Manure Application Rate Calculator
Match manure tons to crop nitrogen need without overapplying.
What it does
Manure feeds crops, but too much pollutes water. The safe rate starts from two lab numbers: how much nitrogen the crop needs, and how much available nitrogen each ton of manure carries. Dividing need by content gives tons to spread per acre.
The tool above takes need, content, and field area and returns rate per acre plus total to haul. The defaults 120 lb per acre need with 12 lb per ton content on 20 acres match the worked example, so the page opens showing 10 tons per acre. Switch to metric for kg per ha.
Content varies widely with animal, bedding, storage, and moisture. Dairy solids near 12 lb N per ton differ from poultry litter near 60 lb per ton. Always use your own manure test, not a book average, and use first year available N, not total N, for the divisor.
Need comes from soil tests and university guides for yield goal, minus credits for legumes, prior manure, and irrigation nitrate. Rate on nitrogen, then check phosphorus. In high P soils, phosphorus often caps the rate below the nitrogen rate.
This page shows the formula, one full hand checked example at 120 lb per acre, a reference table of typical contents, and practice checks that run through the tool in both unit systems.
How to use
- Pick Imperial (lb/acre) or Metric (kg/ha).
- Type crop nitrogen need from your soil test and guide.
- Type manure available nitrogen per ton from your lab test.
- Type field area in acres or hectares.
- Read Application rate per acre or per hectare.
- Read Total to haul and book trucks from that total.
Formula
- need
- Crop N need in lb per acre or kg per ha, after credits.
- content
- First year available N per ton or per tonne.
- area
- Field area in acres or hectares.
Use available N, not total N, for the divisor. If phosphorus limits, compute both N and P rates and spread the smaller. Calibrate the spreader, since fluffy tons differ from compact tons.
Worked example
Example: 120 lb per acre need, 12 lb per ton
Corn needs 120 lb N per acre. Dairy manure tests 12 lb available N per ton. Field is 20 acres.
- Write need = 120 lb per acre, content = 12 lb per ton, area = 20 acres.
- Rate: 120 / 12 = 10 tons per acre.
- Total: 10 x 20 = 200 tons to haul.
- Metric mirror: 134.5 kg per ha / 6 kg per tonne = 22.4 tonnes per ha.
- Metric total: 22.4 x 8.1 = 181.6 tonnes, near 200 short tons.
- Check P: if soil P is high, test whether P caps below 10 tons per acre.
Reference table
| Manure | lb per ton | kg per tonne |
|---|---|---|
| Dairy solids | 10 to 14 | 5 to 7 |
| Beef feedlot | 14 to 20 | 7 to 10 |
| Swine slurry | 8 to 12 | 4 to 6 |
| Poultry litter | 50 to 65 | 25 to 32 |
| Example used here | 12 | 6 |
Limits
- Nitrogen rate only. Phosphorus, salt, and lime needs can cap lower.
- Needs lab tests. Book values swing widely with moisture and storage.
- Ignores losses in handling and uneven spreading. Calibrate equipment.
- Follow setbacks and bans on frozen or wet ground from local rules.
Common mistakes
- Dividing by total N instead of first year available N, which underrates tons.
- Skipping legume and prior manure credits, so need runs high.
- Using book content instead of a current lab test.
- Forgetting phosphorus. High P soils often cap below the N rate.
- Mixing short tons with metric tonnes in one step.
Try it yourself
Quick checks. Answers sit tucked under each question.
Need 60, content 10, area 10 acres?
Rate 6 tons per acre, total 60 tons.
Need 90, content 15, area 5 acres?
Rate 6 tons per acre, total 30 tons.
Questions
How much manure per acre?
Need divided by content. 120 / 12 = 10 tons per acre in the example.
Total for my field?
Rate times area. 10 tons per acre x 20 acres = 200 tons.
Available or total N?
Available first year N for rate math. Total N overstates what crops see this year.
What about phosphorus?
Compute N and P rates and spread the smaller when soil P is high.
Why test each load?
Moisture and feed shift content a lot. A test beats a book value.
Metric version?
Same division in kg per ha over kg per tonne. 134.5 / 6 = 22.4 tonnes per ha.