Why Taking Activity Measurements - PRONOVA Stelzner pH AGRAR 2000 Technical Instructions

MULTI 2000
›
Application tips for measuring pH in
liquids and substrata/soils
When measuring the pH in liquids, pH glass
insertion electrode is held directly in the li-
quid being measured. The value may be read
as soon as measured value has stabilised on
the meter's display. Temperature influences
play a secondary role in agriculture.
The pH of soils and substrata can be measu-
red directly or measured using an extraction
solution. The soil must be moist in order to
take direct measurements in the soil (at least
50 % available water capacity). If the soil is
too dry, the measurement hole can be mois-
tened with deionized water. Be sure to wait
until the water is evenly distributed.
Attention! Drill a measuring hole using the
enclosed dibble (which corresponds to the
length of the glass probe). Insert the glass
probe in this hole for the measurement. Gent-
ly press the soil around the probe. Gently pull
out the electrode after the measurement.
Clean it with distilled water and pat dry.
Stony substrata or soil must be sieved before
the measurement is taken.
A measurement should be taken in a soil
solution when the soil is too dry or rocky.
For this purpose, a 1:5 mixing ratio of subs-
trata to extraction solution should be used.
For mineral soils, the ratio should be 1:2. We
recommend using CaCl
lution. (0.01 molar CaCl
in the case. It should be dissolved in 10 litres
of distilled water. This solution may then be
stored for later.) After the larger particles of
substrata/soil settle out, the pH value can be
measured using the glass probe in the remai-
ning solution.
When taking comparative measurements,
make sure to maintain a similar method
of measuring. Measurements of pH taken
directly in the ground using the glass probe
may be compared to the measured values
achieved from a soil extraction in distilled
water. Values from LUFA (a German agricul-
tural investigation and research institute) are
based on an extraction with calcium chloride
(at pH 6, these values are lower by about 0.4
pH than the values for aqueous extracts).
as the extraction so-
2
powder is enclosed
2
Testing the pH glass insertion
electrode using a mV display
With an attached pH glass insertion elec-
trode, the testing function can be activated
by pressing the MODE button. A mV value
will be displayed. The intact probe must ge-
nerate a differential value of at least 145 mV
in both buffer solutions.
If this difference value is too low (due to
damage or ageing), the electrode must be
replaced.
The electrode slope will appear immedia-
tely after the meter is turned on. It should be
between 45 mV and 59 mV. A reliable measu-
rement is not possible when this value falls
too low.

4.2 Why taking activity measurements

Why measure the nutrient activity in the
soil?
The activity meter MULTI 2000 determines
the activity of the dissolved salts in the soil.
In chemistry, activity means efficiency of
ions. The dissolved salts decompose into
positively-charge cations and negatively-
charged anions. In pure water, the electri-
cally charged ions move freely and react
quickly. In soil, the ions are decelerated and
their range of action is strongly restricted;
this means that their efficiency is severely
impaired in comparison to their efficiency in
water. How much of their efficiency they lose
is described by the activity coefficient accor-
ding to the following equation:
d x fb = a/d = density = concentration in g/l of soil
/fb = activity coefficient
/a = activity in g/l of soil = efficient amount
Activity measurement
A continuous flow of electrical current is
applied to the soil using an electrode. The
electric current will induce the nutrients,
which are all ions, to move to the poles of
the electrode. There, they will be deprived of
their electric charge thus disappearing from
the soil. The correct expression is: the soil is
electrolysed.
GB
(degree of efficiency)
of ions
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