Compost Nutrient Credit Calculator
Before you buy a bag of anything: work out what the compost you already spread is supplying this season, including what is still coming out of the loads you put down in previous years — and see the phosphorus that has been going in alongside it.
Two tools that never spoke to each other
A fertilizer calculator asks what you are growing and tells you what to apply. A soil mix calculator asks how big your bed is and tells you how much compost to fill it with. Both are useful and neither knows the other exists, so the gardener who uses both in the same spring applies the nitrogen twice — once as compost they did not count, and once as a bag they did not need.
This closes that gap, and it draws the crop requirement from the same table the Fertilizer Calculator prescribes against, so the two cannot disagree. Work out the credit here first, then take the shortfall — if there is one — to the fertilizer calculator rather than the whole requirement.
If you are still building the heap that feeds this, the Compost Ratio Calculator gets the carbon and nitrogen balance right, and the Soil Mix Calculator works out how much of everything a new bed takes.
Frequently Asked Questions
Does compost count as fertilizer?
For nitrogen, far more than most gardeners credit it. An inch of ordinary garden compost spread over a 200 square foot bed is roughly 617 pounds of material carrying about 6 pounds of total nitrogen, and the share that becomes available in the first season alone comes to about three times what a vegetable crop at its hungriest actually asks for. The reason it does not feel like fertilizer is that it arrives slowly and invisibly rather than in a measured scoop — but the plants cannot tell the difference, and neither can your wallet when you buy a bag you did not need.
Why does compost from three years ago still matter?
Because only a fraction of compost nitrogen is available in any one season. The rest is locked in organic matter and has to be released by soil life, which takes years. Published figures put roughly 15% of the organic nitrogen into circulation in the year of application and about 10% in each of the two years after. Spread compost every spring and by the third year your bed is receiving this year's release plus last year's plus the year before's, all at once. Credit only the load you just spread and you will under-count by well over half.
Can you put too much compost on a garden?
Yes, and phosphorus is how it shows up. Compost carries phosphorus roughly in the proportion the original plant material had, which is much richer relative to nitrogen than any crop's demand. Spread enough to matter for nitrogen and the phosphorus goes in at many times what the plants remove — every year, with nothing taking it away. It does not leach like nitrate; it accumulates. Very high soil phosphorus locks up iron and zinc, producing deficiency symptoms in plants sitting in soil that looks, on paper, extremely fertile.
Should I stop composting then?
No. Compost does things fertilizer cannot: it feeds soil biology, holds water, and builds the structure that lets roots and air move. The change worth making is smaller and duller — stop treating a compost dressing as free and invisible. Count the nitrogen it supplies before you buy any, and if you have been spreading an inch or more every year for several years, get a soil test for phosphorus before you spread again. Leaf mould and low-phosphorus materials are the answer when you want organic matter without the nutrient load.
How accurate is this?
It is an order of magnitude, not a measurement, and the reason is the compost rather than the arithmetic. Analysis varies enormously between suppliers and between batches from the same supplier, and the mineralisation rate depends on your soil temperature, moisture and biology. The figures here come from published averages, including a Washington State University survey of 126 manure-compost samples. Use it to find out whether you are in the right region — needing nothing, needing a little, or heading for a phosphorus problem — and use a soil test to find out where exactly.
Estimates from published averages, including a Washington State University survey of 126 manure-compost samples. Compost analysis varies enormously between batches; a soil test is the only way to know what a particular bed holds. Not a substitute for one.