Saltpetre is obtained using a spade on a saltpetre spawn, in the Hosidius region of Zeah, the area southwest of the main bank. It spawns at one of five possible locations, moving when it is depleted. It can also be obtained as a reward from the Chambers of Xeric and Wintertodt. You can teleport here by using the skills necklace and going to the woodcutting guild.
Players with a high Mining level can collect 1500+ saltpetre per hour; however, players with level 1 Mining will only be able to get about 850. This is because when you mine saltpetre, you have a chance of receiving two blocks, and this chance is roughly equivalent to your Mining level out of 100. A higher Agility level, energy potions and weight-reducing clothing are also advantageous.
The NPC, Konoo, will follow the saltpetre around, so it's helpful to look for a yellow dot on the minimap when searching for the new spawn. He provides players with a free spade, should you not have one.
Hopping worlds to find the saltpetre location just south of the bank can cut down on banking time.
Saltpetre is used with regular compost to create Sulphurous fertiliser, which can then be used to gain favour with House Hosidius by donating it to the clerk south-west of the bank, at a rate of 0.1% favour per bucket. This also grants 2 Farming experience per bucket of fertiliser made.