Manures and the principles of manuring

by Charles Morton Aikman

That various manures, such as bones, horn, wool, and rape-cake are readily nitrifiable, has been shown by experiment. Laboratory experiments have also been carried out on such different nitrogenous substances as ethylamine, thiocyanates, gelatin, urea, asparagin, and albuminoids of milk. But in all these experiments, how far these bodies have been directly acted upon by the nitrifying organisms, or how far they have first undergone a preparatory change in which their nitrogen has been first converted into ammonia, is impossible to say. It is at least quite probable that all the organic forms of nitrogen have first to be converted into ammonia ere they are nitrified.

Rate at which Nitrification takes place.

A question which is practically of no little importance is the rate at which nitrification takes place. From what has been already said as to the nature of the conditions favourable for the process, it will be at once seen that this will depend on how far these conditions are present in the soil. In point of fact the rate at which nitrification takes place will vary very much in different soils. A greater difference, however, in the rate at which it takes place, will be found even in the same soils at different periods of the year. In this country, where the most favourable temperature for its development is seldom reached, it never goes on at the same rate as in tropical climates. One of the causes of the greater fertility of tropical soils is due, doubtless, to the very much longer duration of the period of nitrification, as well as to its greater intensity. As, however, temperature is not the only condition, and the presence of moisture is quite as necessary, it may be that its development is seriously retarded in many tropical climates by the extreme dryness of the soil during long periods.

Takes place chiefly during the Summer Months.

Although in this climate, as has already been pointed out, nitrification probably goes on during most of the winter months, owing to the fact that the temperature of our soils is only occasionally below the minimum temperature at which the process takes place, yet there can be little doubt that the great bulk of the soil-nitrates are produced during a few months in summer. A fair conception of this amount is afforded by the interesting experiments on the composition of drainage-waters made at Rothamsted, which we shall have occasion to refer to immediately. It may be pointed out, however, that it is not always safe to take the amount of nitrates found in drainage-waters as an infallible indication of this rate, for this amount will depend to a certain extent on the amount of rainfall, and would be misleading in the case of a long period of drought. On the whole, however, it furnishes us with extremely useful data for the elucidation of this important problem.

Process goes on most quickly in Fallow Fields.

It has been shown in the Rothamsted experiments that the process goes on best in fields lying in bare fallow; and in this fact lies the explanation of one of the many reasons why the practice of leaving fields in bare fallow, so common in past times, and still practised in the case of clay soils in some parts of the country, was so beneficial to the land thus treated. But despite this fact, the practice of leaving soils in bare fallow can scarcely be justified from this point of view, as the loss of nitrates through the action of rain is very great in our moist climate.

Laboratory Experiments on Rate of Nitrification.

Several interesting experiments have been carried out with the object of affording data for estimating the rate at which the process may go on in our soils under certain conditions. An old experiment, carried out by Boussingault, illustrates, in a general way, how rapid the process is under favourable circumstances. A small portion of rich soil was placed on a slab protected by a glass roof, and was moistened from time to time with water. The amount of nitrate of potash formed under these circumstances was estimated from time to time during a period of two months. During the first month (August) the percentage was increased from .01 to .18 (equal to about 5 cwt. of nitrate of potash per acre). The increase during the second month (September) was very much less,—indeed only about a seventh of the amount.[120] The soil experimented with was an extremely rich garden soil, and all the conditions for nitrification were most favourable.

Of recent experiments on the rate of nitrification, the most striking, perhaps, are those by Schloesing. He mixed sulphate of ammonia with a quantity of soil fairly rich in organic matter, and containing 19 per cent of water. During the twelve days of active nitrification no less than 56 parts of nitrogen per million of soil were nitrified per day. Taking the soil to a depth of 9 inches, this would be equal to more than 1 cwt. per acre—an amount of nitrogen equal to that contained in 6 cwt. of commercial nitrate of soda. These experiments are interesting as showing what is probably the maximum rate of nitrification under the most favourable circumstances, and where there is an abundant supply of easily nitrifiable nitrogen. That nitrification ever takes place in our soils to this extent is not to be for a moment supposed.