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.
