crop will require and how he can provide for that demand. 

 Perhaps the greatest of all needs is water. By turning to Cor- 

 nell Experiment Station Bulletin 120, page 419, it will be seen 

 that in a dry season a bushel of potatoes requires about three 

 tons of water for its production. If Mr. White expects 200 

 bu.shels of potatoes per acre, he must somehow manage to pro- 

 vide 600 tons of water for each acre. He has no facilities for 

 irrigation, and his only resource is to make the soil a reservoir. 

 He must store the supply left by winter snows and spring rains, 

 and also the irregular rainfall that comes during the season's 

 growth. Speaking in broad averages, in soils most commonly 

 met with, this storage possibility amounts to about 300 tons of 

 w^ater per acre in the first eight inches of the soil. It must be 

 understood that this amount is not in the form of standing water^ 

 for water standing in the soil for any length of time injures both 

 soil and plant. 



3. The most useful form of water for playits is film moisture. — 

 Water is capable of assuming many forms, such as steam, 

 vapor, ice, or free-moving liquid. The condition most valuable 

 in the soil is none of these, but is in the form of film moisture. 

 This film moisture can be shown by dipping a marble into 

 w^ater and observing the film of water surrounding it on all sides. 

 When each soil-grain is covered with film moisture, as the marble 

 is, the ideal conditions of soil moisture exist. This form of water 

 is largely independent of gravitation and travels readih^ in all 

 directions, as can be .seen b}^ dipping a cube of sugar into a spoon- 

 ful of coffee. It is capable of transporting plant-food to the 

 roots of plants from remote corners, where the roots do not reach. 



It will be observed that film moisture is held only on the sur- 

 face of soil-grains. The more the soil is pulverized, the more 

 soil-grains there wdll be, and therefore the greater amount of sur- 

 face to hold film moisture. 



The difference in the capacity of lumpy and fine soils to hold 

 film moisture is surprising to one who has not given the question 

 study. George W. Cavanaugh, assistant chemist at the Cornell 

 Experiment Station, has very graphically shown this by the fol- 

 lowing experiment : He put some small marbles in a tumbler, as 

 showm by Fig. i, and the total amount of film moisture that the 



