276 ABSORPTION OF WATER 



(2) in the upper layers of any soil after a heavy rain or irrigation, but before 

 the water content of the soil has become attenuated to its field capacity. As 

 the water in the films surrounding the soil particles with which the root tips 

 are in contact becomes depleted, more water moves towards those particles 

 by capillarity. The actual rate of such capillary movement of water through 

 the soil may become a factor influencing the rate of absorption. Root sj'stems, 

 however, are not static, but are more or less continually growing through the 

 soil. The rate of root growth of most species decreases, as a general rule, with 

 increasing wetness of the soil above the field capacity, due to the corresponding 

 reduction in soil aeration. Hence in soils in which capillary movement of 

 water occurs — which are necessarily relatively wet — the rate of elongation of 

 roots is generally less than in otherwise similar but somewhat dryer soils. 

 This continued growth of root tips through the soil brings them into contact 

 with other portions of the soil water, so even if capillary movement to certain 

 points ceases, capillary movement of water to the roots may be re-established 

 by the extension of the root tips themselves into zones of the soil which have 

 not yet been depleted of available water. 



Many plants, much of the time, grow in soils at water contents between 

 the wilting percentage and the field capacity. In this range of soil water 

 contents capillary movement of water is negligible. Once most of the film 

 water present on the soil particles with which the root tips are in contact 

 has been absorbed it cannot be replaced in any significant quantity by capillary 

 movement from adjacent regions of the soil if the soil water content is below 

 the field capacity. Neither does water move in vapor form through soils 

 towards the absorbing regions of roots at appreciable rates. Under such con- 

 ditions the absorbing region of every rootlet often becomes surrounded with 

 a narrow cylindrical zone of soil which has been depleted to a water content 

 much below that of the surrounding soil. 



Since in soils at a water content below the field capacity movement of 

 water towards the roots is inappreciable, the only method by which the roots 

 come in contact with additional increments of water is by continually growing 

 through the soil (Burr, 191 4). Mature root systems of many species of plants 

 bear millions of root tips. Each of these numerous root tips may be pictured 

 as slowly, although often intermittently, progressing through the soil and 

 absorbing most of the water present in the smaller interstices between the 

 soil particles with which they come in contact (Livingston, 1927). That 

 large quantities of water can be absorbed in this way, at least by some species 

 of plants, is shown by the results of Dittmer (i937) on the extent of the 

 root system ol a winter rye plant. The total length of all the roots on a four 

 months old plant of this species was found to be 387 miles. This means that, 



