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TENSIOMETER IRRIGATION SCHEDULING GUIDE
Important Note:

Monitoring the rate of change of the soil moisture tension, is just as important as the actual reading used to schedule the irrigation.

 

Soil Texture
 

Fine:

Clay,

Silty Clay,

& Sandy Clay

Moderately Fine:

Clay Loam, Silty Clay Loam,

& Sandy Clay Loam

Medium:

Silt,

Silt Loam, Loam,

& Very Fine Sandy Loam

Moderately Coarse:

Fine Sandy Loam &

Sandy Loam

Coarse:

Loamy Sand & Sand

Tensiometer Reading at the Time of Irrigation

(Units: cenitbars of soil moisture tension)
12" Tensiometer is installed in the

6" - 9" depth of the root zone

70 - 80

60 - 70

50 - 60

40 - 50

30 - 40
 
% Available Water Depleted at given Tension

25 - 30%

30 - 35%

35 - 40%

40 - 45%

45 - 50%
Average Available Water

2.0"/ft.

2.5"/ft.

2.25"/ft.

1.5"/ft.

1.0"/ft.
Approximate Soil Moisture Tension at Field Capacity

30 cb

25 cb

20 cb

15 cb

10 cb
 
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Soluble Salts

Soluble salts contribute additional soil moisture tension than that measured by the tensiometer (i.e., it measures matric potential, which is water adsorbed to the individual soil particles and held between the small capillary pores). Thus, when using a irrigation water containing an electrical conductivity (ECiw) of 1.0 mmho/cm (i.e., about 640 mg of soluble salts/liter of water), the soil solution (ss) at the time of each irrigation will develop an equilibrium electrical conductivity (ECss) value of about 2 mmhos/cm (i.e., when using a 2" application). This value corresponds to an additional soil moisture tension of about 72 cb (e.g., an electrical conductivity of 1 mmho/cm is approximately equivalent to 36 cb of tension). Therefore, the total soil moisture tension can range between 100 to 150 cb, for coarse thru fine soils respectively. Consequently, the typical soil analysis lab results for soluble salts will range between an electrical conductivity of the soil saturation extract (ECe) of 0.5 - 1.5 mmhos/cm; this is for coarse thru fine soils respectively when applying a 2" irrigation of the given quality.

NOTE: For most soils with a two foot root zone, the 0 - 12" depth represents about 70 % of the total plant uptake of soil moisture. The ceramic tip of the tensiometer tube is about 3" in length and is placed in the 6" - 9" root zone depth. Here it measures the soil moisture tension at this depth only. Consequently, the above soil in the 0 - 6" depth will be at a greater soil moisture tension (i.e., the soil is drier and can have values considerably > 80 cb which the tensiometer cannot read). Also, the 12" - 24" root zone depth where about 30% of the total plant uptake of soil moisture occurs, will have soil moisture tension that are lower than that measured in the 6" - 9" depth (i.e., the soil is wetter with low cb readings which the tensiometer easily measures). A 24" tensiometer is recommended for placement in the 18"- 21" root zone depth for monitoring water movement below the root zone.

 

Technical Questions about HIT may possibly be quickly answered by contacting:
Rudy Garcia
Natural Resources Conservation Service
Soil Conservationist & Water Quality Specialist
e-mail: rgarcia@nm.nrcs.usda.gov
or call: 1-505-522-8775, extension 116

 

Note: Technical Questions may possibly be quickly answered by contacting USDA, Natural Resources Conservation Service Soil Conservationists:

Holistic Irrigation Technology (HIT) Rudy Garcia, Soil Conservationist (Water Quality),  rgarcia@nm.nrcs.usda.gov or call: (505) 522-8775, ext. 116

Remote Sensing & GIS Technology Dave Christenson, Soil Conservationist (Remote Sensing), dchriste@nm.nrcs.usda.gov or call: (505) 522-8775, ext. 115

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Copyright © 1999 Regional Precision Farming Pilot Project
Last modified: April 28, 2000