Nutrient Cycling in Organic Systems
How nitrogen, phosphorus, and potassium move through organic farming systems — and how to manage each cycle for maximum fertility.
Nutrient Cycling
In organic farming, nutrients are not bought in a bag — they cycle through living systems. Understanding these cycles lets you manage fertility with minimal or zero purchased inputs.
The Nitrogen Cycle (Organic System)
Atmospheric N₂ (78% of air — the largest N reservoir)
↓ Biological Nitrogen Fixation (BNF)
Legume roots + Rhizobium bacteria → Organic N in plant tissue
↓ Plant dies or residue incorporated
Organic N → Ammonification (bacteria) → NH₄⁺ (ammonium)
↓ Nitrification — Nitrosomonas, Nitrobacter
NH₄⁺ → NO₂⁻ → NO₃⁻ (nitrate — plant-available)
↓ Plant uptake
Plant protein → Eaten by animals → Excretion → Back to soil
Key lever: Legumes + Rhizobium can fix 50–300 kg N/ha/year for free from air. A well-nodulated soybean or pigeonpea crop fixes more N than most farmers spend on fertilizer.
Nitrogen Fixation Rates by Legume:
| Crop | N Fixed (kg/ha/year) |
|---|---|
| Soybean | 80–200 |
| Groundnut | 72–124 |
| Chickpea | 40–140 |
| Pigeonpea | 40–200 |
| Dhaincha (Sesbania) | 60–120 |
| Sesbania rostrata | 100–200 |
Managing the N Cycle:
- Include legumes in every rotation
- Always inoculate legume seeds with correct Rhizobium species (species-specific!)
- Don't apply N fertilizer with legumes — it suppresses fixation
- Incorporate legume residue — N is released in 2–6 weeks
The Phosphorus Cycle (Organic System)
Phosphorus is special: it doesn't cycle through the atmosphere (unlike N). All P comes from mineral weathering or recycled organic matter.
Rock phosphate minerals (Ca₃(PO₄)₂, FePO₄, AlPO₄)
↓ Weathering + organic acids
Inorganic P in soil (mostly insoluble and unavailable)
↓ PSB (Phosphate Solubilizing Bacteria) + Mycorrhizal fungi
Plant-available P (H₂PO₄⁻, HPO₄²⁻)
↓ Plant uptake
Organic P in plant tissue → Animal → Excretion/Death
↓ Decomposition
Back to soil as organic P → Slowly re-mineralized
The challenge: 65–80% of P in Indian soils is already present but locked in insoluble forms. The organic approach is to unlock existing P rather than add more:
- PSB inoculants solubilize locked P
- Mycorrhizal fungi access P in pores too small for roots
- Lowering pH slightly (compost, sulfur) unlocks P in alkaline soils
- Rock phosphate + PSB = slow but effective P supply
Potassium, Calcium, Magnesium, Sulfur
These nutrients are released from mineral weathering and organic matter decomposition:
| Nutrient | Organic Source | Release Rate |
|---|---|---|
| Potassium (K) | Wood ash, rock powders, compost | Wood ash: fast; rock: very slow |
| Calcium (Ca) | Dolomite lime, eggshells, bone meal, gypsum | Months to years |
| Magnesium (Mg) | Dolomite lime, compost | Months |
| Sulfur (S) | Gypsum, compost, mustard cake | Slow |
Rock powders (granite dust, basalt): Slow-release K, Ca, Mg over 5–10 years. Excellent long-term amendment. Free from quarry waste in many regions.
Closing the Loop
The most important nutrient management principle: return everything you take off the farm.
- Crop residues returned to soil (don't burn)
- Animal manure returned to fields (don't let it wash away)
- Kitchen/food waste composted and returned
- Green manure incorporated before cropping
Every kg of produce leaving the farm takes nutrients with it. If you don't replace them, soil fertility declines — organically or chemically.
Next: Cation Exchange Capacity