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Water monitoring Devices: these are many
and varied - they do not all do what they are expected to
do! You cannot beat good agronomic knowledge and advice -
do not be persuaded by the plausable salesman - talk to people
with experience. If you put 20 different devices in the soil
you may well get 20 different suggestions - they are not all
right!
The 'right' equipment used badly is no
better than bad equipment. ' An essential tool is the Spade,
to be kept at all times in the truck! Even if you have the
best water monitoring system in the world (see later), you
need to check whether the information you have collected applies
to the whole field or just those particular spots. Also, when
your monitoring system tells you something you do not understand
it is important to go to the field, maybe with your adviser,
and dig - you will learn a great deal.
Computer
Models: Are all based on the Pennman-Monteith equation,
which takes weather data and calculates evapotranspiration
rates for crops by adding crop factors, based on crop cover,
stage, soil type etc. Much better than nothing as at least
they make the grower think! Are as accurate as the information
supplied! But, it is not easy to identify when a 'soil is
Full' or what percentage of water pumped is actually getting
into the soil as needed. When used in conjunction with good
point measurement they can be useful.
Gypsum blocks, TDR's,
and other buried devices: These can be useful, but
need to be installed very carefully (water can run down the
cables and by digging holes to install, the soil structure
can be changed, giving misleading data). You need to know
what volume of soil they are measuring, whether this is useful
or misleading and whether they are actually accurate - some
most definitely are not!
Tensiometers:
These come in various forms - basic, quick draw, electronic
etc. They all work on the same principle - measuring the 'suck'
(usually in Kilopascals) required to take water from the soil
as a plant may do. When used by scientists they can give useful
information, but most farmers find them confusing and often
misleading.
As can be seen from Tensiometer graphs,
the 'moisture' reading follows an exponential curve and once
one gets above 30kp there is very little change. Identifying
when to water is not clear. Soil structure will effect the
readings and generally it is found that they tend to over
estimate both dry and wet. A scientist will tell you that
you should water every time you get to 30 or 40 or 50 or 60
depending on the 'expert', but it is very difficult to know.
Most people will be watering too soon to get maximum water
use from the soil water that is 'free', hence wasting water.
There are quite a few situations where water use increases
as the soil dries, probably due to increases in oxygen, so
watering too soon will prevent this. When you need to 'see'
what is happening at multiple depths, they become more confusing
as they are difficult to install in one plane, which is important.
Better than nothing, but not ideal. Speak to a 'real expert'.
Neutron Probes
- only portable: Are considered a standard around the world.
They measure using a radioactive source, at multiple depths
and in deep soils with cheap labour are very successful. However,
here in the UK they are operated by companies that come round
once, or occasionally more, per week, collect the data, interpret
it and make recommendations. But there are a few drawbacks:
- Multiple sites become expensive. -Not too good near the
soil surface. -Measure a large volume of soil - can be good
and sometimes not so good. -The volume of soil measured can
vary according to the amount of water in the soil. -By measuring
large volumes of soil, can average wet and dry, which will
miss the true picture. -Complicated to own, due to regulations.
Capacitance
Probes - permanent or portable: These measure electronically,
through plastic tubes inserted in the soil, to multiple depths
and can be logged to monitor continuously. But, beware, some
are very good and some not so good! Some are very accurate
and some very inaccurate - check 'scientific accreditation'
and also with anyone who has been using them for some time.
Check sphere of influence (some are little better than tensiometers,
which rely on touch
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