110 



AVAILABLE MOISTURE. 



In his investigations as to the relation of atmospheric precipita- 

 tion, esj^ecially rainfall, to the plants and the soil,Wollny shows that 

 the percentage of water in the layer of soil containing growing plants 

 increases from above downward as soon as the downward movement 

 of the rain water in the soil ceases, but that the percentage increases 

 from below upward while the rain is falling and so long as the water 

 continues to be penetrating doAvnward. The frequency of rainfall 

 is of even greater importance than the quantity. Slight rainfalls 

 that only wet the soil to the depth of a few millimeters do but little 

 good to the vegetation, because the greater part of the water is 

 quickly evaporated back again into the atmosphere. If it should 

 rain daily 2 millimeters during the three summer months, then, even 

 with this abundant precipitation the plants might die for want of 

 water, whereas if this total of 180 millimeters were uniformly 

 divided into ten or twelve rains during the three summer months it 

 would be considered a remarkably favorable growing season, since 

 under these conditions the earth would be wet down to a considerable 

 depth and the water thus stored up is protected from evaporation. 

 Therefore, for equal quantities of rain its value for agriculture 

 increases as the number of rainy days diminishes, and diminishes as 

 the number of rainy days increases, at least up to a limit that varies 

 with the character of the soil. 



In order to attain precise ideas on this subject, Haberlandt set out 

 a series of glass tubes full of dry earth ; each received at the begin- 

 ning a certain quantity of water, and by weighing these from day to 

 day he determined the loss due to evaporation. These losses are 

 given in the folloAving table, in percentages of the original quantity 

 of water, which latter may be considered as a rainfall whose depth is 

 given at the top of the column : 



Initial rainfall September ; 

 Loss by evaporat ion 



September 21 



September 22 



September 23 



September 24- 



September 25 



September 26 



September 27 



September28 



September 29 



September 30 



October 5 



October 10 



[) in millimeters. 

 in percentages. 



94.75 

 5.68 



Total in 20 days J00.43 



39.51 

 17.02 

 18.85 

 12.16 

 7.29 

 3.04 

 1.82 



26.34 

 10.22 

 14.87 

 14.56 



5.89 

 5.58 

 4.34 

 2.48 

 2.79 



14.78 

 10.09 

 13.39 

 11.82 

 7.30 

 8.17 

 3.48 



1.74 

 5.55 

 2.09 



9.81 



7.75 

 10.33 



1.86 

 1.76 

 6.31 



2.89 



7.48 

 9.05 

 8.09 

 7.05 

 6.70 

 3.48 

 3.04 

 2.61 

 2.00 



2.95 



