It partly depends on the color. I would assume that soil is dark-colored while sand is more light-colored. In general, dark-colored objects absorb heat much faster than light-colored objects. Dark colors tend to absorb more light/heat and don't reflect as much (hence the dark colors that our eyes see).

Now, you have to be careful in making this conclusion because if the soil is wet, then it will be more difficult to heat it up. So assuming we have dry soil and dry sand, the soil will heat up much faster.

As you can see, it depends on a lot of different factors (is it wet/dry?, what color is it?, etc.), so we need to make sure we know what we're dealing with before making any conclusion.