Walking through a gardening Store I sometimes wonder, why some types of soil, like Compost and Potting Soil, are measured in Liters but other types like sand and gravel are not. To get a good understanding of this topic beyond the most obvious answers, one can come up at the spot, I looked a bit more into this issue. If you only here to get a quick answer to these questions then here you go:
Why is Compost measured in Liters? Generally, it’s measured in Liters to avoid inconsistency related to weight caused by water-concentration in the Soil. Unlike Stones, water does play a huge role in the weight of compost, therefore measuring in Volume is preferred. Additionally, most gardeners care about filling up certain Areas with soil, conversion from Liters to m³ can be done much faster than from Kilo. Generally speaking, one liter of Compost can be converted to roughly 0,6kg (1,32lbs).
Now with the short answer out of the way lets take a closer look into why Compost is measured in Liters.
Why Compost measured in Liters
As mentioned previously it is mostly measured in liters to keep a consistent unit so packing can be optimized without further complications. As seen in the image above, the compost currently is damp, but pretty dry – it hadn’t rained for a couple of days back then.
Looking at compost directly after some rain will quickly show a huge difference in weight, with only little variation in volume – due to the weight of the water taking effect on the air pockets in the soil.
What makes the conversion so hard
There are a lot of contributing factors you have to consider when converting soil volume to weight. In general, you can use a density of 0,6kg (1,32lbs) per liter of compost. Assuming your compost prides itself with optimal composition – this is also the number I would advise to use when buying commercial compost.
For compost which consists mostly of moist components, is compacted, relatively fresh -abundant with kitchen or organic waste- and lacks brown materials. You can assume a higher bulk density ranging from 0,7-0,9kg (1,55-2lbs) per liter.
On the other hand, very dry, air-filled compost, abundant in brown materials will weight much less and will be anywhere between 0,3-0,5kg (0,66-1,1lbs) per liter.
Why you sometimes need both kilo and liters?
There are a couple of instances where it makes sense to be able to convert these numbers. One such example is buying in bulk. Most large scale operations will have an industrial scale and sell their compost per weight.
Another example would be if you want to fill Pots for planting, which on a later occasion will be moved. Your life will be easier if you consider the weight of said pot beforehand, and not only once you want to move it.
Similarly, you can optimize your trips to your local gardening center if you know how much your car or truck will be able to handle. In most cases, the limiting factor will be the limit to loading weight. Converting this number to soil liters can give you a general idea of how much soil you can transport per trip.
With that out of the way let’s get straight to converting soil liters to kilos.
How to get calculate your composts density?
The easiest way to calculate your soil density is to use a bucket with a volume you know (5 or 10 liters) and filling it up completely with soil. Measure its weight before and after filling it with your soil. Once you know the difference you can just divide this weight by the volume you used.
For example if my bucket of 5 liters weighs 0,3kg (0,66lbs) before and 3,3kg (7,27lbs) after filling it up. I can calculate the density of my soil by dividing (3,3kg-0,3kg)/5liter = 3kg/5liter = 0,6kg(1,32lbs)/liter
When calculating the density of your soil try to get a good mixture of components, the density of compost can vary greatly so it’s best to mix it thoroughly beforehand.
How much soil do I need for a 10ft x 5ft x 1ft raised bed? This raised bed will have a volume of 50ft³ to fill it up you will need 1415liters or roughly 850kg (3120lbs) of soil.
How much area can I cover with a liter of soil? One liter can be used to cover 100cm² or 15,5in². When looking at volume one liter can cover 1000cm³ (0,001m³) or 61in³ (0,035ft³).