Compost can be a valuable source of organic matter for gardeners and farmers. It can improve the soil’s ability to absorb and store water as well as hold plant nutrients. Applied at heavy rates, compost can also contribute nutrients and salts to the soil.
Testing compost helps growers to make informed management decisions. We recommend that farmers and gardeners test compost at a testing lab. If buying in bulk from a composting facility, growers should ask the seller for a compost analysis report.
The University of Minnesota Soil Testing and Research Analytical Laboratory (STRAL) provides compost testing services to compost producers, researchers and growers. While the STRAL is not licensed to certify compost for commercial purposes, the lab can provide comprehensive measurements of the material's physical and chemical properties.
This page provides an overview of a few key compost analyses that growers should be aware of, and how to interpret the results from these tests.
Any time you are adding compost to your soil, consider taking a soil test first. Read more about soil testing for farms and managing soil and nutrients in gardens.
Total solids
Total solids is reported alongside total moisture, and together these numbers equal 100%.
Compost can vary substantially in moisture content. If the goal of using compost is to add organic matter to the soil, compost with a higher percentage of total solids will provide more organic matter compared to a compost that is mostly water.
Compost that has a high moisture content (over 60%) may be hard to spread, whereas compost with a low moisture content (less than 40%) may have a dusty texture.
Carbon to nitrogen ratio (C:N ratio)
An ideal range for the carbon to nitrogen ratio of compost is about 25:1 to 30:1. A higher C:N ratio (more carbon rich) will result in microbes tying up or immobilizing plant-available nitrogen. This nitrogen is not necessarily gone, but may not be released in time for the crop to use it.
If the C:N ratio is low (more nitrogen rich), the compost breaks down quickly and there is plenty of leftover nitrogen for plants to use. But if this low C:N compost decomposes with no roots to take it up, plant-available nitrogen can be lost from the field.
The C:N ratio varies substantially across different sources of compost.
Total organic carbon (C)
Many gardeners use compost for the purpose of adding organic matter to their soil. Organic carbon represents approximately one half the amount of total organic matter in compost. So, if your total organic carbon is reported as 30%, the compost contains around 60% organic matter.
The remaining material in compost typically includes soil particles, minerals, and dissolved nutrients like nitrogen, phosphorus, potassium, calcium, and many others.
Nitrogen (N)
Ammonium - NH4-N
NH4-N is ammonium, a form of inorganic nitrogen that is readily available to plants after compost is applied to the soil. When considered together with nitrate-N (NO3-N), this test measures a portion of the readily plant-available nitrogen in the compost.
Nitrate - NO3-N
NO3-N is nitrate, which is another form of inorganic N readily available to plants. Nitrate is highly mobile, and so it can be a potential pollutant in ground and surface water.
Organic N
In most labs N analysis is done in two steps. Inorganic N is extracted and measured from a sample in the form of nitrate and ammonium. Total N is measured from a different sample by combustion (total N = inorganic N + organic N). Organic N is calculated by subtracting the inorganic N from the total N (organic N = total N - inorganic N).
Organic N is bound in organic matter, and is not immediately available to plants. Organic N will become plant-available over time as microbes in the soil break down the organic matter in the compost. This fraction of N is the reason why only a portion of the total N in composts or fresh manure is available in the first year of application. The organic fraction is what gives the compost (or cover crop, manure, or any other organic soil amendment) a N credit in the second year after application.
Too much N causes plants to over grow during the vegetative stage and, depending on the crop, applying too much N might reduce yields.
For example, compost N might be 70% available in the first year, with about 5% being available in the second year after application. This credit should be used when planning for N application to avoid overapplication and potential negative environmental impacts.
pH
pH is a measure of acidity. A pH of 7 is considered neutral; anything below 7 is acidic, and anything above 7 is basic or alkaline.
For most plants, the ideal pH of soil or growth media is between 6 and 7, with the exception of a few crops such as blueberries, which thrive in soil with a pH around 4-5.
The pH of plant-based composts can be highly variable (either alkaline or acidic), and manure-based composts are often slightly alkaline.
Combining compost with soil or media may affect the pH of the soil or media. But most agricultural and garden soils in Minnesota are strongly buffered against pH changes; therefore, compost application at rates that would supply the plant needs for nutrients may not result in significant changes to the soil pH.
Soluble salts
The measurement of electrical conductivity (EC) reflects the concentration of soluble salts in the compost. The higher the conductivity measurement, the higher the salt content or salinity.
The STRAL reports electrical conductivity in millimhos per centimeter (mmhos/cm) using a saturated paste extract. The higher the number, the higher the salt content. Electrical conductivity is sometimes reported in units of dS/m or mS/cm; these units are interchangeable with mmhos/cm.
Some labs may report results using different methods such as a 1:1 or a 1:2 soil to water dilution. These results cannot be directly compared with results obtained from a saturated paste extract.
Compost with high salinity should be used with caution, especially if it is being used to fill raised beds, inside high tunnels, or if large quantities of compost are added to the soil in the spring. High salt levels can inhibit germination and cause plant stress. This negative effect can quickly build up over time in conditions such as inside the high tunnels or beds that do not receive significant rainfall to flush the excess salts.
Soluble salts readily leach from soil with rain water. If the soluble salt levels in your compost are high, consider mixing the compost with soil to dilute the salts, or letting it sit outdoors for a season so that the salts have a chance to leach out before applying it.
Relative salt sensitivity levels in saturated paste extract
| mmhos/cm | Description | Effect on crops |
|---|---|---|
| 0-2 | Non-saline | None |
| 2.1-4 | Very slightly saline | Sensitive crops restricted |
| 4.1-8 | Moderately saline | Many crops restricted |
| 8.1-16 | Strongly saline | Most crops restricted |
| more than 16 | Very strongly saline | Few plants tolerant |
Soluable salt test values and relative salt tolerance of fruit and vegetable crops*
| 0-2mmhos/cm (nontolerant) | 2-4mmhos/cm (slightly tolerant) | 5-7mmhos/cm (moderately tolerant) | 8-16mmhos/cm (tolerant) |
|---|---|---|---|
| Blueberries | Apples | Broccoli | Asparagus |
| Carrots | Cabbage | Beets (table) | Swiss chard |
| Green beans | Celery | Cucumbers | |
| Onions | Grapes | Muskmelons | |
| Radishes | Lettuce | Squash | |
| Raspberries | Peppers | Tomatoes | |
| Strawberries | Potatoes | Spinach | |
| Sweet corn |
*Plants can be successfully grown at these test levels or lower.
Tables published in Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota.
Phosphorus (P)
Like nitrogen, phosphorus (P) is present in inorganic forms and also as organic P in compost. Inorganic P can be found as dissolved phosphate and precipitated as P minerals, mainly attached to calcium or in a mineral called struvite made up of nitrogen, magnesium, and phosphorus.
The solubility of the many different minerals that can be found in the compost will depend on the compost pH. The more alkaline the compost the less soluble P will be, whereas the more acidic the compost the more soluble the minerals will be.
But, as pH decreases below 6, then P can start to be tied up with aluminum and iron that are also present in the compost. Organic P has to be mineralized by enzymes present in the soil before it can become available for plant uptake.
The STRAL reports the results for total P in % on a dry weight basis, so it is important to know the compost moisture so that accurate nutrient rates can be calculated.
Other labs may report P as part per million (ppm), which is the same as mg/kg. The report's unit of "mg/kg compost" means milligrams of P per kilograms of compost.
To figure out the phosphorus contribution of your compost from a nutrient management perspective, follow these steps:
- If the report shows P concentration in ppm, first you need to convert ppm to percent P by dividing the ppm by 10,000 (move the decimal 4 places to the left).
- Divide the %P by 100 again to figure out pounds of P per pound compost.
- Most nutrient requirement tables report the P needs in the oxide form, phosphate (P2O5). To convert lbs of P to lbs of P2O5, multiply by 2.3.
Example: If your compost report says 28,400 ppm P, the material is 2.84% P (30,000 ppm P 10,000 = 2.84%) on a dry basis. There are 0.0284 lbs P per lb compost, and 0.066 lbs P2O5 per lb of dried compost (2.84% 100 = 0.0284, 0.02842.3=0.0653 lbs). If the fresh compost has 50% moisture content (for example), then it would have only 0.0326 lbs P2O5 per lb of fresh compost.
This is equivalent to approximately 32.6 lbs of P2O5 per cubic yard of fresh compost, assuming a cubic yard of that fresh compost weighs 1,000 lbs (0.0326 lbs K2O*1000 lbs=32.6 lbs K2O/cu yd). Your compost will likely not weigh exactly 1,000 pounds per cubic yard; this amount of compost may range from 800 to 1500 pounds depending on the moisture content and other factors. To get a proper estimate of the weight, consider weighing a subset of the compost (e.g. 1 cubic foot).
The organic portion of the total P will have to be mineralized before it can be taken up by the plant. For most gardens, assuming 100% of total P is available should help keep soil test P from building to excessively high levels. The weight of a cubic yard of compost will depend on many things, but most importantly on the total solids, or moisture content, of the compost.
Potassium (K)
The University of Minnesota Soil Testing and Research Analytical Lab reports the results for total K in % on a dry weight basis, so it is important to know the compost moisture so that accurate nutrient rates can be calculated.
Other labs may report K as part per million (ppm), which is the same as mg/kg. The report's unit of "mg/kg compost" means milligrams of K per kilograms of compost. All of the K from compost can be considered 100% available for plant uptake as soon as it is applied because almost 100% of K in compost is either dissolved or in mineral form.
To figure out the potassium contribution of your compost from a nutrient management perspective, follow these steps:
- If the report shows K concentration in ppm, first you need to convert ppm to percent K by dividing the ppm by 10,000 (move the decimal 4 places to the left).
- Divide the percent K by 100 to figure out pounds of K per pound compost.
- Most nutrient requirement tables report the K needs in the oxide form, K2O. To convert lbs of K to lbs of K2O, multiply by 1.2.
Example: If your compost report says 30,000 ppm K, the material is 3% K (30,000 ppm K 10,000 = 3.00%) on a dry basis. There are 0.03 lbs K per lb compost, and 0.036 lbs K2O per lb of dried compost (3.00% 100 = 0.030, 0.0301.2 =0.036 lbs K2O). If the fresh compost has 50% moisture content (for example), then it would have only 0.018 lbs K2O per lb of fresh compost.
This is equivalent to approximately 18 lbs of K2O per cubic yard of fresh compost, assuming a cubic yard of that fresh compost weighs 1,000 lbs (0.018 lbs K2O*1000 lbs=18 lbs K2O/cu yd). Your compost will likely not weigh exactly 1,000 pounds per cubic yard; this amount of compost may range from 800 to 1500 pounds depending on the moisture content and other factors. To get a proper estimate of the weight, consider weighing a subset of the compost (e.g. 1 cubic foot).
Rosen, J. Eliason, R. 2005. Nutrient Management for Commercial Fruit & Vegetable Crops in Minnesota. University of Minnesota Extension. https://hdl.handle.net/11299/197955
Sullivan, D., Bary, A., Miller, R., Brewer, L. 2018. Interpreting Compost Analyses. Oregon State University Extension. https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em9217.pdf
Reviewed in 2023