You'll probably break the apron. Even if you only go with half a load. Dirt will be close to double the weight of manure for a given volume.... and as noted already... dirt is not slippery. Even mud will not slide very well in a box spreader. I've hauled the last scrapings of a compost pile that was probably half dirt or more in a hydraulic push spreader. That was some of the most miserable stuff I've ever hauled in it. Dirt just doesn't slide and it makes probably twice the friction in addition to double the weight so it's 4 times as hard to push out of the spreader box.