64 



is distinguished from capillary water in that it does not move by capillary 

 action or otherwise and can not be absorbed by the plant roots in sufficient 

 quantities to be of any practical value to sustain normal plant growth. 



Capillary water is that which exists as a thickened film around each soil 

 particle and partially fills the pore spaces. It is held in the soil against 

 gravity and is not drawn out by drainage. It moves in the soil in any direc- 

 tion, and rises in the soil between the soil particles because of the same 

 action which draws the oil up a lamp wick. Capillary water is of greatest 

 importance to plant growth. It covers the plant roots and is the water on 

 which the growth of plants depends. 



Capillary water moves from a wet soil to a drier soil and this is what 

 causes the water to spread laterally and to be drawn from the wetter sub- 

 soil to the surface where it is evaporated if the evaporation is not checked 

 by a mulch. The water will rise to a greater height in a soil whose texture 

 is fine than in a soil whose texture is coarse, but the rapidity of rise is 

 greater for a soil of coarse texture than for one of fine texture. For in- 

 stance, Professor Hilgard found that in a sandy soil the maximum height to 

 which water rose was 17 inches in 6 days, while the maximum height for 

 a clay soil was 46 inches in 195 days. In the sandy soil it took 1 hour to 

 rise 8 inches while in the clay soil it took 12 hours to rise the same height. 



Gravity water is that water which moves downwards through the soil 

 pores because of gravity. When the soil is saturated the pores are entirely 

 filled with water and that water which fills the space between the pores not 

 occupied by capillary water, is gravity water. Gravity water is not retained 

 by the soil if there is natural or artificial drainage. It passes downwards 

 and supplies capillary water to the soil below and the excess reaches the 

 water table or a drainage channel. When there is an excess of gravity 

 water, it passes down to a depth which is too far below the ends of the 

 plant to be drawn up by capillarity and is wasted. 



A soil which is saturated contains gravity, capillary and hygroscopic 

 water. A soil which is drained contains capillary and hygroscopic moisture 

 only. A soil which is air dried in the sun contains hygroscopic moisture 

 only and a soil which is dried by artificial heat contains no moisture. 

 Percentage of free moisture in soil for ji'ant viroutli. 



The pores for most cultivated soils will average from 25 to 50 per cent, 

 by volume of the entire volume, being smallest for sandy soils and greatest 

 for clay soils. For plants to grow it is necessary that they have air as well 

 as water and for best growing condition it has been found that the capillary 

 water in the soil should range from 40 to 60 per cent, of the pore space. 

 This leaves about an equal space for air. In other words, for maximum 

 growing condition a very sandy soil containing 25 per cent, pore space 

 should have 10 to 15 per cent, by volume of free moisture, while a heavy 

 clay soil containing 40 to 60 per cent, pore space should have 20 to 30 

 per cent, by vo'ume of free moisture. Expressed in per cent, by weight, 

 for a very sandy soil air dried weighing 110 pounds per cubic foot, the free 

 moisture content should be from 5.7 to 8.5 per cent.; for a stiff clay soil 

 weighing 75 pounds the moisture content should be 16.6 to 25 per cent, 

 and for an average sandy orchard loam, 10 to 15 per cent. Professor 

 Loughridge found that on a sandy soil where the moisture content in the 

 fourth foot of the soil was 15 per cent, by weight, there was an excess of 

 moisture which crowded out the air and caused the trees to suffer. In this 

 case the correct moisture content should have been about half of the pore 

 space or about 10 per cent, by weight. 



