Unit3 - Subjective Questions

Soil Fertility Evaluation is the process of assessing the nutrient-supplying capacity of the soil to ensure optimal crop production. It involves diagnosing nutrient deficiencies or toxicities to provide fertilizer recommendations.

Approaches to Evaluation:

1. Visual Symptoms of Deficiency/Toxicity: Identifying nutrient status based on visible signs on plant leaves and stems (e.g., chlorosis, necrosis).
2. Plant Analysis:
   • Total Elemental Analysis: Laboratory analysis of plant parts.
   • Rapid Tissue Tests: Field tests using plant sap.
   • Biological Tests: Using indicator plants or microbial methods (e.g., Azotobacter plaque test).
3. Soil Testing: Chemical analysis of soil samples to determine available nutrient status.
4. Greenhouse Pot Culture: Controlled experiments (e.g., Neubauer seedling method).
5. Field Experiments: The most reliable but expensive method involving actual crop growth under field conditions.

Concept:
The critical level is the concentration of a nutrient in the soil below which the crop will respond profitably to the application of that nutrient, and above which the response is diminishing or nil. It is often defined as the nutrient concentration corresponding to 90% of the maximum yield.

Cate-Nelson Graphical Method:

1. Plotting: A scatter diagram is plotted with soil test values on the X-axis and relative yield percent on the Y-axis.
2. Partitioning: A transparent overlay with crossed lines (perpendicular to each other) is moved over the scatter plot.
3. Optimization: The cross is positioned to maximize the number of points falling in the positive quadrants (low soil test/low yield and high soil test/high yield) and minimize points in the negative quadrants.
4. Result: The point where the vertical line intersects the X-axis represents the Critical Level.

Potassium exists in the soil in four distinct forms, which are in dynamic equilibrium.

Forms of Potassium:

1. Solution K (): Potassium ions () dissolved in soil water. It is immediately available for plant uptake but constitutes a very small fraction (1-2 mg/L).
2. Exchangeable K (): Electrostatistically held on the negative exchange sites of clay and organic matter. It is readily available and replenishes solution K.
3. Non-Exchangeable / Fixed K (): trapped between the lattice layers of 2:1 clay minerals (like illite and vermiculite). It is slowly available.
4. Structural / Mineral K (): Component of primary minerals like feldspars and micas. It constitutes the bulk of soil K (90-98%) but is unavailable until weathering occurs.

Dynamic Equilibrium:
The equilibrium can be represented as:

As plants remove , moves into the solution to replace it. Over time, may release K to the exchangeable pool.

Definition:
Indicator plants are specific crops or plants that exhibit characteristic deficiency symptoms of a particular nutrient earlier and more distinctively than other plants. They are used in biological soil evaluation to detect specific nutrient deficiencies.

Examples:

1. Nitrogen (N): Maize, Cauliflower, Cabbage.
2. Phosphorus (P): Rapeseed, Mustard, Tomato.
3. Potassium (K): Potato, Lucerne, Tobacco.
4. Zinc (Zn): Maize (shows 'White Bud').
5. Boron (B): Sunflower, Lucerne.
6. Molybdenum (Mo): Cauliflower (shows 'Whip tail').

Definition:
Rapid plant tissue testing (also known as sap testing) is a semi-quantitative field diagnostic method used to estimate the concentration of unassimilated soluble nutrients (e.g., , , ) in the plant sap.

Principle:
The test relies on the fact that nutrients absorbed by roots are transported in the inorganic form in the sap before being assimilated into organic structures. High sap concentration indicates sufficiency, while low concentration indicates deficiency.

Procedure (General):

1. Sampling: Select active conducting tissue (petioles or midribs) from specific leaves.
2. Extraction: Sap is extracted by crushing the tissue or cutting it into fine bits.
3. Reagent Application: Specific chemical reagents are applied to the sap on a spot plate or filter paper.
4. Color Development: The intensity of the color developed is compared with a standard chart.

Advantages:

• Immediate results in the field.
• Low cost and simple equipment.
• Useful for troubleshooting during the growing season.

Limitations:

• Less precise than laboratory analysis.
• Influenced by time of day, moisture stress, and sunlight.

Concept:
The Nutrient Index (NI) is a value calculated to evaluate the overall fertility status of a region (like a district or block) based on a large number of soil samples. It was introduced by Parker (1951).

Calculation:
Soils are categorized into three classes based on soil test values: Low (), Medium (), and High ().

Where:

• = Number (or percentage) of samples in the Low category.
• = Number (or percentage) of samples in the Medium category.
• = Number (or percentage) of samples in the High category.
• = Total number of samples.

Interpretation:

• NI < 1.67: Low Fertility
• NI 1.67 – 2.33: Medium Fertility
• NI > 2.33: High Fertility

Hidden Hunger:
This occurs when a plant is deficient in a nutrient, but the deficiency is not severe enough to show visible symptoms. The plant yields are reduced, but the farmer is unaware of the deficiency. On the growth curve, this falls in the zone where nutrient concentration increases slightly, and yield increases significantly, just below the critical level.

Luxury Consumption:
This refers to the absorption of a nutrient by the plant in excess of what is required for optimum yield. The nutrient content in the plant increases without a corresponding increase in crop yield. This is common with Potassium () and Nitrogen (). On the curve, this is the zone beyond the sufficiency range.

Macy's Critical Percentage:
The transition point between poverty adjustment (hidden hunger) and the luxury consumption zone is often determined by the critical percentage.

(Note: A diagram showing Yield on Y-axis vs. Nutrient Concentration on X-axis, illustrating the zones of deficiency, hidden hunger, sufficiency, and toxicity, is recommended).

Introduction:
DRIS was developed by Beaufils (1973). Unlike the critical level approach which looks at single nutrient concentrations, DRIS evaluates the nutritional status based on the ratios or balance of nutrients (e.g., N/P, N/K, K/P) relative to high-yielding norms.

Key Concepts:

1. Nutrient Balance: Plants yield best when nutrients are in specific ratios, irrespective of absolute concentration.
2. Indices: DRIS calculates indices for each nutrient (e.g., Index N, Index P).
   • A negative index indicates deficiency (relative to other nutrients).
   • A positive index indicates excess.
   • An index near zero indicates balance.
3. Procedure:
   • A database of high-yielding populations is created.
   • Norms (mean ratios) are established.
   • The sample's ratios are compared to the norms to calculate indices.

Advantages:

• Less sensitive to the age of the plant (sampling time) than critical level methods.
• Ranks nutrients in order of limiting importance (e.g., N > P > K means N is the most limiting).

Methods of Soil Sampling:

1. Tools: Use a screw auger, tube auger, or spade (khurpi).
2. Technique (Zig-Zag Method): Traverse the field in a zig-zag pattern to cover the variation.
3. Depth:
   • Field crops: 0–15 cm (plough layer).
   • Deep-rooted/Horticultural crops: Different depths (e.g., 0-15, 15-30, 30-60 cm).
4. Composite Sample: Collect 10–20 scrapes/cores from the field, mix them thoroughly in a clean plastic bucket, break clods, remove stones/roots, and reduce the sample size to ~500g using the Quartering Method.
5. Precautions: Avoid sampling near bunds, shade of trees, irrigation channels, or spots where manure was recently heaped.

Importance of Representativeness:
Soil testing analyzes only a tiny fraction (a few grams) of the soil to represent hectares of land. If the sample is not representative, the analytical results, no matter how precise, will lead to erroneous fertilizer recommendations (errors in sampling contribute more to inaccuracy than analytical errors).

Forms of Phosphorus:

1. Organic P: Associated with humus, inositol phosphates, nucleic acids (30-50% of total P).
2. Inorganic P:
   • In Acidic Soils: Generally bonded with Iron () and Aluminum () as variscite and strengite.
   • In Alkaline/Calcareous Soils: Bonded with Calcium () as apatites, dicalcium phosphate, etc.
   • Soil Solution P: (predominant in acid soil) and (predominant in alkaline soil).

Extractants for Estimation:

1. Bray’s No. 1 Method (Acidic Soils):
   • Extractant: .
   • Mechanism: Fluoride complexes with Al and Fe, releasing P.
2. Olsen’s Method (Alkaline/Neutral Soils):
   • Extractant: adjusted to pH 8.5.
   • Mechanism: Bicarbonate precipitates Ca as , increasing the activity of P in solution.

Concept:
The STCR approach, also known as the "Targeted Yield Approach," was developed by Ramamoorthy in India. It links the soil test values with the crop response to fertilizer application to achieve a specific yield target.

Basis:
It is based on three parameters:

1. NR (Nutrient Requirement): Kg of nutrient required to produce 1 quintal of grain.
2. CS (Contribution from Soil): Efficiency of soil available nutrients (%).
3. CF (Contribution from Fertilizer): Efficiency of fertilizer nutrients (%).

General Equation:
To calculate the Fertilizer Dose () in kg/ha:

(Simplified linear form: , where A and B are constants derived from field trials).

This method ensures balanced nutrition and economic profitability.

Nitrogen in soil exists in two major categories: Inorganic and Organic.

1. Inorganic Forms (Available to plants):

• Ammonium (): Held on exchange sites or fixed in clay lattice. Preferred by crops like rice.
• Nitrate (): Highly mobile in soil solution. The primary form absorbed by most upland crops. Prone to leaching.
• Nitrite (): Usually present in negligible amounts; toxic to plants.
• Elemental Nitrogen (): Inert gas in soil air, available only via nitrogen fixation.

2. Organic Forms (>90% of Total N):

• Proteins and Amino Acids: Derived from plant and animal residues.
• Amino Sugars: (e.g., hexosamines) from microbial cell walls.
• Complex N: Associated with stable humus/lignin.

Note: Mineralization converts organic N to inorganic forms ( and ), while immobilization converts inorganic N back to organic forms.

Biological methods use living organisms (plants or microbes) to assess soil fertility.

Microbiological Tests:
These tests use specific microorganisms that are sensitive to specific nutrients. The growth of the microbe in the soil sample is proportional to the available nutrient.

1. Winogradsky’s Azotobacter Plaque Test:

   • Used for: Phosphorus (P), Calcium (Ca), and Potassium (K) deficiency.
   • Method: Soil is mixed with starch and made into a plaque. If colonies of Azotobacter develop, the soil has sufficient P/K/Ca. Lack of colonies indicates deficiency.

2. Aspergillus niger Test:

   • Used for: Micronutrients (specifically Copper, Zinc, Molybdenum).
   • Method: The mycelial growth or spore color of the fungus Aspergillus niger is observed. For example, black spores indicate sufficiency of Copper; lighter colors indicate deficiency.

3. Cunninghamella Plaque Method:

   • Used for: Phosphorus.
   • The diameter of the fungal colony is measured.

Common Extractants:

• Hot water: For Boron ().
• DTPA: For Zn, Fe, Mn, Cu.
• Ammonium Acetate: For Mo (sometimes).

DTPA Method (Lindsay and Norvell, 1978):

• Extractant: (Triethanolamine) buffered at pH 7.3.
• Principle:
  1. Chelation: DTPA (Diethylene Triamine Pentaacetic Acid) is a chelating agent that combines with free metal ions (, , etc.) in solution, reducing their activity.
  2. Replenishment: In response, ions desorb from the soil exchange sites to replenish the solution.
  3. Simulation: This mimics the action of plant roots, which excrete organic chelates to extract micronutrients.
  4. pH Buffer: TEA maintains the pH at 7.3 to prevent the dissolution of carbonates.

The critical limit is not a fixed universal value; it varies due to several factors:

1. Crop Species and Variety: Different crops have different nutrient requirements (e.g., legumes need more P than cereals; hybrids need more nutrients than local varieties).
2. Soil Texture: For example, the critical level of K is usually lower in sandy soils compared to clay soils due to differences in buffering capacity.
3. Soil pH: Affects the availability and chemical form of nutrients (e.g., P fixation varies with pH).
4. Cation Exchange Capacity (CEC): Soils with high CEC have higher buffering capacity and may have different critical limits.
5. Method of Extraction: The critical limit depends strictly on the chemical extractant used (e.g., Bray P limit $
   eq$ Olsen P limit).
6. Yield Target: A higher target yield may imply a higher critical threshold.

Total Plant Analysis:

• Method: Laboratory digestion (wet oxidation) of the whole plant or specific parts to determine total elemental content.
• Significance: Provides a precise quantitative measure of nutrient uptake. Used for diagnosing chronic problems, calculating nutrient removal (nutrient audit), and research.
• Choice: Choose when precision is required, for post-mortem diagnosis (why a crop failed), or for calculating fertilizer requirements for the next season.

Tissue Testing (Rapid Test):

• Method: Field analysis of fresh plant sap for soluble/unassimilated nutrients.
• Significance: Provides a quick "snapshot" of current nutrient supply.
• Choice: Choose for immediate troubleshooting in the field to apply corrective measures (like foliar sprays) during the current growing season, or when laboratory facilities are inaccessible.

Definition:
The Neubauer Seedling Method is a biological method of soil fertility evaluation based on the principle that seedlings of certain plants (like Rye) have a strong extractive power for nutrients (P and K) in their early growth stage.

Procedure:

1. 100 rye seedlings are grown in 100g of the test soil mixed with sand for 17 days.
2. A control is run using only sand (no soil).
3. After 17 days, the plants are harvested and analyzed for total P and K.

Calculation:
The nutrient uptake from the test soil is calculated by subtracting the nutrient content of seedlings grown in sand (control) from those grown in soil.

This value represents the available nutrient status of the soil.

While critical levels vary by state and crop, the general ratings used for fertilizer recommendations in India are:

Note: Values often vary slightly based on specific state agricultural university guidelines.

Correct sampling is vital because nutrient concentration varies with plant age and the specific plant part.

General Guidelines:

• Sample generally at the blooming/flowering stage or active growth stage.
• Collect fully expanded, mature leaves (usually the 3rd or 4th leaf from the top).
• Do not sample dead, diseased, or insect-damaged tissues.

Specific Parts:

1. Maize (Corn):
   • Seedling stage: All growing parts.
   • Silking stage: Ear leaf (the leaf adjacent to the corn cob).
2. Rice:
   • Tillering/Panicle Initiation: The 3rd fully opened leaf from the top (Y-leaf).
3. Cotton:
   • Early Growth: Youngest fully mature leaves on the main stem.
   • Peak Flowering: Petiole of the 4th leaf from the top.