

sie 
Automatic Watering 
Dr. Kenneth Post 
Department of Floriculture and Ornamental Horticulture 
Cornell University, Ithaca, N.Y. 
PRINCIPLES BACK OF AUTOMATIC WATERING 
When a bench of soil is thoroughly 
watered from the surface and allowed to drain, 
the water remaining is held by capillarity. 
It surrounds the soil particles and the roots, 
and is in many of the small pores between the 
soil particles. As the soil becomes drier 
the films of water are held tighter around 
the soil particles. The tensiometer reading 
is greater indicating an inorease in the cap— 
illary tension. 
In a bench of soil that has dried some- 
what, these films of water are larger at the 
bottom than at the top of the bench because 
of the action of gravity. The oapillary ten 
Sion is greater near the top of the soil than 
near the bottom. 
At first these films of water are all 
connected. As water is removed from one area, 
more water moves by capillarity to replace 
the loss. By capillary movement the filme 
of water tend to remain equal in size through- 
out the soil volume, If the soil becomes 
quite dry, the capillary films are broken and 
water no longer moves to the dry spots. 
The oapillary movement of water is rapid 
at low tensiometer readings. All automatio 
watering should, therefore, be at low tension 
to take advantage of this fact. Capillary 
movement is most rapid in fine sand and poor 
est in olay and silty soils; by maintaining 
low capillary tension it can be maintained in 
any soil type. 
See also Cornell Bulletin 793. 
HOW AUTOMATIC WATERING WORKS 
Methods of automatio watering employ 
these principles of capillary movement. The 
lower layer of soil is wet and water moves 
toward the top of the soil by capillarity. 
The soil is never saturated with water, and 
oxygen can diffuse into it readily from the 
surface. 
A water-tight bed level lengthwise is 
necessary. It is best to put tile ov some 
other conductor in the bottom and to place a 
thin layer of gravel, from 1 to 2 inches, in 
the benoh to provide easy movement of the 
water under the soil. 
When the soil reaches a oapillary ten- 
sion of 1 inch, only enough water is injected 
in the bottom of the bench to reduce the cap- 
illary tension to 0, Usually from 3 to 5 
pints to each square foot of bench area are 
enough. It will all be absorbed by the soil 
in about two hours. The soil acts as a wick 
pulling the water upward and satisfying the 
Oapillary tension. Water moves laterally as 
well as upward and moistens soil at the side 
of the benoh. 
APPLICATIONS OF AUTOMATIC WATERING 
Several variations in the application of 
automatio watering to cut-flower crops have 
evolved. (1) The benches may be piped and 
valved individually. When the tensiometer 
reads 1 inch, the valve is opened and the 
water flows for a period of time sufficient 
to inject the correct amount of water (about 
two quarts to the square foot). (2) An elec= 
tric valve wired to a tensiometer and the cur= 
rent run through a time switch operates the 
valve automatically (see wiring diagram). 
(3) a constant water level may be maintained 
in the bench by the use of a poultry watering 
float valve (see diagram), (hy As many benches 
may be connected to one unit as are of the 
same elevation, and any of these systems of 
getting water to them can be used. 
The float-valve arrangement giving a con- 
stant water level requires the least equipment 
and is liked by many growers. This arrange- 
ment has been tried the shortest time of any 
method, First installations have been in 
operation for about one year. It looks good 
and should give satisfaction if the water 
table is kept Just in contact with the soil, 
If the water is too high in the soil, anaero= 
bic bacteria will become active, marsh gas 
will arise, and plants will be injured. 
This system is at a disadvantage if the 
bench leaks. If bench leaks cannot be plugged, 
the injection system may be used to provide 
enough water at each injection to take care 
of the leak and to moisten the soil, 
TIME CLOCK 





RELAY 
Ni VAC UUM 
sce é Sate 
WATER 
MAIN 
To TO 
BE NCH TENSIOMETER 
cup 
WIRING DIAGRAM FOR COMPLETE AUTOMATIC WATERING 
OF BENCH CROPS 
WON 1 Ade tb SHE Hh a Ht Ht Oe ab at a a 
THE SOIL TENSIONETER 
The soil tensiometer was adapted as a 
guide to kmow when to water. It does not 
measure the moisture content of the soil but 
doses measure the tension with which the water 

