Manures, Composts and Organic Farming Inputs
This domain covers bulky organic manures, composting, and the inputs permitted in certified organic production, with emphasis on C:N ratio and nutrient availability rather than total content.
Core rules
- FYM averages about 0.5% N, 0.2% P2O5 and 0.5% K2O; only a fraction of that N (roughly 20–30%) is available in the first season. Vermicompost is richer, around 1.5–2.5% N.
- C:N ratio drives the direction of net mineralisation. Below about 20, net release of N; 20–30 roughly balanced; above 30, microbial immobilisation of soil N.
- Green manures such as sunhemp (Crotalaria) and dhaincha (Sesbania) add 50–100 kg N/ha through biological fixation and improve structure.
- Oil cakes (neem, castor, groundnut) are concentrated organics; neem cake also slows nitrification through its terpenoid content.
- Composting is controlled humification; a proper heap heats to 55–65 °C, which kills pathogens and weed seeds - raw manure does neither.
- Organic certification (NPOP in India) prohibits synthetic fertilizers and pesticides, requires buffer zones, and applies a conversion period before harvest can be sold as organic.
Worked example
1000 kg of crop residue contains 45% C and 0.9% N: C = 450 kg, N = 9 kg, C:N = 50. That residue will immobilise N. To bring the ratio near 24, add about 10 kg N (450/24 = 18.75 kg N total needed). This is why high-carbon residues are composted or supplemented with N before incorporation.
Traps that cost marks
- Treating FYM as a direct substitute for fertilizer N on a kilogram-for-kilogram basis.
- Ignoring that fresh manure may carry weed seeds and pathogens.
- Assuming organic equals nutrient-free or automatically residue-free.