Unit 5 - Soil Fertility and Plant Nutrition | MSc Horticulture & Agronomy

Soil Fertility and Plant Nutrition

( HORSS-201 & AGRMI-202 )

UNIT V

Soil Fertility Evaluation and Soil Test Crop Response (STCR) Correlations

Soil Fertility Evaluation

Overview
Soil fertility evaluation is the systematic assessment of a soil’s ability to supply essential nutrients for optimal plant growth and yield. Accurate evaluation is fundamental for effective nutrient management, sustainable agriculture, and maximizing crop productivity.

Objectives of Soil Fertility Evaluation

• Determine nutrient availability and deficiency/toxicity.
• Assess soil’s productive capacity.
• Provide fertilizer recommendations for crops.
• Prevent nutrient losses and environmental pollution.
• Aid in site-specific nutrient management (SSNM).

Major Methods of Soil Fertility Evaluation

1. Visual Diagnosis

• Observation of Deficiency Symptoms: Identifying characteristic signs of nutrient deficiencies or toxicities in crops (e.g., chlorosis, necrosis, stunted growth).
• Limitations: Symptoms may be confused with those caused by pests, diseases, or environmental stresses.

2. Plant Tissue Analysis

• Chemical Analysis: Measuring nutrient concentrations in plant tissues to assess current nutrient status and diagnose hidden hunger before visible symptoms appear.
• Diagnostic Ratios: Use of nutrient ratios (e.g., N:P:K) to interpret balance and sufficiency.

3. Soil Testing

• Laboratory Analysis: Quantitative assessment of soil samples for available macronutrients (N, P, K), micronutrients, pH, organic matter, and cation exchange capacity.
• Soil Test Calibration: Relating soil test results to crop responses for fertilizer recommendations using approaches like the Soil Test Crop Response (STCR) and target yield equations.
• Modern Techniques: Use of infrared spectroscopy, geostatistical analysis, and digital soil mapping for high-resolution fertility assessment.

4. Biological Tests

• Indicator Plants: Growing specific plants sensitive to certain nutrient deficiencies to detect soil fertility status.
• Microbiological Assays: Using microorganisms (e.g., Azotobacter, Aspergillus niger) to estimate nutrient availability based on their growth response.

5. Field and Pot Experiments

• Field Trials: Testing different fertilizer treatments on crops to determine optimal types and rates for specific soils and regions.
• Pot Culture Methods: Controlled experiments using indicator crops to assess nutrient status in small soil samples.

6. Remote Sensing and GIS

• Spatial Mapping: Integration of soil test data with GPS and remote sensing to generate fertility maps and zoning for precision agriculture.

Modern Approaches

• Soil Test Crop Response (STCR): Establishes fertilizer recommendations based on the relationship between soil test values and actual crop yield, aiming for targeted yield levels.
• Diagnosis and Recommendation Integrated System (DRIS): Uses nutrient ratios and mathematical models for balanced nutrient management.

Interpretation of Soil Test Results

Nutrient Status	Level in Soil	Fertilizer Recommendation
Low	Below critical level	High recommendation
Medium	Around critical level	Moderate recommendation
High	Above critical level	Low or no recommendation

• Critical level: Nutrient level below which crop response to fertilizer is likely.
• Nutrient Index (NI): developed by Parker et al. in 1951, calculates a single value for soil fertility based on the number of samples falling into low, medium, and high nutrient status categories.

Nutrient Index (NI) = (1 * L) + (2 * M) + (3 * H) / TNS

Where L = low, M = medium, H = high fertility samples and TNS = Total number of samples..

Factors Affecting Soil Fertility Evaluation

• Sampling method and depth
• Soil heterogeneity
• Time of sampling
• Calibration with crop response
• Climatic and crop management practices

Summary Table: Soil Fertility Evaluation Methods

Method	Description/Use
Visual Diagnosis	Field observation of plant symptoms
Plant Tissue Analysis	Lab analysis of nutrient content in plant tissues
Soil Testing	Chemical analysis of soil samples
Biological Tests	Indicator plants, microbial assays
Field/Pot Experiments	Fertilizer trials for recommendation
Remote Sensing & GIS	Digital mapping and spatial analysis

Soil Test Crop Response (STCR) Correlations

Concept and Purpose

Soil Test Crop Response (STCR) correlations are scientific approaches that establish quantitative relationships between soil test values, crop yield responses, and fertilizer requirements. The main objective is to develop precise fertilizer recommendations tailored to specific soil fertility levels and crop yield targets, thereby optimizing nutrient use efficiency, sustaining soil health, and minimizing environmental impact.

Objectives of STCR Approach

• To optimize fertilizer use efficiency.
• To achieve a targeted yield level.
• To make fertilizer recommendations based on existing soil nutrient status.
• To promote site-specific nutrient management.

Basic Concept of STCR

The STCR approach is based on the principle:

"Fertilizer recommendation should depend on both the fertility status of the soil and the desired yield target."

It involves three key parameters:

1. Nutrient Requirement (NR): Amount of nutrient required to produce one quintal of economic yield (e.g., grain).
2. Soil Efficiency (SE): Percentage of nutrient supplied by soil contributing to yield.
3. Fertilizer Efficiency (FE): Percentage of applied nutrient used by the crop.

Key Components of STCR Correlations

• Soil Testing: Rapid chemical analysis to determine the available nutrient status (N, P, K, etc.) of the soil before planting. This provides a baseline for fertilizer recommendations.
• Crop Response Studies: Field experiments are conducted to measure how crops respond to varying fertilizer doses across different soil test values. These studies help derive the relationship between soil nutrient status and crop yield.
• Fertilizer Prescription Equations: Using the data from soil tests and crop response trials, yield-targeting equations are developed. These equations calculate the exact amount of fertilizer needed to achieve a specific yield goal, accounting for:
• Soil’s native nutrient supply
• Fertilizer use efficiency
• Nutrient contributions from organic sources (e.g., FYM)

Methodology

• Gradient and Test Crop Experiments: These are designed to generate a range of soil fertility levels and observe crop responses, forming the basis for STCR equations.
• Targeted Yield Approach: For a desired yield, the nutrient requirement is estimated, and fertilizer doses are adjusted based on soil test values and nutrient recovery rates.
• Verification Trials: Field trials validate the accuracy and applicability of the developed equations under local agro-climatic conditions.

Advantages and Applications

• Precision Fertilizer Recommendations: STCR provides site-specific, quantitative guidance for fertilizer application, improving yield and nutrient use efficiency.
• Sustainability: By matching fertilizer inputs to actual crop needs and soil supply, STCR helps maintain soil fertility and reduces environmental risks.
• Adaptability: The approach can be customized for different crops, soils, and regions, and is compatible with precision agriculture tools like variable-rate fertilizer application.

General STCR Fertilizer Equation

F = (T×NR−S×SE) / FE​

Where:

• F = Fertilizer dose (kg/ha)
• T = Targeted yield (q/ha)
• NR = Nutrient requirement per q of yield
• S = Soil test value (kg/ha)
• SE = Soil efficiency (%)
• FE = Fertilizer efficiency (%)

Steps in STCR Approach

1. Conduct field experiments across different fertility gradients.
2. Measure initial soil test values for N, P, K.
3. Apply different levels of fertilizers.
4. Record crop yield and nutrient uptake.
5. Develop regression equations based on the relationship between soil test values, applied fertilizers, and yield.
6. Derive fertilizer adjustment equations for yield targets.

Advantages of STCR

• Highly scientific and quantitative method.
• Helps in achieving targeted yields.
• Improves fertiliser use efficiency.
• Reduces chances of over- or under-application.
• Environmentally friendly and cost-effective.

Limitations of STCR

• Requires extensive field experimentation for calibration.
• Equations are crop- and region-specific.
• Less adaptable in multi-cropping or diversified systems.
• Does not account for residual or carry-over effects.
• Needs accurate and timely soil testing.

Examples of STCR Equations

For rice in an alluvial soil region:

• N = 4.38T - 0.34SN
• P₂O₅ = 2.95T - 4.12SP
• K₂O = 2.27T - 0.31SK

Where:

• T = yield target (q/ha)
• SN, SP, SK = Soil test values of available N, P, and K

Summary Table: Steps in STCR Correlation

Step	Purpose/Outcome
Soil Testing	Assess available nutrient status
Crop Response Trials	Quantify yield response to fertilizer rates
Develop Prescription Eq.	Calculate fertilizer doses for targeted yield
Field Verification	Confirm recommendations under local conditions

If notes don't match with the syllabus, visit our Website for an Updated Version or message us on Telegram for corrections.

---

Soil Fertility and Plant Nutrition- Click here for Notes of all units

Want a PDF of all the notes? Send a DM to me: https://www.instagram.com/puspendrps