Popular Science Monthly/Volume 41/May 1892/Energy as a Factor in Agriculture
By Dr. MANLY MILES.
THE rapid development of science and its numerous applications in the industrial arts are leading to a general recognition of its importance as a factor in the material and intellectual progress of the age. The aid of science is now invoked in every department of human activity, and, judging from what has already been accomplished, we can not perceive any indications of a limit to its useful applications in the industries.
While the general outlook encourages optimistic views in regard to the present and prospective advantages that may be realized from the applications of science, we should not overlook the shadows involved in its progress, which seriously interfere with its own advancement, and at the same time increase the difficulties attending original investigations relating to many industrial problems.
The scope and extended range of modern science, that necessitate a subdivision of its lines of research into numerous branches, each of which requires a lifetime of diligent study for its mastery, are serious obstacles in the investigation of a certain class of problems that can only be solved by contributions from the entire circle of the sciences.
Prof. Huxley has sounded a note of warning which should be heeded, especially by those who are engaged in conducting experiments for the advancement of agricultural science. In his retiring address as President of the Royal Society he says: "Of late years it has struck me with constantly increasing force that those who have toiled for the advancement of science are in a fair way of being overwhelmed by the realization of their own wishes. We are in the case of Tarpeia, who opened the gates of the Roman citadel to the Sabines, and was crushed under the weight of the reward bestowed upon her. It has become impossible for any man to keep pace with the progress of the whole of any important branch of science. If he were to attempt to do so his mental faculties would be crushed by the multitude of journals and voluminous monographs which a too fertile press casts upon him. This was not the case in my young days. A diligent reader might then keep fairly informed of all that was going on without demoralizing his faculties by the accumulation of unassimilated information. It looks as if the scientific, like other revolutions, meant to devour its own children; as if the growth of science tended to overwhelm its votaries; as if the man of science of the future were condemned to diminish into a narrower and narrower specialist as time goes on.
"I am happy to say that I do not think any such catastrophe a necessary consequence of the growth of science; but I do think it is a tendency to be feared, and an evil to be most carefully provided against. The man who works away at one corner of Nature, shutting his eyes to all the rest, diminishes his chances of seeing what is to be seen in that corner; for, as I need hardly remind my present hearers, that which the investigator perceives depends much more on that which lies behind his sense-organs than on the object in front of them.
"It appears to me that the only defense against this tendency to the degeneration of scientific workers lies in the organization and extension of scientific education in such a manner as to secure breadth of culture without superficiality; and, on the other hand, depth and precision of knowledge without narrowness."
From the exceeding complexity of many of the problems in agricultural science, and the number of factors that require consideration in attempts to solve them, there is especial need of guarding against the dangers attending the exclusive prosecution of special lines of research, which are so forcibly stated by Prof. Huxley with reference to the general advancement of science.
In almost every problem in agriculture the complex phenomena of life are directly concerned, under various forms and activities, which can not be expressed or formulated in chemical terms, from the self-evident truth that the part can not contain the whole. The significance and interdependent relations of the biological factors in agriculture are unavoidably obscured by the exclusive consideration of specific details which, with the advance of knowledge, may prove to be but incidents in the manifestations of general laws.
The solution of these Protean problems can only be secured by abstract researches to determine the relations of the several factors to each other, and to the general laws of which they are the expression. The principles of science that are admitted to be of general application are the only safe guides in developing an improved and rational system of agriculture, while the purely empirical lines of research that aim to discover specific rules of practice, and thus gain immediate practical results, retard the march of progress by the delusive importance assigned to nonessential details.
The truth of these statements may be illustrated by the remarkable progress of the physical sciences in the past quarter of a century, and the rapid development of the industrial arts through the recognition and applications of the principle of the conservation of energy, which Faraday looked upon as "the highest law in physical science which our faculties permit us to perceive," and Huxley refers to, in connection with evolution, as "the greatest of all of the generalizations of science."
The principle of the conservation of energy, which is now generally admitted to be a prime factor in Nature's operations, has not received adequate attention in agricultural science. It is true that in general terms it has been incidentally referred to as a factor in biology, more particularly with reference to mechanical work, but the dominance of purely chemical considerations has prevented its real significance in all organic processes from being fully recognized.
More than twenty-five years ago, Dr. William B. Carpenter pointed out to physiologists the "distinction between the dynamical and the material conditions; the former supplying the power which does the work, while the latter affords the instrumental means through which that power operates."
The material conditions have, however, continued to receive a predominant, and almost exclusive, share of attention, and the manifestations of energy in the processes of vegetable and animal nutrition have practically been ignored.
In the applications of science to agriculture, and especially in planning and conducting experiments, the transformations of matter have been looked upon as the sole factors requiring attention, and a simplicity in organic processes has been assumed that is not warranted by our present knowledge of the conditions that have a decided influence on the nutrition and well-being of plants and animals.
An approximate quantitative estimate of the expenditure of energy in certain processes of Nature involved in growing a field crop will serve to illustrate its importance in biological science and farm economy, and a preliminary review of some of the salient points in the economy of plants will simplify the problem we have to deal with.
A growing crop, in common with other living organisms, requires certain conditions of environment for the healthy and vigorous exercise of its vital activities, among which may be enumerated as essential, a suitable temperature, a certain supply of moisture, and a sufficient food-supply; and to these must be added soil conditions that promote an extended root development and distribution.
Plants differ as to the temperature required for active growth, but there is for each a minimum, below which growth ceases; a maximum, above which life is destroyed; and between these an optimum temperature which is most favorable for the activity of the processes of nutrition. The temperature of the atmosphere, which is an incident of seasons, need not be noticed here, but it may be remarked that it is of less practical importance than soil temperatures, which depend on conditions that, to some extent, may be controlled.
Plants obtain their supply of water from the diffused moisture of the soil, which is retained by capillary attraction. In fertile soils this capillary water is kept in constant circulation by the drafts made upon it by growing plants, and by the evaporation which takes place from the surface soil, and an equilibrium is thus maintained in the distribution of soluble soil constituents, and in the processes of soil metabolism.
To say nothing of other important considerations, it is evident that soil conditions favorable for the extended distribution of the roots of plants are necessary to enable them to obtain their needed supplies of water from the comparatively limited amount present in the soil. As the water evaporated from the surface soil is replaced from below by capillary attraction, its influence on soil metabolism and the transportation of soluble soil constituents toward the surface strata should receive attention as a factor in the economy of plant growth that is closely related to that presented "by the water absorbed by the roots of plants and exhaled by their leaves.
Energy has been defined as "the power of doing work, or overcoming resistance," and its varied transformations into heat, motion, electricity, etc., without gain or loss, are expressed by the general term conservation of energy. In the nutrition and growth of plants an expenditure of energy is evidently required in the work involved in a number of distinct, but correlated, processes, the most important of which are—constructive metabolism, or the building of organic substance; the exhalation of water by the leaves, which is constantly taking place in their processes of nutrition; the evaporation of water from the surface soil; and the warming of the soil to provide optimum conditions of temperature.
The energy expended in constructive metabolism, or tissuebuilding, is stored up as potential energy, and reappears as heat when the plant is decomposed by any process, as, for example, when it is burned. The mechanical force exhibited by growing plants is a phase of the constructive process that has often been noticed. President Clark's squash raised a weight of 4,120 pounds in its processes of growth. Sprouts from the roots of a tree pushing their way through an asphalt pavement have been observed by myself, and many similar exhibitions of the force exerted by growing plants are often seen.
These obvious manifestations of energy in constructive metabolism are, however, so familiar that they require but a passing notice, and we will proceed to consider the much larger expenditures of energy involved in vaporizing the water exhaled by the leaves of plants and evaporated from the surface soil, as these unobtrusive and incidental processes, as they might be termed, are quite as significant factors in plant growth as the direct work of building organic substance, to which the attention of physiologists is more particularly directed. In field experiments the results obtained with manures must largely depend on the expenditure of energy, under the prescribed conditions, in the work of exhalation by the plants and the evaporation of water from the surface soil. The supply of plant food in the manure may, in fact, be a matter of secondary importance to the growing crop.
Experiments at Rothamsted, England, and on the continent by Hellriegel, on the exhalation of water by a variety of farm crops, including wheat, oats, peas, beans, and clover, show that about three hundred pounds of water are exhaled by the leaves for each pound of dry organic substance formed by the plants. It was estimated by Lawes and Gilbert that the average annual exhalation from the wheat grown on some of the experimental plots at Rothamsted was at the rate of 1,680,000 pounds of water per acre, or the equivalent of 7·4 inches of rainfall; and on the same "basis the exhalation from a crop of Indian corn, of 60 bushels per acre, would be equivalent to about 8·5 inches of rainfall.
So far as the expenditure of energy is concerned, it matters not whether water is changed to vapor in the process of exhalation by the crop or in evaporation from the soil, and the same standard of measurement will, therefore, be applicable in both cases.
Energy is measured in heat-units, and work is expressed in foot-pounds or in kilogramme-metres. For convenience of illustration we will make use of another standard adoj)ted by engineers, which, although not as definite, is sufficiently accurate for our purpose.
From experimental data it has been found that, under favorable conditions, one pound of coal will evaporate from 6·60 to 8·66 pounds of water from an initial temperature of 32 Fahr., according to the quality of the coal used. If we assume that one pound of coal will evaporate 8·5 pounds of water under the conditions presented in crop-growing, our standard will be considerably above what is realized in ordinary steam-engines.
The energy required to vaporize the water exhaled by one acre of corn in its processes of growth, with a yield as above estimated, would, therefore, be represented by the heat produced in burning 226,500 pounds of coal, or over 113 tons. This may be expressed in another form, which will, perhaps, be more readily understood. We are told that "a good condensing engine of large size, supplied with good boilers, consumes two pounds of coal per horse-power per hour." The work involved in the process of exhalation from one acre of corn would, therefore, be equivalent to the work of more than twenty-five horses day and night, without cessation, for six months.
The same standards of measurement applied to the energy expended in evaporating water from the soil will give quite as striking results. With a sufficient rainfall to supply the requirements of a crop, the amount of water evaporated from the soil will vary, within certain comparatively narrow limits, with the amount and distribution of the rainfall, the capacity of the soil for heat, and the atmospheric conditions that influence evaporation, as temperature, humidity, and the character of the prevailing winds.
From the best evidence I can obtain, which need not here be discussed in detail, it appears safe to estimate the soil evaporation in the Middle States at approximately twice the amount exhaled by a growing crop of fair luxuriance. Of an annual rainfall of thirty-two inches, or over, fairly distributed, we may then assume, with apparent good reason, that about sixteen inches will be disposed of by evaporation from a fertile, well-drained soil, and about eight inches by exhalation from a growing crop, or an aggregate of about twenty-four inches will be disposed of in the form of vapor from soil and crop, involving an expenditure of energy represented by the heat produced by burning 320 tons of coal per acre, or the equivalent of the work of seventy-three horses, day and night, without intermission, for six months. If to this is added the energy expended in constructive metabolism and in warming the soil, which we will not now estimate in specific terms, the sum would represent the normal demands for energy in growing a crop of one acre.
This enormous expenditure of energy appears to be quite as essential to the well-being of the crop as the supply of food constituents, to which attention has been too exclusively directed, and any conditions that tend to materially increase or diminish it must be looked upon as injurious.
From this standpoint the principle of the conservation of energy furnishes most satisfactory data for discussing the philosophy of farm drainage. On undrained, retentive soils, the rain that falls in excess of the normal requirements of the crop and soil metabolism must be removed by evaporation, and this calls for a very considerable expenditure of energy that on drained land might be made available in useful work, to say nothing of the influence of removing surplus water by evaporation on the physical and biological characteristics of the soil.
For each inch of surplus rainfall removed from the soil by evaporation, the energy expended would be represented by 26,600 pounds of coal per acre. With an annual rainfall of forty inches, which is not unusual in the Middle States, and is considerably exceeded in some localities, there would be sixteen inches of water in excess of the normal demands of an ordinary farm crop, and to remove this by evaporation would require the equivalent of about 213 tons of coal per acre, representing the continuous work of forty-eight horses, day and night, for six months. The removal of this surplus water by drainage would obviate the necessity for this enormous expenditure of energy, besides other incidental advantages which we need not notice here.
In the economy of animals the manifestations of the law of the conservation of energy are quite as striking and significant. The potential energy of their food is the sole source of the energy expended in work, and in their processes of nutrition and growth. Animals have been looked upon as machines for converting the vegetable products of the farm into animal products of greater value, and this in the light of the law of the conservation of energy may be interpreted as the conversion of the potential energy of field crops into the useful work of an animal machine. Considered as machines for the manufacture of definite products, the efficiency of animals must depend upon the amount of work performed for a given supply of energy in their food.
An ordinary steam-engine formerly converted less than one tenth of the potential energy of the fuel consumed into useful work, and the attention of engineers has been directed to improvements in construction to secure greater economy and efficiency in the work performed, by a more complete utilization of the potential energy supplied in the form of coal or other fuel. The remarkable industrial development of the past few years, resulting in a material reduction of the cost of production and transportation, is largely owing to improvements in the steam-engine which have been brought about by a more intelligent application of the principle of the conservation of energy.
There are good reasons for the belief that the animal machine works with greater economy than the steam-engine, even in its improved form, but, according to the most favorable estimates, only a small proportion of the potential energy of foods is utilized in useful work, and there is a broad margin for improvement, even in what we call our improved breeds, to secure a more efficient expenditure of energy.
The problem of paramount interest in animal husbandry is essentially the same the mechanical engineer has been dealing with in his efforts to improve the steam-engine. It is simply to obtain the largest net returns in useful work from the potential energy of the food consumed. It is evident that improvements in the animal machine itself must be the leading object to receive attention, and the breeders of pure-bred stock must recognize this principle in their efforts for improvement. The form and proportions in which the chemical constituents of food are provided are of far less importance than the inherited capacity and capabilities of the animal machine to utilize and economize energy in the work involved in the manufacture of animal products.
When speaking of foods we should bear in mind the fact that there is but a limited demand in the animal economy for the so-called nutritive constituents, aside from their agency in the transformations of energy involved in the metabolism of the system. But a small proportion of the chemical constituents of foods are stored up in the body, even during the period of growth, when the demands for new materials in constructive metabolism are most active, while an abundant supply of energy in an available form must be provided as an essential condition of the manifestations of life. It must not, on the other hand, be assumed that the potential energy of foods may be considered as a reliable index of their physiological value. Biological processes are exceedingly complex, and, in calling attention to energy as a dominant factor in vital activities, we do not lose sight of other important considerations which can not here be noticed.
Protean transformations of energy are constantly carried on in all the metabolic tissues. The energy expended in building organic substance in animals, as in plants, is stored up in the form of potential energy as an essential condition of its constitution, and it is again liberated in the form of heat in the correlative processes of destructive metabolism which are taking place without cessation in the work performed in every operation of the system.
Dr. Foster tells us that what is really meant by the phrase, "living substance, is not a thing, or body, of a particular chemical composition, but matter undergoing a series of changes." These metabolic changes are brought about, in the main, at the expense of energy, and they represent in fact successive transformations of energy from the active to the potential form, and a final reconversion to heat, which leaves the body in various ways.
The animal machine is in effect a heat-engine that is constantly being worn out by the work performed, and as constantly repaired by its own processes of nutrition, and the heat leaving the body (animal heat) represents the energy that has been used in internal work, and finally liberated through the agency of destructive metabolism.
We must not, however, carry the analogy of the heat-engine so far as to assume that the food consumed by animals is disposed of by a process of combustion, like the fuel burned under a steamboiler. There is no evidence that anything like a combustive oxidation of the food constituents, or of the tissues, takes place in the animal economy. The building of organic substance and storing of potential energy (constructive metabolism) is accompanied by parallel processes of disintegration (destructive metabolism), in which the stored potential energy is changed to heat; and these alternate, or possibly simultaneous, transformations of energy which take place in living tissues must be regarded as manifestations of vital activities that differ widely in their characteristic features from the processes of combustive oxidation that take place in non-living matter.
From what is now known in regard to animal physics it will be safe to assume that from four fifths to five sixths of the potential energy of the food consumed and digested by working animals is expended in vaporizing the water thrown off by the skin and lungs, and in the internal work performed by the metabolic tissues in their constructive processes of nutrition, and the energy used in this internal work finally leaves the body as animal heat, a very large proportion of which is the result of muscular and glandular metabolism.
The work performed in twenty-four hours by the heart alone of a man weighing 150 pounds is estimated at 75,000 kilogramme-metres, an expenditure of energy sufficient to raise his own weight to a height of 3,600 feet, and the work performed by other internal organs, and in vaporizing the water thrown off by the skin and lungs, is quite as significant.
The energy expended in some of the unobtrusive operations of Nature that are likely to escape attention may exceed in amount the more obvious expenditures in mechanical work. We readily recognize the demands for energy by an animal moving a heavy load when working eight or ten hours a day, while we fail to notice that from two to three times as much energy is expended by the same animal in the course of twenty-four hours in vaporizing the water thrown off by the lungs and skin. As this energy is all derived from the food consumed, it must be taken into the account as a significant factor in discussing the physiology of nutrition.
Another important fact should not be overlooked. In the reconstructive processes that are carried on without intermission in the living tissues of the animal machine, a supply of energy, as we have seen, must be constantly provided to replace that which is thrown off from the system in the form of heat, or expended in vaporizing water and in external work; but new materials are not required to replace all the disintegrated constituents of the tissues, as there is a rearrangement, to a certain extent, in the processes of repair of the elements of which they are composed. This is especially the case with muscle, which constitutes so large a proportion of the proteid substance of the body. The work performed by muscle is not at the expense of its nitrogenous substance, and its energy is, to a great extent, if not exclusively, derived from the carbohydrate elements of the food. The demands of the proteid substance of muscle for nitrogen are, therefore, limited, and the available supplies of energy in the various elements of the food determine the efficient activity of the animal machinery.
Energy as a factor in animal physics seems to be entirely overlooked in the application of the popular theory of nutritive ratios. There is a wide difference in the potential energy of feeding rations that have been formulated for the same specific purpose, with practically the same nutritive ratio. On the same page of a popular agricultural paper I find two rations for milk- production, the one having a ratio of 1 to 5, and the other of 1 to 5·1, but there is a difference in potential energy in the two rations equivalent to over 2,411,000 kilogramme-metres of work, or one and a quarter horse-power in the day's rations.
In two other rations for milk-production with nutritive ratios of 1 to 5 and 1 to 5·1, the difference in potential energy would be represented by 3,112,000 kilogramme-metres, or 1·6 horse-power for the day's feed.
There are likewise rations with exactly the same nutritive ratio (1 to 5), prescribed for Jersey cows giving milk, in which the difference in potential energy is equivalent to 1,123,600 kilogramme-metres, or more than one half of a horse-power for the day's feed. There are also rations for horses, with nutritive ratios 1 to 6, and 1 to 6·4, which have a difference in energy of 2,834,000 kilogramme-metres, or the equivalent of over one and a quarter horse-power for the day.
It is unnecessary to cite further instances of the obvious fallacies in rations that have been formulated in accordance with a theory which ignores the significance of energy in animal nutrition. The facts already presented must be sufficient to show that the law of the conservation of energy should be recognized as an important factor in the nutrition and growth of both plants and animals, and that it should receive due attention in planning and conducting experiments for the promotion of agricultural science, and in interpreting their results. In the development of a rational system of farm economy the applications of this general law must have a dominant influence in determining the most profitable and consistent methods of practice.