Soil pH and Vegetable Crops

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.

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.

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:

Vegetables	pH requirement
Asparagus	6.0-8.0
Beets, cabbage, muskmelon, peas, spinach, palak, summer squash	6.0-7.5
Sweet corns, pumpkin, tomatoes,	5.5-7.5
Snap bean, lima bean, carrot, cucumber, parsnip, peppers	5.5-7.0
Eggplant, watermelon	5.5-6.5
Potato	5.0-6.5

 pH Tolerance of Vegetables:

Less tolerant (pH 6.8-6.0)	Moderate tolerant (pH 6.8-5.5)	Very tolerant (pH 6.8-5.0)
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	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.