Water-Content of Plants 55 



tendency toward saturation apparently encourages a type 

 of bacterial action which may promptly result in great 

 harm to many agricultural plants. 



36. Exceptional plants. - The preceding statements 

 relative to the optimum water-supply are to be understood 

 as applying to a great maj ority of cultivated plants ; how- 

 ever, as an example of an exceptional crop the cranberry 

 may be cited. This plant grows to the greatest advantage 

 in typical bog situations. As ordinarily cultivated, 

 drainage is given this crop in such manner that the sur- 

 face soil will not contain free water ; yet under ordinary 

 circumstances the soil approaches saturation on account 

 of the low water-table, - - at the same time bog conditions 

 are such as to retard oxidation. The cranberry and many 

 other bog plants are therefore adjusted to the peculiar 

 conditions of their habitat. These bog conditions are, in 

 fact, extremely interesting, but it is unnecessary to go into 

 this subject further at this point. 



37. Unavailable water. - The plant is unable to with- 

 draw all of the film moisture in contact with the soil par- 

 ticles. If at any time the plant is unable to obtain from 

 the soil the water it requires, wilting will ensue. The 

 water then remaining in the soil is unavailable or non- 

 physiological. When this point is reached, the soil is dry to 

 the touch, yet an appreciable percentage of water remains. 

 For any plant the film water unavailable in a variety of 

 soils is proportional to the water-holding capacity of these 

 soils ; that is, the greater the water-holding capacity the 

 greater the pull against the plant when the content is low. 



Under ordinary agricultural conditions with loamy 

 soils, there will be from 5 to 12 per cent of water unavail- 



