Imágenes de página






Director of Public Gardens and Plantations.

Vol. II.



[blocks in formation]

The last legacy of Death is the first food of Life.

This axiom of Nature, when referred to the nitrogen contained in living things, finds its justification and realisation in fact by the relationship of nitrates to the decay and the re-creation of the materials of which all forms of life are composed. The last stage in the decay of nitrogenous matter from plants and animals is that of the nitrates formed in the soil, and this again is the first stage in which the dead matter is taken up afresh by the growing plant and re-created into the substance of living matter.

In the tropics this cycle is appreciably accelerate by conditions favourably adapted towards swiftness of decay and a regeneration frequently miraculous in the speed of its attainment.

A knowledge of the process of nitrification underlies an intelligent comprehension of the principles of soil management, of cultivation and manuring.

Since the hurricane, the favourable conditions for the progress of nitrification have painted the entire vegetation of Jamaica a darker and fuller green from the free supply of nitrates which our plants and trees have recently obtained

The most valuable commercial form of nitrogenous fertilizers for use in a tropical country of free rainfall, as in the cultivated areas of Jamaica, is Sulphate of Ammonia.

An attempt will be made to give an account of the process whereby ammonia derived from organic decay or the commercial fertilizer is converted into nitrate for the direct nutrition of a crop.


Although plants are able to feed upon ammonia dissolved in the soil water, they rarely do so under normal conditions. It has been found that during the growing season of the year, i.e. continuously in

the tropics, ammonia is being steadily converted into nitrates in the soil, and that plants avail themselves of these soluble salts for their supply of nitrogenous nourishment It is therefore necessary in considering the manurial effect of sulphate of ammonia to go beyond the stage of its absorption by the soil and to study the conditions under which the nitrogen of the ammonia is converted into nitrate. This latter process is known as nitrification, and although its practical working has been recognised for centuries in the manufacture of nitre, it was only by the assistance of modern bacteriology that the exact cause and mechanism of the process have become clear and intelligible

The change of ammonia into nitrates in the soil early attracted the attention of chemists. Boussingault studied this change as occurring in cultivated soil, and found the amount of nitrate present at any one time to be dependent on the rainfall. After heavy rains the nitrates were washed out, while during dry weather an accumulation again took place. In 1846 Dumas concluded from his experiments that nitrification was a purely chemical process of oxidation, and regarded nitrate as the direct outcome of the combined action of chalk, oxygen and ammonia. Other chemists ascribed the action to the porous organic matter in the soil which was supposed to have the same powers of oxidation as that possessed by the aërated pores of charcoal.

In 1862 Pasteur suggested that this change was analagous to that of the souring of beer or the oxidation of alcohol to acetic acid by the vinegar ferment, and that living organisms were involved in the production of nitrates from ammonia. Experimental proof of this idea was published by Schloesing and Müntz in 1877, and from this date all doubt ceased as to the biological nature of nitrification. Schloesing and Müntz showed

(1) nitrification in a soil could be absolutely stopped by such an
antiseptic as chloroform, which is known to have a powerful
effect on living organisms.

(2) a soil which has been deprived of its nitrifying properties by
treatment with chloroform recovered them again on being in-
oculated with a small fragment of ordinary soil.

Warington extended these researches and published several valuable memoirs on the conditions affecting nitrification and the distribution of the active nitrifying agency in soil and water. Attempts to isolate specific organisms, however, were for many years abortive. Munro showed in 1886 that nitrification could take place in the entire absence of organic matter, and it gradually became apparent that the failure of all the early attempts to isolate the organisms of nitrification had been due to the use of nutritive media containing organic matter which was then supposed to be an essential food-material of all micro-organisms.

At this juncture the brilliant Russian physiologist, Winogradsky, undertook the investigation of the matter, and by new and ingenious methods was successful in isolating the nitrifying organisms and in cultivating them in the pure state. His medium for cultivation consisted of pure silica jelly, which was solidified by the addition of a minute proportion of the sulphates of potash, magnesia and ammonia and carbonate of soda. The exclusion of organic matter and the use of the solid jelly were the two secrets of his success in separating the

[ocr errors]

nitrifying organisms from the hosts of other micro-organisms which teem in ordinary fertile soil. The first organism thus isolated was only capable of oxidising ammonia to the state of nitrite, and was only responsible for the first stage of the process of nitrification. Winogradsky completed his discovery in 1891 by the isolation of a second type of organism which possessed the power of transforming the partially oxidised compound -the nitrite into the fully oxidised product -nitrate.

The biological agents in this cycle of change had now been isolated and described, and nitrification was raised from the status of a mysterious and elusive phenomenon to that of a definite outcome of bacterial activity.


Stage I. Conversion of Ammonia into a Nitrite.

The organisms responsible for this, the first stage in the general process of nitrification, belonging to the familia class of minute vegetable organisms known as bacteria. Although several distinct species have been isolated which possess the power of converting ammonia into an alkaline nitrite under suitable conditions, they may be conveniently classified under the two groups (a) nitrosomonas, and (b) nitrosococcus.

Nitrosomonas.-This type is peculiar to the soils of the old worldEurope, Asia and Africa, and is distinguished as possessing marked powers of locomotion through the activity of a 'cilium' or long motile appendage. Only one species has so far been isolated from European soils and this appears in the form of minute, briskly motile cells provided with a 'tail' or flagellum. For bacteria, these organisms are of large size and vary from 20,000 to 40000 of an inch in length and about two-thirds as much in breadth. If cultivated in a suitable liquid medium, the individual cells eventually become quiescent and undergo a distention of cell-wall which causes them to collect at the bottom of the liquid in adherent masses of a grey, gelatinous appearance. (Zoogloea).

Nitrosococcus.-Varieties of this type occur in South American and Australian soils and differ from the nitrosomonas class in two important features; (1) absence of cilia or organs of locomotion, (2) non-formation of gelatinous masses of zoogloea.

Distinct species of nitrosococcus from Quito and Brazil have been studied by Winogradsky and found possessed of exceptional vigour as producers of nitrite from ammonia. As the name implies, these organisms are spherical in shape. Their size is large as compared with many other bacteria and reaches 12,000 of an inch in diameter in some



Stage II.-Conversion of Nitrite into Nitrate.

No organism of the class we have just considered has the power of extending the oxidation of ammonia beyond that of the nitrite, and a second distinct order of living workers is responsible for the final stage of the process of nitrification.

Nitrobacter is the term generally employed to describe the nitrateproducing bacteria which differ so markedly from the nitroso' or

« AnteriorContinuar »