Manure as fertilizer
Manure supplies nitrogen, phosphorus, potassium and organic matter. Used well, it replaces much of a crop’s purchased fertilizer. Used without a calculation, it often over-supplies phosphorus and nitrogen, which can leach into groundwater or run off into streams.
The nitrogen-based rate
Only part of the nitrogen in manure is available to the crop in the year of application; the rest is in organic forms that release slowly over the following years.
Rate = crop nitrogen need ÷ (nitrogen per unit of manure × first-year availability)
The unit is a ton for solid manure and 1,000 gallons for liquid manure.
Example
A corn crop needs 150 lb of N per acre. Solid dairy manure contains 10 lb of total N per ton, and about 40% is available the first year:
- rate: 150 ÷ (10 × 0.40) = 37.5 tons per acre;
- total nitrogen applied: 375 lb per acre;
- phosphate applied at 6 lb P₂O₅ per ton: 225 lb per acre, several times what the crop removes.
Typical first-year availability
| Manure | Available N, first year |
|---|---|
| Liquid swine, injected | 60% to 80% |
| Liquid dairy | 40% to 50% |
| Solid beef or dairy with bedding | 25% to 35% |
| Poultry litter | 50% to 60% |
| Composted manure | 10% to 20% |
Incorporating or injecting manure quickly reduces ammonia losses and increases the available share.
Phosphorus limits
Because manure contains relatively more phosphorus than crops need, applying for nitrogen often builds up soil phosphorus over the years. Many state nutrient management plans switch to a phosphorus-based rate on fields with high soil test phosphorus or a high risk of runoff. Enter the cap from your plan in the calculator to see the rate it allows and the nitrogen to complete with commercial fertilizer.
Get the numbers right
- Test the manure: nutrient content varies widely with species, diet, bedding and storage.
- Calibrate the spreader: weigh a load and measure the area it covers, or use a tarp test.
- Account for credits: manure applied in previous years and legume crops add nitrogen.
Regulations and timing
Large animal operations need nutrient management plans, and many states restrict application on frozen or snow-covered ground, near wells and streams, and on steep slopes. Apply close to the time the crop needs the nitrogen, ideally in spring.