Re: soil moisture meter system required

[Steve Evett <srevett@ag.gov>]

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Concerning  Jac Le Roux's reply to Naas Terblanche:

Jac, I think your four points are well made except for number 3 concerning
the EnviroSCAN system, which is not a TDR system. It is a capacitance
system and has the usual susceptibility to soil texture and salinity of
those systems - a point you make in your number 2. We have calibrated it
for a clay loam soil and found the factory calibration to be off by about
0.2 m^3/m^3 at the wet end. Also, our calibration was quite different (by a
factor of 2) from that published by Paltineanu and Starr in Soil Science
Soc. Amer. Journal for several U.S. soils. Introducing saline water into
the calibration barrels caused the reported water content to jump up by 50%
when the soil was saturated. The system gives quite interesting soil water
dynamics and may well be useful if calibrated for a particular soil and if
the soil salinity doesn't change much. But, it has the expected problems of
a capacitance system, not those of a TDR system.

Best regards,

Steve

At 08:12 AM 8/3/99 +0000, you wrote:
>
>2. Cheap capacitance sensors seem to work in sandy soils but run off the
>scale in heavier soils. I know it's a matter of calibration; but my
>ultimate feeling is that this is not the way to go if you are serious about
>soil moisture and irrigation management.
>
>3. If the application is critical, such as growing potatoes in sand (the
>kind of sand where only a 4x4 will go) then a sophisticated TDR system such
>as Enviroscan or Adcom is probably your best bet. This will give you real
>time soil moisture variation and make it possible to optimise production
>(provided the management is in place).
>
>4. If you have a large number (20+ ?) of different fields, blocks, crops or
>soil types  I suggest a portable probe such as a neutron probe, in
>conjunction with an irrigation scheduling program (such as Probe Schedule,
>Waterman or NPS). I am very fond of my neutron probe(s) because I can
>calibrate it for the soil type I am dealing with and get results that are
>marvelously repeatable.   It might seem odd that you could have control
>over soil moisture and irrigation with sporadic measurements but if your
>software can interpolate with some accuracy what happens between
>measurements it is possible to come within 95% accuracy. This is the system
>I use and can refer you to many satisfied growers who have either increased
>yield by 20%, improved internal fruit quality or in the most extreme case
>cut irrigation by 50%. 
___________________
Steve Evett, Soil Scientist
USDA-ARS, 2300 Experiment Staton Rd., Bushland, TX 79012 USA
806-356-5775, FAX: 806-356-5750
srevett@ag.gov, http://www.cprl.ars.usda.gov/programs/