ENERGY AS A FACTOR IN AGRICULTURE. 93 



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 as- 

 sume, 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 aggre- 

 gate 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 con- 

 stituents, 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 philos- 

 ophy 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 ne- 

 cessity 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. 



