Soil pH

pH is the single most important measurement you can make about your soil. Get this wrong, and no amount of fertilizer — organic or chemical — will work properly.

pH Scale: 0 (extremely acidic) → 7 (neutral) → 14 (extremely alkaline)

Optimal for most crops: pH 6.0–7.0 (slightly acidic to neutral)

Why pH Controls Nutrient Availability

pH determines whether nutrients that are present in your soil are actually available to plants. The same soil can have abundant phosphorus at pH 6.5 and zero available phosphorus at pH 7.8.

Nutrient	Problem Condition
Nitrogen (N)	Low availability below pH 5.5
Phosphorus (P)	Locked up below pH 5.5 AND above pH 7.5
Potassium (K)	Available across a wide range (6.0–8.0)
Iron (Fe)	Deficient above pH 7.5 (causes yellowing in young leaves)
Manganese (Mn)	Toxic below pH 5.0
Boron (B)	Deficient above pH 7.0
Zinc (Zn)	Deficient above pH 7.5 — critical problem in most of India

Phosphorus is the most pH-sensitive nutrient: It gets locked in calcium complexes (Ca₃(PO₄)₂) in alkaline soils and in iron/aluminum complexes in acid soils. This is why 65%+ of Indian soils are phosphorus-deficient despite heavy phosphate fertilizer application.

India's Soil pH Map

pH Range	Soil Type	Region	Key Problem
<5.5	Strongly acid	Northeast India, Kerala, parts of WB, Odisha	Mn toxicity, low nutrient availability
5.5–6.0	Slightly acid	Assam, Bihar hills, Western Ghats	Moderate — lime helps
6.0–7.0	Ideal	Parts of Punjab, Haryana, UP	Maintain with compost
7.0–7.5	Slightly alkaline	Central India, parts of Maharashtra	Organic matter focus
7.5–8.5	Alkaline	Rajasthan, Gujarat, Vidarbha, most Deccan	Zn & Fe deficiency common
>8.5	Saline-alkaline (usar)	Haryana flatlands, UP Terai, Reh lands	Crop-hostile, multi-year reclamation

Critical fact: Most of India's agricultural land is in the pH 7.0–8.5 range — moderately to strongly alkaline. This means Zinc deficiency is the most widespread micronutrient problem in Indian agriculture (67% of soils deficient, ICAR).

Organic pH Correction

For Acidic Soil (pH <6.0)

Dolomite Lime (Calcium-Magnesium Carbonate):

• Rate: 1–4 t/ha depending on severity
• Also supplies Ca and Mg — two secondary macronutrients
• Apply 1–2 months before planting for best effect
• Repeat test after 3–6 months

Wood Ash:

• Rate: 500 kg – 1 t/ha
• Contains K₂O (6–10% K), CaO, raises pH
• Fast-acting but short-lived
• Free byproduct of biomass burning

Compost (all pH ranges):

• The ultimate pH buffer — buffers both acidic and alkaline soils toward neutral
• Works slowly but permanently
• 5–10 t/ha/year consistently brings pH toward 6.5–7.0 over 3–5 years

For Alkaline Soil (pH >7.5)

Sulfur:

• Rate: 500 kg – 1 t/ha
• Soil bacteria (Thiobacillus) convert sulfur → sulfuric acid → lowers pH
• Slow-acting: 4–8 weeks for effect
• Combine with irrigation for distribution

Gypsum (Calcium Sulfate):

• Not acidic, but dramatically improves alkaline black cotton soil structure
• Rate: 1–2 t/ha
• Particularly effective for saline-alkaline (usar) soils
• Provides Ca + S; displaces sodium from soil exchange sites

Green Manuring:

• Dhaincha (Sesbania), Sunn hemp decomposition releases organic acids
• Lowers pH locally in the rhizosphere
• Combine with sulfur for larger effect

For Saline-Alkaline (Usar) Soils — pH >8.5

This is the most challenging soil type. Requires multi-year program:

1. Gypsum: 2.5 t/ha — displaces Na⁺ from soil
2. Leaching: Heavy irrigation to push Na⁺ below root zone (need drainage outlet)
3. Green manuring: Dhaincha is remarkably tolerant of usar soils
4. Biogas slurry or organic matter application
5. DSR (Direct Seeded Rice) — rice is one of few productive usar crops initially
6. Repeat for 3–5 years. Gradual reclamation is the realistic timeline.

Quick Field Test for pH

Vinegar Test (accuracy ±1 pH unit):

• Add 2 teaspoons of vinegar to dry soil
• If it fizzes → alkaline (pH >7)

Baking Soda Test:

• Add 2 teaspoons baking soda dissolved in water to soil
• If it fizzes → acid (pH <7)
• No reaction to either = roughly neutral

For serious farming decisions, always use a lab test (₹50–100 from any KVK or soil testing lab).

Soil Salinity (EC) — pH's Companion Problem

Electrical Conductivity (EC) measures soluble salt concentration in soil — a separate but related problem to pH. High EC and high pH often occur together (saline-alkaline soils) but not always — you can have acidic saline soil too.

EC Scale (dS/m — deciSiemens per metre):

EC (dS/m)	Classification	Crop Impact
<1.0	Normal	No restriction
1.0–2.0	Slightly saline	Sensitive crops affected (onion, beans)
2.0–4.0	Moderately saline	Most field crops show yield decline
4.0–8.0	Saline	Only tolerant crops (paddy, barley, cotton) viable
>8.0	Strongly saline	Crop-hostile; reclamation needed

Field test: Mix soil with water 1:2 by weight, let settle 30 minutes, measure with an EC meter (₹500–1,500, available from agri-input shops). Lab test costs ₹50–100 alongside pH.

Organic EC management:

• Gypsum (2–5 t/ha) displaces sodium from soil exchange sites — same treatment as alkaline soil correction
• Leaching irrigation with good-quality water pushes salts below root zone (needs drainage outlet — doesn't work in poorly drained fields)
• Organic matter dilutes salt concentration per unit volume and improves structure so leaching works better
• Avoid manure/compost from animals on high-salt feed or near coastal areas — can reintroduce salts

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Next: Organic Carbon — The Foundation of Soil Fertility