Unit 3: Soil fertility evaluation and diagnosis

1. Soil Fertility Evaluation

Soil fertility evaluation is the process of estimating the nutrient-supplying power of a soil. Its primary objective is to determine the optimal amount of fertilizer required for maximum economic crop yield and to identify specific nutrient deficiencies or toxicities.

Approaches to Evaluation

1. Visual Diagnosis: Identifying nutrient status based on visible symptoms (chlorosis, necrosis, stunting) on the growing plant.
2. Plant Analysis: Determining the nutrient content within plant tissues.
3. Biological Tests: Using growing plants or microorganisms (e.g., Azotobacter plaque test) to assess fertility. Field trials are the standard but are time-consuming.
4. Chemical Soil Analysis (Soil Testing): Rapid chemical analysis to estimate available nutrient fractions.

2. Soil Testing

Soil testing is a rapid chemical analysis to assess the available nutrient status, salinity, and elemental toxicity. It is the most common proactive method for fertility management.

Phases of Soil Testing

1. Sample Collection: The most critical step. Errors here cannot be corrected in the lab.
   • Composite Sampling: Mixing sub-samples from 15–20 spots in a uniform field area to form one representative sample.
   • Depth: Usually 0–15 cm for shallow-rooted crops; deeper for horticulture.
   • Pattern: Zig-zag method is preferred to minimize bias.
2. Extraction (Analytical Analysis): Using specific chemical reagents to extract the "plant-available" portion of nutrients, not the total elemental content.
3. Interpretation: Correlating the analytical value with crop response data (calibration).
4. Recommendation: Formulating fertilizer dosage based on the interpretation.

Common Extraction Methods

• Organic Carbon: Walkley and Black method (wet oxidation).
• Available Nitrogen: Alkaline Permanganate method.
• Available Phosphorus:
  • Bray No. 1: For Acidic soils.
  • Olsen’s Method: For Neutral to Alkaline soils.
• Available Potassium: Neutral Normal Ammonium Acetate extraction.
• Micronutrients (Zn, Fe, Mn, Cu): DTPA (Diethylene Triamine Pentaacetic Acid) extractant.

3. Forms of Nutrients in Soil

Nutrients exist in the soil in various chemical and physical forms, categorized generally into three pools: Solution (immediately available), Exchangeable (readily available), and Non-exchangeable/Fixed (slowly available).

Macronutrients

Micronutrients

Most micronutrients are absorbed as ions or chelates.

• Iron (Fe): (Ferrous - available), (Ferric - unavailable).
• Manganese (Mn): .
• Boron (B): (Boric acid).
• Molybdenum (Mo): . Note: Mo availability increases with pH, unlike other micronutrients.

4. Critical Levels of Nutrients in Soil

The Critical Level is the concentration of a nutrient in the soil below which the crop will respond economically to added fertilizer, and above which the response is diminishing or nil.

Soil Fertility Ratings (General Interpretation)

Values may vary by region and extraction method.

1. Organic Carbon (Indicator of N availability)

• Low: < 0.5%
• Medium: 0.5 – 0.75%
• High: > 0.75%

2. Available Nitrogen (Alkaline KMnO4 method)

• Low: < 280 kg/ha
• Medium: 280 – 560 kg/ha
• High: > 560 kg/ha

3. Available Phosphorus ()

• Low: < 10 kg/ha
• Medium: 10 – 25 kg/ha
• High: > 25 kg/ha

4. Available Potassium ()

• Low: < 110 kg/ha
• Medium: 110 – 280 kg/ha
• High: > 280 kg/ha

5. Micronutrients (DTPA Extractable - Critical Limits in ppm)
If values fall below these limits, the soil is considered deficient:

• Zinc (Zn): 0.6 ppm
• Copper (Cu): 0.2 ppm
• Iron (Fe): 4.5 ppm
• Manganese (Mn): 2.0 ppm

5. Plant Analysis

Plant analysis (or tissue testing) involves determining the concentration of nutrients in plant samples. It serves as a check on the soil test and evaluates the actual uptake efficiency of the plant.

Objectives

1. Diagnosis: Confirming visible deficiency symptoms.
2. Monitoring: Identifying Hidden Hunger (nutrient levels are low enough to reduce yield but not low enough to cause visible symptoms).
3. Optimization: Determining if nutrients are in the "luxury consumption" range.

Sampling Guidelines

Nutrient content varies by plant part and age. Standardization is crucial.

• Timing: Usually at maximum active growth or flowering (e.g., Silking stage in maize).
• Plant Part: usually the most recently matured leaf (MRML).
  • Wheat/Rice: Flag leaf at bloom.
  • Corn: Ear leaf at silking.
  • Soybean: Uppermost fully developed trifoliate leaves.

Interpretation Concepts

1. Critical Concentration: The nutrient concentration in the tissue associated with 90% or 95% of maximum yield.
2. Sufficiency Range: The range of concentration where yield is maximized.
3. DRIS (Diagnosis and Recommendation Integrated System): An advanced method that uses ratios of nutrients (e.g., N/P, N/K) rather than absolute concentrations to determine relative sufficiency.

6. Rapid Plant Tissue Tests

These are semi-quantitative chemical tests performed on fresh plant tissue (usually sap) in the field. They provide an immediate "Yes/No" or "High/Medium/Low" diagnosis.

Characteristics

• Sample: Fresh plant sap (petioles or midribs).
• Location: Performed in the field (Field Kits).
• Speed: Results in minutes.
• Accuracy: Lower than laboratory dry ashing; good for troubleshooting but not for precision prescription.

Common Rapid Tests

1. Nitrate Nitrogen: Uses diphenylamine reagent.
   • Blue color: High Nitrate.
   • Colorless/Pale: Low Nitrate.
2. Phosphorus: Uses ammonium molybdate + stannous chloride.
   • Dark Blue: High Phosphorus.
   • Light Blue/Green: Low Phosphorus.
3. Potassium: Turbidity test using sodium cobaltinitrite.
   • High turbidity: High K.
   • Clear solution: Low K.

Limitations

• Influenced by time of day (photosynthesis affects sap concentration).
• Affected by moisture stress.
• Subjective interpretation of colors.

7. Indicator Plants

Indicator plants are specific species that exhibit distinct and rapid deficiency or toxicity symptoms for specific nutrients. They are used as biological detectors of soil fertility status.

Criteria for Indicator Plants

• High susceptibility to the specific nutrient deficiency.
• Rapid growth habit.
• Distinct, easily identifiable symptoms (not confused with diseases).

Examples of Indicator Plants