ICT Role in 21st Century Education & its Challenges.pptx
Pract 5.pptx
1. Pract. 5.:
STUDY OF DETERMINATION OF
SOIL MOISTURE CONTENT BY
GYPSUM RESISTANCE BLOCK
METHOD
2. Gypsum resistance blocks are used for indirect
measurement of soil moisture content.
Resistance blocks work on the principle that the flow
of electricity between two electrodes in a porous
block, embedded in soil, depends on the moisture
content of the soil.
Resistance to the flow of electricity in a porous
medium is inversely proportional to the moisture
content.
The most common material used to make the block is
gypsum (Plaster of paris) but nylon and fiberglass
have also been used.
3. The commonly used electrical resistance
instrument was developed by Bouyoucos (1949)
and hence, they are called as ‘Bouyoucos
moisture meter’.
The instrument uses a specially designed
wheatstone bridge for resistance measurement.
Generally these read about
400-600 ohms at field capacity
50,000-75,000 ohms at wilting point.
7. PREPARATION OF GYPSUM RESISTANCE BLOCKS
MATERIALS REQUIRED
• Gypsum block assembly board (Frame/mold),
• Single core cable,
• Plaster of paris (dental grade),
• Water,
• Beaker,
• Glass rod,
• Screwdriver,
• Knife,
• Bucket of water and
• Bouyoucos moisture meter.
8. PROCEDURE
1. Expose about 5.0 cm of single core cable and dip it in molten tin.
2. Set two electrodes in each compartment of the frame parallel to
each other.
3. Prepare slurry of plaster of paris with water (about 1:1
proportion) and mix thoroughly by stirring.
4. Pour the slurry into the compartment in one stroke and fill it.
5. Remove air bubbles if any by gentle tapping.
6. Allow blocks to set hard.
7. Remove blocks from the frame and dry it for about fortnight in the
shade.
9. Standardization of gypsum resistance blocks
1. Lower the block gradually in a bucket of water and allow to
saturate for about half an hour.
2. Take the block out and expose to the atmosphere for about 10
minutes. Lower the block again in the bucket. This helps to
remove the entrapped air if any.
3. Record the resistance reading by connecting the wire leads to
the Bouyoucos moisture meter while the block is still in water.
4. Record the reading of different blocks as above.
5. Reject the blocks, which show deviation of more than five per
cent of the mean resistance reading.
10. INSTALLATION OF GYPSUM RESISTANCE BLOCKS
MATERIALS REQUIRED
Gypsum resistance block, post-hole augur and Bouyoucos
moisture meter.
PROCEDURE
Sink a bore with posthole auger to the depth of installation.
It should be in the root zone and preferably just on the side
of a crop line.
Place the block inside and close the bore in small portions
tapping the soil with a metal rod or a stick.
Fill the whole bore with soil in natural order. 3-4 blocks
could be placed in each bore at different depths.
Measure resistance with the help of Bouyoucos moisture
meter. When the resistance comes to the recommended
level, irrigate the field.
11. Note
1. Gypsum blocks are not sensitive under water
logged or saline soil conditions.
2. In Clayey soils the gypsum blocks lose contact with
the soil due to shrinkage.
13. Maximum water holding capacity (M.W.H.C.) is the
amount of soil moisture expressed in per cent on oven
dry basis held by a soil when it is completely saturated
with water.
It is also called as ‘Maximum water retention capacity
or Saturation capacity (S.C.)’.
It is the upper limit of water holding capacity. At this
moisture content, all the pore spaces are completely
filled with water and no space for air.
The tension of moisture at saturation capacity is almost
0.0 to 0.1 atm. and it is equal to free water surface.
Water present in this condition is of limited use for
plant growth.
14. MATERIALS REQUIRED
Pestle and mortar,
0.5 mm sieve,
Keen’s cup (Keen Raczkowski box with perforated bottom
and ring),
watch glass,
filter paper,
scissor,
spatula,
small brush,
petri plate,
water bottle,
glass rod,
knife, balance, soil sample and
hot air oven.
17. Keen’s cup + a filter paper +
Moist Soil
‘b’
Oven Dry wt
‘c’
18. PROCEDURE
1. Air dry the soil sample, break the aggregates as far as possible and
pass the soil through a 0.5 mm sieve.
2. Weigh the Keen’s cup along with a filter paper (placed inside on
the perforated bottom) on a watch glass (a).
3. Pour the sieved soil sample into the cup. While pouring the soil
sample just tap the box gently on the working table with each
addition of soil for uniform placing. When the cup is full slice off
the extra quantity of soil with the help of knife. Tap the box, add
little more quantity of soil and again remove the surplus stuff to
ensure a level.
4. Place the Keen’s cup containing soil in a water tray and leave it for
saturation over night. Next day, remove the cup, dry the outer
surface by using filter paper and record its weight along with watch
glass (b)
19. d =
y - x
5
5. After weighing, dry it in hot air oven at 105oC till constant
weight is obtained. Later cool the Keen’s cup and weigh along
with watch glass (c).
6. Apply correction factor for the amount of water absorbed by the
filter paper (d) as follows.
Weigh five filter papers of uniform size of that of Keen’s cup
diameter together (x). Saturate them with water. Roll gently with a
glass rod over them to squeeze out excess water uniformly. Weigh
again (y). Calculate the average amount of water held by one paper
(d) using the formula.
7. Calculate the maximum water holding capacity (M.W.H.C.) of
the soil sample by using the formula.
20. M.W.H.C. (%) =
Weight of moisture
x 100
Oven dry weight of soil
M.W.H.C. (%) =
[(b-a) – (c-a)] – d
x 100
(c-a)
M.W.H.C. (%) =
b – c - d
x 100
c – a
7. Calculate the maximum water holding capacity (M.W.H.C.) of
the soil sample by using the formula.
21. Pract. 7.:
STUDY OF DETERMINATION OF
FIELD CAPACITY AND PERMANENET
WILTING POINT OF SOIL
22. I. FIELD CAPACITY OF SOIL
Field capacity (F.C.) is the amount of soil moisture
expressed in per cent on oven dry basis when it is
completely saturated with water and the downward
movement of excess water has practically ceased.
Such a stage is reached generally 48-72 hours after
saturation. Sandy soils reach F.C. earlier than clayey
soils.
At field capacity, moisture is retained in the soil against
the force of gravity. Soil moisture tension at F.C.
varies from soil to soil but it generally ranges from 0.1
to 0.3 atm.
Field capacity is the upper limit of available soil
moisture range for plant growth.
23. MATERIALS REQUIRED
Straw or polythene sheet,
spade,
water,
screw auger,
moisture can,
balance and
hot air oven.
24. PROCEDURE
Select a representative spot in the field. Ensure that
water table is not within two meters.
Bund an area of about one square meter on all four
sides and remove all weeds.
Pour water till the desired depth gets sufficiently wet.
Spread straw or polythene sheet on the surface.
After 48-72 hours, draw soil samples from the desired
layer with a screw auger and determine the moisture
content.
Repeat these on successive day/s till the values become
nearly the same.
25.
26. II. PERMANENT WILTING POINT OF SOIL
Permanent wilting point (P.W.P.) is the amount of soil
moisture expressed in per cent on oven dry basis at which
nearly all plants wilt and do not recover in a dark humid
chamber unless water is added from an external source.
Permanent wilting point is the lower limit of available
soil moisture range for plant growth. The force with which
the moisture is held by the soil at this stage corresponds to
15 atm.
It is the characteristic of the soil and not the plant.
Sunflower plant is commonly used as an indicator plant for
the determination of permanent wilting point.
27. MATERIALS REQUIRED
2000 grams capacity can with lid,
sunflower seeds,
glass tube (0.5 cm diameter),
cotton plug,
wax,
bell jar,
water tray,
moisture can,
soil sampler,
balance and
hot air oven.
28. PROCEDURE:
1. Make three holes on the lid of the can so as to pass through the
plants and also a glass tube. In addition, provide a drainage hole at
the bottom of the can.
2. Fill the cans with soil taken from a plot of which permanent
wilting point is to be determined. Sow five seeds of sunflower and
allow them to germinate by providing adequate moisture.
3. After emergence, thin out the plants to two. Allow them to pass
through the holes of the lid and place the lid.
4. Avoid heating the cans due to insolation, by keeping them in moist
sawdust or straw. Grow them for about six weeks or till they
develop three pairs of leaves.
5. Add sufficient water to bring it to field capacity.
6. Insert a glass tube in the soil through the lid and plug it with cotton
wool.
29. PROCEDURE ……… contd.
7. Seal the lid and also the drainage hole with sealing wax. After
sealing the can further irrigation should be stopped.
8. Paint the bell jar with black colour or paste a black colour paper
on the bell jar so as to restrict the entry of light in to the bell jar.
9. Allow plants to wilt in the open atmosphere and when they show
wilting symptoms, transfer them to a dark humid chamber (keep
the cans in a small tray containing water and place the dark
coloured bell jar over it). Leave the plants to recover overnight. If
they gain turgidity, transfer them back to atmosphere till they show
wilting and then transfer back to dark humid chamber. Repeat till
they do not recover.
10. At this stage, remove the lid, cut the plants and remove the
roots. Draw a small quantity of soil from the can and determine the
soil moisture content.
30. OBSERVATIONS
•Draw neat diagrams of the layout of field capacity determination
and permanent wilting point experiment.
•Determine the field capacity and permanent wilting point of
given soil sample. Comment on observations.
STUDY QUESTIONS
☺Mention the factors affecting field capacity and permanent
wilting point.
☺What are the points to be considered while estimating field
capacity?
☺Is permanent wilting point characteristic of soil or plant?
State reasons.
☺Why sunflower plant is selected for wilting point studies?
☺Why the plants have to be kept in dark humid chamber?
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