Vegetables
are one of the important food-plant species that comes under the human food-chain
due to their eminent nutritive value. Like other plant species vegetables are
also bound to take their nutrition from soil. One of the important factor that
controls the availability of these nutrients is the Soil pH.
Now
what is pH? In brief, pH is the soil reaction measurement of acidity or
alkalinity. In chemistry the pH was defined as the negative logarithm of
hydrogen ion (H+) concentration, i.e.
–log[H+]. Now, the pH scale has a range of 0 to 14 units which
defines the acidity or alkalinity. The mid-point of the scale is 7.0 and
counted as the neutral unit which is neither acidic nor alkaline.
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| Soil pH scale |
The
degree of proximity from 7.0 to 0 determines the extent of acidity while from
7.0 towards 14.0 determines the extent of alkalinity. Concept of soil pH stands
with the same principle.
Now, why soil pH is important to the crop?
Soil
pH determines the availability of almost all essential nutrients of soil. Plant
uptakes the nutrients as consumable metal ions which may be counted as plant
food. The availability of most nutrients are highest near the neutral point.
Towards acidity many of the essential nutrient specially phosphorus (P) becomes
unavailable for plant consumption due to phosphate fixation. Nutrients like
Aluminium (Al) and manganese (Mn) becomes toxic to the plant. The beneficial
microorganisms contained in soil become unavailable, even sometimes the
activity of adverse microorganism increased in acidic soil which is not desirable
as it affects plant health.
On
the other hand, alkaline/basic soil prevents the availability of some nutrients
like copper (Cu), Zinc (Zn), Iron (Fe) etc. Some nutrients like manganese,
phosphorus which shows unavailability in strong acidic soil also becomes
unavailable in alkalinity. Iron dependent plants specially leafy vegetables
suffers most in alkaline soil.
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| Availability of nutrients in soil |
How Soil pH can change?
1. Leaching through irrigation and
rain water
2. Formation of weak acids by reaction
of CO2 with water evolved from decomposed organic matter as well as respiration
in rhizosphere.
3. Acid rain
4. Use of some fertiliser which
cause reaction to develop strong acids.
Optimum pH requirement of some vegetables:
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Vegetables
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pH requirement
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Asparagus
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6.0-8.0
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Beets, cabbage, muskmelon,
peas, spinach, palak, summer squash
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6.0-7.5
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Sweet corns, pumpkin,
tomatoes,
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5.5-7.5
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Snap bean, lima bean, carrot,
cucumber, parsnip, peppers
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5.5-7.0
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Eggplant, watermelon
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5.5-6.5
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Potato
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5.0-6.5
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pH Tolerance of Vegetables:
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Less tolerant (pH 6.8-6.0)
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Moderate tolerant (pH
6.8-5.5)
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Very tolerant (pH 6.8-5.0)
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Asparagus, beet, broccoli,
cabbage, cauliflower, celery, palak, swiss chard, cress, Chinese cabbage,
leek onion, lettuce, muskmelon, New Zealandspinach, okra, orach, parsnip,
salsify, spinach, water cress.
|
French bean, lima bean, Brussels
sprout, carrot, collard, cucumber, brinjal, garlic, horse radish, kale,
Knol-Khol, parsley, pea, chilli, pumpkin, radish, rutabaga, summer squash,
winter squash, tomato, turnip
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Chicory, dendellion, endive,
fennel, potato, rhubarb, shallot, sorrel, sweet potato, watermelon
|
Examples of Physiological disorders of some vegetables associated with pH:
A. Tip
Burn in Chinese cabbage:
This disorder primarily happens due to Calcium deficiency which is predominant
in acidic soils. 2 floiar application of cycocel @1120 ppm or 4 application of
CaCl2 reduces the appearance.
B. Browning
in cauliflower:
This disorder is associated with Boron deficiency. Boron deficiency is common
when soil is alkaline, i.e. pH goes
beyond 6.5. Application of Borax/Sodium Borate/Sodium tetra borate @ 20kg/ha as
soil application reduces boron deficiency. Boron becomes toxic to the plant in
acidic soils where pH goes below 5.5.
C. Whiptail
in cauliflower:
cauliflower plants shows whiptail symptoms due to the Molybdenum deficiency.
The molybdenum deficiency commonly occurs in acidic soil with a pH below 6.5.
Application of lime to increase the pH and application of Sodium/Ammonium Molybdate
@ 1.0 kg per ha reduces the problem.
D. Akashin
of radish:
Akashin disorder in radish is associated with the boron deficiency which is
common in alkaline soil. Spraying of 1-2 ppm boric acis may reduce the problem.
E. Brown heart in turnip: This disorder generally
happens when the soil is too acidic where boron is either in toxic or deficient
form. Application of Borax @10-15 kg/ha in soil along with 0.2% boric acid
spray as foliar application @ 2-3 times controls the problem.