Nitrate and phosphate mines were discovered on land around the same time that the guano islands were depleted
Peruvian guano from the Chincha Islands was the gold standard for natural fertilizers. Nearly two hundred feet deep in the early 1840s, it was mined out by the late 1870s.
However, by the mid-19th century, inland nitrate and phosphate mines had been discovered in Peru and Chile, as well as in North America and Europe. These finds permitted blending of the various minerals for balanced synthetic fertilizers. Some of these mines remain in use today.
Today, the United States retains ownership of several remote Pacific and Caribbean islands first claimed in the mid-19th century for the guano trade.