68 



Elementary Principles of Agriculture 



air space, because the capillary films will not be thick 

 enough to fill the spaces between the grains, espe- 

 cially if the soil is coarse grained. This is the condition 

 most favorable to the growth of roots, because both 

 water and air are present. (Fig. 39.) 



(c) Hygroscopic water does not differ essentially from 

 capillary water, except that it is held more firmly to 

 the grains. Air-dry soil may still contain from one to 

 ten per cent of hygroscopic water, that is, water which 

 may be driven off only by heating to the temperature 

 of boiling. Clay soils, in particular,- often contain large 

 amounts of hygroscopic moisture. 



1 OOa. Rate of Percolation of Water Through Soils. Prepare lamp 

 chimneys as in ^ 95b, filling them two-thirds full, using different 

 kinds of soil. Quickly fill all the chimneys full to the top with water, 

 and then notice the time required for water to begin dripping at 

 the lower end. It will be well to place small -mouthed bottles 

 under each chimney to collect the drippings. In this way the amount 

 of water percolating through the different soils may be estimated. 

 Which would be preferable in field conditions, for the water to per- 

 colate rapidly or slowly? Discuss this question. 



101. The Amount of Capillary Water which a soil may 

 retain varies with the soil. This is a measure of the power 

 of a soil to store up water. The following table, taken 

 from Schubler*, who first investigated this property 



*See Johnson, How Crops Feed. 



