THE WATER-COST OF DRY MATTER 143 



ing is almost indispensable. Fallowing may be replaced 

 by crops such as corn or sugar beets, which receive culti- 

 vation throughout the season and thereby set free plant- 

 food for the following crop. It is probable, however, that, 

 even under conditions of irrigation, as in the West, 

 where land is plentiful and water scarce, it may in the 

 end be profitable to observe the occasional clean fallow of 

 the land. The resting period not only helps to destroy 

 weeds, plentiful under irrigation, but enables the soil to 

 resume a natural physical condition, to set free plant- 

 food and to start again a favorable bacterial flora. The 

 value of fallowing is well shown is another of the Utah 

 experiments. One series of soils had been cropped steadily 

 for four years; another series had been cropped only 4hree 

 out of four years, and still another series had been cropped 

 only one year out of four. These three series of soils were 

 left exposed to the elements for three years; that is, they 

 received a three-years' fallow. They were then all sown 

 to corn, which grew and flourished well. The transpira- 

 tion ratios, determined for each series of soils, were almost 

 identical. This shows that the three years of fallow had 

 restored the three soils to an approximate equality of 

 fertility, so far as water-consumption was concerned, 

 although at the beginning of the period, they had been 

 left widely different by the various treatments they 

 had received. The fallow period, objectionable chiefly 

 because of the chance it gives the organic matter to be 

 oxidized by the air, has great advantages in restoring the 

 soil to a condition where crops may be produced at a low 

 water-cost. 



101. The vigor of the plant. Whenever the seasons, 

 the nature of the soil, the available plant-food, the treat- 

 ment of the. soil ; the factors above discussed, favor vigor- 



