I get regular questions about both the cost of composts and quality of composts (or manure) in my visits with organic producers and conventional producers or both. There seems to be this mystery about compost including, what is actually in the compost and how much it is worth? Certainly, there is some mystery since composts do have organic matter and as a result also contain microbes that generally are not measured by the labs. These two ingredients add a lot of value to a compost but in general we have trouble quantifying or putting a $$ value on their presence in the compost.
Almost any company that makes and/or sells a compost product will have an analysis for you to know what is in their product. Typically, they should be taking samples on a regular basis or at least as the supply source changes. Compost is generally made from manure and manure is made from feed that livestock eat. As a livestock producer changes what their animals eat this can drastically influence what nutrients end up in the manure and ultimately the compost.
If you get an analysis then the first thing to look at is the moisture% in the compost. A recent analysis sent to me showed the compost to be 52.08% moisture. So, one ton of compost would be about 50% compost and 50% water. This is not unusual but it does make a difference on the analysis and so the price. This analysis showed 53.6 lbs of nitrogen per ton but in the fine print you are told to convert the pounds of nutrient/ton as received by multiplying pounds of nutrients as reported by (100-moisture%)/100. So 53.6(100-52.08)/100 or 53.6 * 0.4792 = 25.685 lbs of nitrogen per ton of compost! Now this compost doesn’t look as good as it did for nitrogen or any other nutrient on the analysis.
What we really want is an analysis based on dry matter not with water added so we can compare to commercial fertilizer costs. This gives us a compost value or even a way to compare one compost to another.
I did this analysis on two different companies selling two different products. One was a compost, like you see in the picture above, and the other was a pelleted compost which is very common now. The pellet had an analysis that was based on an as received basis so those results were based on a true ton not a ton minus the water. A very important distinction and one rarely discussed.
This picture of my spreadsheet above shows an analysis of the cost of ingredients based solely on Nitrogen, Phosphorus from P2O5, Potassium from K2O and Sulphur from Ammonium Sulphate. You can see the current cost of those nutrients is based on commercial fertilizer prices so that we get a value to compare composts to each other. In the top example, the $78 compost seems to be a bargain even though the nutrients are slightly less than the second example. But, the water (%moisture) lowers the actual value down significantly to $53.82 per ton. This example is meant to show that you could pay about $54 per ton for the top sample and feel good that you didn’t pay more than the current price of conventional fertilizer. And, in the second example when you pay $200 you are getting more conventional fertilizer nutrients than you should, plus you get lots of micros and organic matter and microbes. Generally you would have to buy about 3 tons or $238 of the $78 compost to equal the amount of nutrients in the $200 compost. This may still be a bargain for you based on the other nutrients we did not analyze or maybe the convenience of one over the other, etc. The point is to do a little comparison shopping before you just look at price per ton, there are a lot of things in the ton you may have never thought about before!