(Pulls out lecture podium...)
Perhaps a history lesson is in order...
When Gunpowder was originally invented, so little was used, that one could use Bat guano from caves, to refine for saltpeter.
However, by the time the Age of Sail reached its glory years, the supply from caves was pretty much exhausted, as the bats weren't crapping enough over 1 year, to replace the thousands of years of doodoo that had been mined out.
Thus, the world began looking for new sources of droppings.
And, in time, they found them - Tiny little islands, often too small and rocky for plant life, mostly along the mid-ocean ridges, where the long-range sea birds would stop, rest, and sometimes raise their young.
But, these islands full of feces were often thousands of miles away from the countries claiming them, leading to harsh words and threats between politicians to said islands, and eventually leading to wars -
Literally, nations were fighting over

- or at least who got to mine and keep it. It wasn't until the revolutionary advances in chemical engineering in the first decade of the 1900s, that scientists finally figured out how to synthesize the relevant materials in a laboratory, in sufficient quantities and cost to be economically viable. But, it took decades for that to spread to all nations (as, after all, the ability to create ammunition more easily, at home, would be a military secret of highest priority).
And, if in school, looking at a globe, you ever wondered why all those countries "owned" islands barely big enough to be considered more than reefs, half-way around the world - THIS is why they did, and why so many had conflicting claims of ownership.
This resulted in the "colonial" pressures of the 1910s between those with large empires, and those with few claims elsewhere, over mining and "mineral" rights(especially for military necessities like guano), becoming one of the pivotal forces behind forging the alliances that made World War One occur.
That war, as a result, became "The War to End All Wars for Seagull Shi...." Well, you get the idea.
By the time the Germans came back for Round Two, in the 1930s, everyone was making the stuff synthetically in chemical plants.
********
So, given that we don't have 1900s chemical tech, and most likely the Crown controls all caves that give guano, Saltpeter's going to be a resource that is found and traded by those traveling the seas....

:gun:
