Calculate how much compost, lime, sulfur, or other soil amendment you need from your bed size, working depth, target blend percentage, and soil test results.
Soil Amendment Formula
The calculator runs four separate modes. Each one uses its own formula.
Blending an amendment into a bed:
- V = amendment volume needed, in cubic feet (divide by 27 for cubic yards)
- A = bed area, in square feet
- D = tilling depth, in inches
- P = amendment share of the finished mix, in percent by volume
- W = waste and settling allowance, in percent
Spreading a topdress layer:
- T = layer thickness, in inches
Lime or sulfur to move soil pH:
- M = material needed, in pounds
- R = base rate for the soil texture, in pounds per 1,000 square feet per 1.0 pH unit
- F = material factor relative to agricultural limestone or elemental sulfur
- dpH = size of the pH change you want, as a positive number
Compost to reach an organic matter target:
- C = compost needed as delivered, in pounds (divide by compost bulk density for cubic yards)
- BD = soil bulk density, in pounds per cubic foot, near 81 for a typical loam
- OM1 and OM2 = current and target soil organic matter, in percent by weight
- RF = first year retention factor, the share of applied organic matter that is still there after a season
- DM = compost dry matter fraction
- OMc = organic matter fraction of the compost dry weight
The blend mode treats P as a share of the finished volume, so 25 percent means one part amendment to three parts existing soil. The topdress mode also reports what that same layer would become as a blend percentage if you later till it in at 4, 6, or 8 inches. The pH mode caps each application at a safe single dose and tells you how many passes to split the total into. The organic matter mode is the only one that works in weight rather than volume, because soil test organic matter is reported by weight, not by volume.
Amendment Densities, Organic Matter, and Typical Blend Rates
Bulk density is what converts a volume into a weight, and organic matter content is what decides how much a material actually moves a soil test number. Bagged and bulk products vary, so use these as starting points and use a supplier figure when you have one.
| Material | Bulk density (lb/cu yd) | Organic matter (% dry) | Typical C:N | Common blend rate |
|---|---|---|---|---|
| Finished compost | 800 to 1,200 | 40 to 60 | 15:1 | 20 to 30% |
| Composted manure | 900 to 1,300 | 35 to 55 | 15:1 | 10 to 25% |
| Worm castings | 1,100 to 1,300 | 30 to 50 | 13:1 | 5 to 10% |
| Leaf mold | 500 to 800 | 60 to 80 | 40:1 | 20 to 40% |
| Peat moss | 300 to 500 | 90 to 98 | 60:1 | 10 to 30% |
| Coco coir | 400 to 600 | 90 to 95 | 80:1 | 10 to 30% |
| Bark fines | 400 to 600 | 85 to 95 | 120:1 | 10 to 20% |
| Screened topsoil | 1,800 to 2,200 | 2 to 6 | 12:1 | bulk filler |
| Coarse sand | 2,400 to 2,800 | 0 | none | up to 20% |
| Perlite | 100 to 160 | 0 | none | 10 to 30% |
| Vermiculite | 130 to 190 | 0 | none | 10 to 25% |
Anything with a C:N ratio above about 30:1, such as bark fines or fresh coir, pulls nitrogen out of the soil while it breaks down. If you blend those in at more than about 15 percent, plan on a nitrogen source in the first season.
Lime and Sulfur Rates by Soil Texture
These rates are pounds per 1,000 square feet to move pH a full 1.0 unit when the material is worked into the top 6 to 7 inches. Scale them linearly for smaller changes.
| Soil texture | Ag limestone to raise 1.0 pH (lb) | Elemental sulfur to lower 1.0 pH (lb) |
|---|---|---|
| Sand | 25 | 10 |
| Sandy loam | 35 | 13 |
| Loam | 50 | 16 |
| Silt loam | 60 | 19 |
| Clay loam | 70 | 22 |
| Clay | 80 | 24 |
Material factors adjust those base rates. Calcitic and pelletized limestone are 1.0, dolomitic limestone is about 0.95 and also supplies magnesium, oyster shell lime is about 1.1, hydrated lime is about 0.74 and acts fast enough to burn roots, and wood ash is about 2.0 because its neutralizing value is roughly half that of limestone. On the acid side, aluminum sulfate takes about 6 times the weight of elemental sulfur and iron sulfate about 8 times. Gypsum is not on either list because calcium sulfate does not change pH at all.
Example Problems
Example 1: blending compost into a raised bed. The bed is 20 feet by 4 feet, you plan to till 8 inches deep, and you want compost to be 25 percent of the finished mix. Leave the waste allowance at the default 10 percent.
- Area = 20 * 4 = 80 square feet
- Total bed volume = 80 * (8 / 12) = 53.3 cubic feet, or 1.98 cubic yards
- Compost before allowance = 53.3 * 0.25 = 13.3 cubic feet
- Compost with allowance = 13.3 * 1.10 = 14.7 cubic feet, or 0.54 cubic yards
- Spread depth before tilling = 8 * 0.25 = 2 inches
- Bags at 1.5 cubic feet each = 14.7 / 1.5 rounded up = 10 bags
Example 2: lowering pH for blueberries. The plot is 500 square feet of sandy loam, the soil test reads pH 7.5, and the target is 6.5 using elemental sulfur.
- Base rate for sandy loam = 13 lb per 1,000 square feet per 1.0 pH unit
- Rate needed = 13 * 1.0 * 1.0 = 13 lb per 1,000 square feet
- Total sulfur = 13 * (500 / 1000) = 6.5 lb
- Safe single dose = 10 lb per 1,000 square feet, so 13 needs 2 passes at least 8 weeks apart
FAQ
How much compost should a new bed get?
For a bed you are building from scratch, 25 to 30 percent compost by volume in the top 8 to 12 inches is the usual target. For an established bed you are maintaining, a 1 to 2 inch topdress once a year is enough, which works out to about 3 to 6 cubic yards per 1,000 square feet. Going past roughly 30 percent compost by volume tends to backfire, because the mix holds too much water, settles hard as the organic matter breaks down, and often carries excess phosphorus and salts.
Why does 2 inches of compost not raise organic matter by 2 percent?
Two different measurements are getting mixed up. A soil test reports organic matter as a percentage of soil dry weight, not as a share of volume, and the top 6 inches of a 1,000 square foot area weighs roughly 20 tons. On top of that, most of the carbon in fresh compost is food for soil microbes and gets respired away within a year, so only about a third of what you apply becomes lasting organic matter. That is what the retention factor in the calculator accounts for. A 1 inch annual application on a typical loam usually moves a soil test by something like 0.2 to 0.5 percentage points per year, which is why reaching a target normally takes several seasons rather than one.
Should you add sand to clay soil?
Only at high rates, and usually it is the wrong move for a garden bed. Small additions of sand to a clay soil fill the pore space between clay particles and can make drainage worse rather than better, and getting a real texture change takes upwards of 50 percent coarse sand by volume, which is far more material than most people are prepared to haul. Adding organic matter improves clay structure at a fraction of the volume, so the blend mode with compost at 25 to 30 percent is the practical route. If you do use sand, keep it coarse and pair it with an organic amendment rather than using it alone.