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
- The series of chemical, physical, and biological changes taking place in the soil, or in the processes of vegetable and animal nutrition, are conveniently expressed by the general term metabolism, and they are frequently designated as metabolic processes.