🧪Soil Fertility Evaluation: Methods, Approaches & Land Classification

Why Fertility Evaluation Matters: A Farmer’s Perspective

A sugarcane farmer in Uttar Pradesh wants to achieve a target yield of 100 tonnes/ha. Simply applying a blanket dose of fertilizer will not do — his soil has high K, medium N, and low P. Using the Soil Test Crop Response (STCR) targeted yield approach, the agronomist calculates the exact doses of N, P, and K needed for his specific soil and target. The result: higher yield with lower fertilizer cost. This is fertility evaluation in action.

Soil Fertility vs. Soil Productivity

Agricultural example: The coastal saline soils of Sundarbans have adequate NPK (fertile) but the high salt concentration prevents crops from growing well (unproductive). After reclamation with proper drainage and gypsum application, these soils become productive.

Soil Testing Research in India: Key Milestones

Standard Soil Testing Methods

Soil Test Rating Chart for Available Nutrients

Crop Response Data

• Soil test data is correlated with crop response, measured as percent yield or percent yield increase
• These represent the ratio of yield in unfertilized (deficient) soil to yield in fertilized (sufficient) soil

• Percent yield is used when comparing native fertility across different locations
• Percent yield increase is used in multi-location trials or pot experiments with soils from different locations
• The highest yield with all fertilizer nutrients applied = Maximum attainable yield

Approaches to Fertility Evaluation

1. Agronomic Approach (Blanket Recommendation)

• Based on fertilizer rate experiments conducted at many locations
• The level producing highest yield is averaged and recommended for a crop in a given agro-climatic region
• Gives a single blanket recommendation — does not account for soil-to-soil variability

Agricultural example: A blanket dose of 120-50-50 kg/ha of N, P₂O₅, K₂O is recommended for rice across a region. But a farmer with high-K soil wastes money on unnecessary K fertilizer.

2. Critical Limit Approach

• Developed by Waugh and Fitts (1965)
• Mainly for less mobile nutrients (P, K, micronutrients) where soils vary in fixing capacities
• Critical limit = the soil test value below which an economic response to added fertilizer is expected

Steps:

1. Incubation study: Soil incubated for 72 hours with graded doses of soluble P (as monocalcium phosphate)
2. Extracted P vs. applied P is plotted — for high-fixing soils, more fertilizer P is needed
3. The amount of P fixed is estimated as the X-value
4. In pot experiments, fertilizer P is added at 0, 0.5X, X, 2X levels
5. Percentage yield vs. soil test value is plotted and grouped using the Cate and Nelson (1965) method
6. The soil test value where the vertical line crosses the x-axis = soil critical limit

Agricultural example: In a laterite soil of Kerala that fixes 80% of applied P, the X-value is high. The farmer needs to apply much more P than a farmer on alluvial soil of Punjab where fixation is low.

3. Targeted Yield Approach (STCR)

• The most precise approach — provides site-specific, crop-specific, and target-specific recommendations
• Based on Soil Test Crop Response Function (STCR)
• Founded on Liebig’s Law of Minimum and the significant linear relationship between grain yield and nutrient uptake, first advocated by Troug (1960)

How It Works:

• For a specific yield target (T in q/ha), a definite quantity of nutrient must be taken up
• This is determined by the nutrient requirement (NR in kg) to produce one unit of target yield

Fertility Evaluation by Plant Analysis

Deficiency Symptoms (Visual Diagnosis)

• Careful inspection of plants can identify specific nutrient stress
• Characteristic symptoms relate to the nutrient’s function in the plant
• Limitation: visual symptoms may be caused by more than one nutrient deficiency

Tissue Tests (Semi-quantitative)

Agricultural example: A sugarcane farmer in Maharashtra tests the tissue sap of leaf sheaths every 5 weeks to monitor N and K levels, adjusting fertigation in real time.

DRIS: Diagnosis and Recommendation Integrated System

• DRIS identifies all nutritional factors limiting crop production
• More robust than critical value approaches because it considers the balance between nutrients, not just individual concentrations
• Based on nutrient ratios (e.g., N/P, N/S, K/Mg, K/Ca, Ca+Mg/K)

• When the ratio is too low, the nutrient in the numerator may be limiting
• When the ratio is too high, the reverse is true
• Common ratios: N/S, K/Mg, K/Ca, Ca+Mg/K, N/P

Crop Logging

Biological Tests for Soil Fertility

Tests Using Higher Plants

Microbiological Methods

Faster, simpler, and require less space than higher-plant methods.

Land Capability Classification (LCC)

Land Capability Classification groups soils based on their capability to produce common cultivated crops without deterioration over time. Developed by USDA and adapted in India by ICAR.

Classes I-IV: Suitable for Agriculture

Classes V-VIII: Not Suitable for Cultivation

Summary Table: Fertility Evaluation at a Glance

- Tisdale, S.L., Nelson, W.L., Beaton, J.D., Havlin, J.L. 1997. Soil Fertility and Fertilizers. 5th ed. Prentice Hall of India, New Delhi.
- Singh, S.S. 1995. Soil Fertility and Nutrient Management. Kalyani Publishers, Ludhiana.
- Maliwal, G.L. and Somani, L.L. 2011. Soil Technology. Agrotech.
- IARI Toppers Soil Science Part-9 (6th Edition 2025).

Summary Cheat Sheet