Water Capacity of Soils. 



135 



This table shows very clearly that the amount of water 

 a soil can retain by capillarity is very materially influenced 

 by the distance it is above the zone of complete saturation 

 or of standing water in the ground. The decrease of water 

 upward is most rapid in the coarsest sand and it is least 

 rapid in the finest soil. 



It is remarkable that in sands so coarse as those used 

 water should continue to drain away during more than two 

 years from so short a vertical column and that so small an 

 amount of water was retained in the upper sections of the 

 columns. .It is not probable that drainage had become 

 complete from the two soils although it may possibly have 

 been, as there was no percolation during the last five days 

 of the trial. 



161. Proportion of Soil- Water Available to Crops. Not all 

 the water which soils will retain is available to plants. A 

 certain amount must be left overspreading the soil grains 

 which the roots of plants are unable to use. The amount 

 found in one field soil, when corn and clover ceased to grow 

 and when the leaves curled early in the day, is given in the 

 table below. In the same table is also given the moisture 

 of adjacent fallow ground determined at the same time and 

 which contains the least amount of water which, for this 

 soil, will permit maximum crops. 



Soil moisture relations when growth is brought to a standstill. 



The moisture contained in the fallow ground shows hmv 

 much this type of soil may retain, against evaporation and 

 percolation, during a dry season, and it liappi-ns to stand 



