Irrigation in Orchards 205 



some heavy soils summer irrigation must be discontinued 

 much earlier. All new wood should be well matured be- 

 fore frost, and the grower need not be alarmed if the ground 

 becomes so dry that some of the foliage turns yellow and 

 begins to drop before this time, for the trees will not suffer. 



How Water is held in the Soil 



Water may be found in the soil in three forms: hydro- 

 scopic, capillary, or free. The first two forms of water are 

 held as a film about the soil particles, and are sometimes 

 referred to as "film moisture." The hydroscopic water is 

 the part of the film nearest the soil particle, and it probably 

 serves the plant very little. It is even present in road dust, 

 and can be driven off only when the temperature is raised 

 to the boiling point. 



The outer part of the film on the soil grain represents the 

 capillary water, and this is the direct water-supply for the 

 plant. It does not flow under the influence of gravity, but 

 passes from particle to particle, and thus tends to keep the 

 soil uniformly moist. Soil is in ideal condition for plant 

 growth when it is well supplied with capillary water. 

 Such a soil is moist enough to stick together when pressed 

 in the hand, and yet will crumble when broken apart. 



Free water is that which fills the spaces between the soil 

 particles. It flows under the influence of gravity, and is 

 that part of the soil water carried off by drains. When it 

 occupies the part of the soil where roots normally grow, 

 it is actually detrimental to the growth of most plants. 

 When well below the surface, it serves as a reservoir from 

 which water may be drawn by capillary action. Soils 

 that contain free water stick together when pressed in the 



