Have you ever wondered how many nutrients you are supplying with your compost? Sometimes it’s way more than you might think! We have written a calculator to help you understand exactly how much of what you might be putting on, especially if you have a compost test report for your compost.
Compost is great stuff for the garden-farm. It supplies:
- Carbon. This is usually the most depleted element in chemically farmed soils due to carbon removal with the crop, soil carbon depletion due to excess nitrogen applications and tillage. Without carbon, life doesn’t happen. Carbon is readily depleted from soil and regular compost applications help put it back.
- Increased cation exchange capacity (CEC). Organic substances in compost, such as humic and fulvic acids, have the ability to capture and hold cation nutrients such as ammonium, calcium, magnesium and potassium.
- Microbial inoculation. Compost is full of the decomposing microbes and can greatly stimulate the ability of the soil to capture and cycle nutrients. This is a huge subject indeed, one worth looking into.
- Moisture retention. Organic matter in the soil holds water. Compost is also a great mulch. Composts that are not finished or that have a high carbon to nitrogen ratio are good candidates for mulch.
- Nitrogen, phosphorus, potassium, and a host of other nutrients. The amounts of these nutrients can vary widely depending on what was in the starting material of the compost and how the compost was made. In general, compost will not supply sufficient nitrogen without excess phosphorus and potassium. Composts made from animal manures are higher in nitrogen and phosphorus than those that are strictly plant-based.
Tim McCabe / Photo courtesy of USDA Natural Resources Conservation Service.
One of the myths of organic gardening is that you can never have too much compost. While this might be true if you are only applying compost made from the weeds and plant leftovers from the garden itself, it certainly isn’t true if you are bringing in a lot of compost from off site. In the first case you are simply cycling nutrients back into the soil, so there is little chance of creating an imbalance by doing so. But with offsite nutrients, it is quite possible to inadvertently create excesses.
In the extreme, excess nitrogen and phosphorus leached from compost can cause nutrient pollution in groundwater and downstream waterways. This is more true for immature composts , and in situations where the compost may be water saturated for days . As compost matures, the nutrients in it become bound up in organic compounds and the bodies of microbes and are less water soluble. Saturating the compost with water reverses this trend. Spring runoff is a time when phosphorus can move into waterways.
Even if nutrients aren’t being leached from composts, they can still create long-term nutrient excesses. Because compost contains a lot of available phosphorus and potassium, and not much available nitrogen, it’s easy to add too much P and K. We see this a lot, especially in hoop houses where the soluble nutrients are not easily leached.
Composts can contain a lot of soluble salts such as sodium and chloride (which combine to form table salt), especially if manure is one of the feedstocks, but also other soluble ions and compounds such as sulfate, nitrate, calcium and magnesium. An excess of soluble substances can result in osomotic stress to plants, which are not able to take up nutrients from the soil when there is an excess concentration of ions outside the root compared to within. Or it can result in excesses of sodium and chloride in the soil, blocking nutrient uptake and potentially damaging soil structure. These problems can be mitigated if the compost and soil can be leached with good water, but this may not always be possible. In any case, it’s good to understand what all is in the compost that you want to apply compared to what is already in your soil.
Sometimes compost vendors will have a compost report available, though usually from a batch a few months ago. These are complete digestion analysis, not soil tests. The first thing to look at is the C:N ratio. If it is over 20, it will rob N if dug in, though it is still a good mulch. Especially if placed under a thin layer of straw where it can be kept moist and decaying in place. If the C:N ratio is less than 15 it is very good. The percent sodium (%Na) should be less than 5%. The percent moisture should be around 50%.
EC (electrical conductivity) is a measure of the amount of soluble salts in a soil, or a compost, however there are a number of different methods used to measure EC , and they do not correlate easily with each other. Instead, the Ag Index is used to determine the ratio of the “big three” plant nutrients to sodium and chloride.
- Ag Index is a ratio of weights: (N + P2O5 + K2O) / (Na + Cl).
- Ag Index > 10 is excellent.
- Ag Index < 2 is poor.
The calculator below is designed to answer the question: How many nutrients am I adding with my compost? To use it enter the area you intend to amend, and the amount of compost you intend to amend with, along with a compost analysis.
Questions? See the FAQ’s below the calculator.
If you dig the compost in, the nutrients in it will be available more quickly. But even if you leave it on the surface as a mulch, it will eventually break down and the nonvolatile nutrients will end up in the soil. Pay particular attention to phosphorus and potassium since these are easy to overdo with compost applications.
 The contribution of water extractable forms of plant nutrients to evaluate MSW compost maturity: a case study https://www.nature.com/articles/s41598-020-69860-9#Tab3
 Nutrient Leaching from Compost: Implications for Bioretention and Other Green Stormwater Infrastructure https://ascelibrary.org/doi/10.1061/JSWBAY.0000821
 Soluble Salts in Compost and Their Effects on Soil and Plants: A Review
 Variability of Soluble Salts Using Different Extraction Methods on Composts And Other Substrates https://www.researchgate.net/publication/261728246_Variability_of_Soluble_Salts_Using_Different_Extraction_Methods_on_Composts_And_Other_Substrates
 Interpreting Compost Analyses https://catalog.extension.oregonstate.edu/sites/catalog/files/project/pdf/em9217.pdf