygen, and causing the -liberated* hydrogen to combine' with the nitrogen .of the air. Either or all of these methods might account for the origin of ammonia in decaying vegetable substances. These processes take place most readily in open subsoils where the oxygen of the air does not act very freely ; hence the advantage of drainage and subsoiling. Ammonia washed down by the rains finds its way to the roots of the plants in a watery solution to be absorbed and made use of. Nitric acid at any rate, and probably ammonia also, is produced by the action of electricity in the atmosphere. During a violent thunderstorm' a considerable proportion of nitric acid is formed and washed down by the rain into the soil. Volcanic eruptions also .produce this powerful acid as well as abundance of ammonia. Another very important source of nitrogen is found in the urine of animals. The. compound, is called urea, and is a combination, of carbon, 1 hydrogen, nitrogen, and oxygen. It is quite as important as ammoj nia, and in allowing the liquid portion of i farmyard mangre to run to waste a great mistake is made. Having now briefly dealt with the four most important elements of plants, we shall next turn to the substances found in the ashes when plants are burned. Potash is the first of these to be taken notice of. The metal potassium is extremely light, being only 39 times the weight of hydrogen, and rather lighter than water. In the metallic state it is a soft metal of a colour not unlike that of lead when freshly cut, and as soft as a piece of ordinary soap It oxidises so freely that it can only be kept under rock oil containing no oxygen. It takes fire immediately .on touching water, or more properly it seizes oxygen so vigorously that the heat produced sets fire to the hydrogen liberated. This is how the smart 'Yankee meant to set the Thames on fire, but in order to do that he would require an enormous quantity of metallic potassium. In plants it is never found except in a combined state, chieflyi.as'a phosphate of the metal. These phosphates of potash are very abundant in the ashes of some plants. In tobacco potassium salts have been found to the extent of from 60 to. 80 per cent, of the total ash. The proportion is rarely less than 50 per cent, of the ash of plants, so that the potash compounds must be important as plant food. i Many plants grown far from the sea contain no sodium, though the same plants when | grown near the sea have it. in abundance ; but all plants in all situations contain potassium. ' Potassium may have existed originally in the free state, but in soils it is always found in the shape of phosphates, carbonates, ornitrates. < Sodium 1 is a soft metal very similar to potassium, and slightly heavier. Its action on water is also similar. The compounds of sodium are very widely diffused, the spectroscope proving sodium to exist in every speck of dust, and .showing it to be present in the sun and most of the stars. It exists in enormous quantities in the granite rock in various compounds, <in immense beds of rock salt, and in the. ocean, which is a great storehouse for it. Ordinary table saty'is'knqwn I to the chemist as cHloride* hi '-.s'odatihi. formerly sodium carbonate was prepared fj-om fthe .ashes^oj^ se^weedii b,ut .now, ifcvis more easily and abundantly the chloride; 'or* sea" salt. "It is rifa^'ifornii of these two salts that sodium is found In

plants. It is a disputed - question whether. ; sodium is essential; to plant lifcor not;-. it is indispensable to animal life andplants readily absorb it where it is abundant, and at leaststore it up for the use of animals, thus revealing a wise provision of Nature in provid- , ing for the wants of animals and man. ■ Phosphorus is another" very important element which enters into the composition of plants. It is so well known now in connection'with rabbit poisoning that I need not go • very far in describing it. -.- Suffice it to say that phosphorus is prepared on the mercanr tile scale from the bones of ' animals. ( When bonedu'stris "mixed' mtK- two-thirds of "its weight of sulphuric acid and water, the calcium phosphate of the .bones, is broken ftp, the ealciu'm 'combining with the acid to form calcium sulphate, or gypsum, which settles down as a, white powders-bile the saperphosphate; df^ calcium '^qlfmM'femainJlin a solutio*n f .^ The y chan'geSiri6lerg6ri''e J is' ! 'representqd.thus: calcium phosphatte 'and^hydrogen 'sulphate changes,; to calcium sulphate and calcium' hydrogen- phosphate. TheVsolution of superphosphate is* then evaporated^ to a syrup, and heated to, redness along with powdered charcoal -In an<earthenware.retort, the nozzle, of. Which' ; dips tinder 1 waters The liquid phosphorous is distills condensers under water '.jn to a .sqlid, which "is after wards melted and cast into sticks, as sold in the shops. ; As, phosphorus XtaHes fire 'at a very low temperature, it*. must always be kept in ' cold water. '. The original source of phosphorous is the phosphates in small quantities in the older granite rock, j j By .disintegration and denudation the phos r phases haye got into the soils. Plants build up phosphates into their tissues, and ani> mals derive their supply of phosphorus from plants. Phosphates are so very important- to r animal) life -tljat- we need not marvel at plants storing them up so freely, especially in their seeds, again showing only too clearly the adaptability of Nature. • ■'-