It's a cargo bike: Low centre of gravity, adequate transmission for hauling heavy loads uphill, moving it isn't the issue. You should be more worried about the actual load capacity: A good cargo bike, as in two wheels not four or a trailer configuration or something, usually maxes out at ~150kg load (including driver). Trikes about 250kg, quads or trikes with trailer at 500kg.

But then a kg of soil weighs more like 400g so we're talking more like 80kg. Including driver you might be exceeding load capacity, but not by much (assuming obese people don't ride bikes which I think is a fair assumption) it's probably going to survive, especially if you're careful around kerbstones and stuff. Those load limits are all calculated off some maximum drop distance, if you don't drop your wheels then you can generally go much higher. The frame is very unlikely to break or bend, the axles would probably be the first to fail.

Are you sure about that weight? I can't say I've used a lot of potting soil in my life, but the first google results for it that have weights listed for the bag are all more like quarter to a third of a kilo per litre. That puts the weight of the load at less than 70kg, which is much more reasonable

Once you've got some speed it's easy to stay stable