minetest.register_abm({
    nodenames = {"default:dirt"},
    interval = 1,
    chance = 1,
    action = function(pos, node)
        minetest.env:set_node(pos, {name="default:sand"})
    end
})

minetest.register_on_generated(function(minp, maxp)
    for x = minp.x, maxp.x do
        for z = minp.z, maxp.z do
            for ly = minp.y, maxp.y do
            local y = maxp.y + minp.y - ly
            local pos = {x = x, y = y, z = z}
            if minetest.env:get_node(pos).name == "default:dirt" then
                minetest.env:set_node(pos, {name="default:sand"})
            end
        end
    end
end)