Soil Fertility and Productivity
Available Nutrient Analysis of Soil; To evaluate the nutrient status of soil, available nutrients are extracted using specific extracting reagents and measured using analytical methods.
Extracting Reagents for Major Nutrients:
Nutrient | Extracting Reagent | Method Used |
Available Nitrogen (N) | 0.25% KMnO₄ (Potassium Permanganate) | Alkaline Permanganate Method |
Available Phosphorus (P) | 0.5 M NaHCO₃ (Sodium Bicarbonate), pH 8.5 | Olsen’s Method |
Available Potassium (K) | 1N Neutral Ammonium Acetate (CH₃COONH₄) | Flame Photometry |
Available Micronutrients:
Micronutrient | Low | Medium | High |
Iron (Fe) | < 4.5 ppm | 4.5–9.0 ppm | > 9.0 ppm |
Manganese (Mn) | < 1 ppm | 1–3 ppm | > 3 ppm |
Zinc (Zn) | < 0.5 ppm | 0.5–1.0 ppm | > 1.0 ppm |
Copper (Cu) | < 0.33 ppm | 0.33–0.67 ppm | > 0.67 ppm |
Organic Carbon | < 0.5% | 0.5–0.75% | > 0.75% |
Available Macronutrient Classification (kg/ha):
Nutrient | Low | Medium | High |
Nitrogen (N) | < 250 | 250–500 | > 500 |
Phosphorus (P) | < 20 | 20–50 | > 50 |
Potassium (K) | < 125 | 125–300 | > 300 |
Analytical Methods in Soil Testing
S.No. | Nutrient/Parameter | Method Used |
1 | Total Nitrogen | Kjeldahl Method |
2 | Available Nitrogen | Alkaline KMnO₄ Method |
3 | Available Phosphorus | Olsen’s Method |
4 | Available Potassium | Flame Photometry |
5 | Organic Carbon | Walkley & Black Method / Rapid Oxidation |
6 | Micronutrients | Atomic Absorption Spectroscopy / Colorimetry |
Crop Logging (H.F. Clement)
Definition: “Crop log is the graphical record of the progress of the crop with chemical and physical measurements that reflect its condition and suggest changes in management for better yield.”
- First used in sugarcane fields of Hawaii
- Parameters measured:
- Nutrient content (N, P, K)
- Moisture
- Sugar content
- Sheath tissue analysis
- Helps to adjust fertilizer and irrigation based on plant need.
Soil Fertility vs Soil Productivity
Aspect | Soil Fertility | Soil Productivity |
Definition | Soil’s capacity to supply nutrients | Soil’s capacity to produce yield under a management system |
Scope | Nutrient availability | Yield performance |
Analysis | Laboratory-based | Field-based |
Function | f(Nutrient status of soil) | f(Fertility, Management, Climate) |
Example | All fertile soils aren’t productive | All productive soils are fertile |
Factors | Physical, chemical, and biological | Location, water, management, climate |
Nutrient Efficiency and Fertilizer Use (Datta & Gomez, 1975)
- N-efficiency: Highest in the first rice crop, declines in successive crops.
- K-efficiency: Increases over time, especially in dry seasons.
FAO Fertilizer Recommendations in India:
Cropping System | Fertilizer Use |
Rice-Wheat | N to both, P to wheat, K & Zn to rice |
Rice-Rice-Pulse | N to both rice crops, P to dry season rice, K, S, Zn to second crop |
Maize + Pulses | N to maize, P to both, K, S, Zn to maize |
🔺 Foodgrain to fertilizer input ratio: ~10:1
Fertilizer Use Precautions
- In saline soils, use nitrate forms of N (less volatilization).
- Apply ammonical N in reduced zones and nitrate N in oxidized zones.
- Stop N application before maturity in crops like potato, beet, sugarcane to avoid quality loss.
- In seed crops, apply foliar N during seed development for better germination.
- In determinate crops (rice, wheat): Last N at panicle initiation.
- In indeterminate crops (cotton, sesame): N at flowering + late flowering.
Miscellaneous Terms:
- Rabbing: Burning organic waste on soil to improve fertility.
- Teast soil: Soil high in Molybdenum (Mo).
- Mar: Raw humus in forest.
- Marling: Applying clay to sandy soil to improve texture and fertility.
