Restrictions on imports, by forcing...

The tariff act of 1850 was the first modest step towards the removal of prohibitions and the lowering of tariff rates, especially those on raw materials; the tariff act of 1857 went a great deal further, with the result that almost all prohibitions on imports disappeared and the tariff rates, while stillhigh and distinctly protective, were at least not entirely prohibitive. In contemporary Russian economic literature the opponents of protection have advanced, among others, the argument that restrictions on imports, by forcing foreign ships to call in ballast at Russian ports, added to freights and thus hindered exports.

Micheal Florinsky "Russia. History and interpretation" Could you please intepret in your own words the bolded as a whole, and call in ballast in Russia in a particular? Thanks a lot.