Manure Application Calculator
Work out how much manure supplies the nitrogen your beds need, in pounds, bags and depth.
Manure is a fertilizer with a analysis printed nowhere on the pile, so gardeners dose it by the barrow and hope. This calculator works backwards from the nitrogen your crop actually needs: pick the manure, set a nitrogen target, and it returns pounds, bags and the layer depth that delivers it, using the standard rule that about half of manure nitrogen is available in the first year.
Composted manure is the safe default for beds that will carry food this season. Fresh manure has timing rules, covered below, and the calculator flags them. For bagged fertilizer precision instead, the Fertilizer Calculator works from the NPK on the label.
How this is worked out
Available nitrogen per pound = the manure’s N percentage x 50 percent first-year availability. Pounds needed per 1,000 sq ft = your nitrogen target / available nitrogen per pound. The other half of the nitrogen is not lost; it releases over the following two to three years, which is why beds manured annually often need less than this calculator says after a few seasons. Volume uses about 40 lb per cubic foot for composted manure and 50 for fresh.
Assumptions:
- Analyses are typical book values. A real pile varies with the animal’s diet, bedding, and how much rain has leached it; a covered, composted pile sits at the good end.
- Phosphorus and potassium are fully counted in year one, unlike nitrogen. Repeated heavy manuring builds soil phosphorus high enough to become a runoff problem, which is a real reason to test soil every few years.
- The target is nitrogen only. Manure also adds organic matter and biology that no bag matches, which is most of why gardeners use it.
Worked examples
1. Composted cow manure on 1,000 sq ft of vegetables, target 2 lb N. At 0.7% N and half available, each pound gives 0.0035 lb of nitrogen, so the job takes about 570 lb: fourteen 40 lb bags, or a layer just under 0.2 in deep.
2. Composted chicken manure on the same bed. At 1.5% N the same target needs about 270 lb. Poultry manure is roughly twice as strong, which is why it burns when doubled by habit.
3. A light 1 lb N top-up with rabbit manure. At 2.4% N, about 85 lb per 1,000 sq ft. Rabbit manure is the strongest common garden manure and can go on fresh around established plants in modest amounts.
Typical manure analyses
| Manure | N | P2O5 | K2O |
|---|---|---|---|
| Chicken (fresh) | 1.1% | 0.8% | 0.5% |
| Chicken (composted) | 1.5% | 1.0% | 0.6% |
| Cow (fresh) | 0.5% | 0.2% | 0.5% |
| Cow (composted) | 0.7% | 0.3% | 0.6% |
| Horse (fresh) | 0.7% | 0.3% | 0.6% |
| Horse (composted) | 0.6% | 0.3% | 0.5% |
| Sheep | 0.9% | 0.5% | 0.8% |
| Rabbit | 2.4% | 1.4% | 0.6% |
Use the table for comparisons, not gospel: the same species from two farms can differ by a third. Composting concentrates nutrients per pound as mass rots away, which is why composted figures run higher than fresh for poultry.
Common mistakes
- Fresh manure on a bed about to be planted. Ammonia burns roots and seedlings, and the pathogen clock (90 to 120 days before harvest) is a food safety rule, not a style choice.
- Annual full rates forever. The leftover nitrogen and all the phosphorus accumulate. After a few years of manuring, test the soil and expect to cut the rate.
- Treating every pile as equal. A rain-leached, weed-seed-laden heap from the field edge is not the analysis on this page. Hot composting fixes most of the gap.
- Piling it against stems. Even composted manure heaped on crowns rots them. Spread the layer, keep it an inch or two clear, same as mulch.
Putting the result to work
Spread on a calm day, fork into the top 4 to 6 in on empty beds, and water in if no rain is coming. Time the main application for fall on heavy soil (it mellows over winter) or a few weeks before spring planting on light soil. Keep notes of pounds per bed per year: the rotation of rates is what keeps soil phosphorus sane. Where manure is scarce, the Compost Recipe Builder turns garden waste into the other home-made amendment.
How much manure does my garden need?
- Pick the manure you actually have; its nitrogen percentage sets everything.
- Set a nitrogen target: 1 lb per 1,000 sq ft for light feeders, 2 for most vegetables, 3 for corn and heavy feeders.
- Enter the area. The result is pounds, bags, volume and layer depth at 50 percent first-year nitrogen availability.
- Respect fresh manure timing: fall application, or 90 to 120 days before harvest.
- Retest soil every few years; repeated manuring builds phosphorus that eventually caps the rate.
How much manure do I need per 1,000 square feet?
For a standard 2 lb nitrogen target: about 570 lb of composted cow manure, 380 lb of composted horse, or 270 lb of composted chicken per 1,000 sq ft. The spread is the nitrogen percentage: poultry manure carries roughly double the nitrogen of cow manure per pound, so half the weight does the same job.
Why is only half the nitrogen counted in the first year?
Manure nitrogen is mostly organic and must be mineralised by soil microbes before roots can use it. About half releases in year one in warm, moist soil; the rest follows over two to three years. Synthetic fertilizer nitrogen, by contrast, is nearly all available at once, which is why manure rates look huge next to bag rates for the same target.
Can I use fresh manure in the garden?
Yes, with timing: work it in during fall, or at least 120 days before harvesting crops whose edible part touches the soil (lettuces, carrots, strawberries) and 90 days for other food crops. Fresh manure also carries weed seed and can burn plants with ammonia. Composted manure has none of these limits and is the default for in-season use.
Will manure burn my plants?
Fresh and poultry manures can: their ammonia and salts scorch roots and seedlings on contact, especially in dry soil. Composted manure at the rates this calculator gives does not burn. When in doubt, spread, work in, water, and wait a week before planting into it.
Does manure replace fertilizer completely?
For nitrogen-hungry beds it can, at the weights shown here. But manure is relatively richer in phosphorus than plants are, so manure-only feeding for years oversupplies phosphorus. Many gardeners manure for soil building and organic matter, then fine-tune nitrogen with a smaller fertilizer dose, checking with a soil test every few years.
Analyses are typical published book values for US manures; first-year nitrogen availability of about 50 percent follows standard extension guidance. Fresh manure harvest intervals (90 and 120 days) follow US food safety practice for untreated manure.
