To simulate earth (even in 1/72) I mostly use real earth sifted a couple of times (as Martin described) and add two or three different tones of pulverised pastel chalks.
I think this gives a good balance between the structure and texture of the real earth and the more in scale grain size of the pastel chalks.
I also like to use the woodland fine turf labled "earth" colour, but not to simulate blank earth.
It looks more like earth with tiny plants or very short grass on it, somewhat like there is moss growing on it and not like freshly dug up earth.
I definitely recommend buying fine turf. It gives a nice variety when combined with other earth techniques or grass products (or coarser turf by woodland scenics), and when mixed with scale-leaves it makes a nice forest ground.
I hope this hleped,
ekke
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