In the early 20th century, agriculture dominated Siquijor’s economy so thoroughly that nearly all of the island’s arable land was already in use. One common crop was coconuts, which had been introduced by the Spanish. Siquijodnons also started to grow maguey and sisal, two agave plants cultivated for their fiber, after the Insular Bureau of Agriculture distributed seeds to the farmers. Several hundred acres of the latter crop were planted throughout the island, and so much of it was grown that by 1920, Siquijor had become the main supplier of the crop in the Philippines.

Tobacco was another profitable crop in Siquijor, which outproduced the rest of Oriental Negros. Many people from neighboring islands sought out Siquijor’s tobacco in particular but were unable to obtain it in large quantities because of the Moros. They dominated the island’s tobacco trade, exchanging their dried fish and toñgong (tan bark) for tobacco and corn.

Siquijodnon farmers also grew rice and Indian corn. However, neither Siquijor’s production nor Oriental Negros’s were able to meet local demand, so both were always forced to import both crops from other provinces. Many also produced sinamay and ugpac, two coarse hemp fibers, to make their own clothes, pillows, mattresses, and curtains. These were exclusively cottage industries. Other smaller industries included abaca and sugar. Siquijodnons additionally raised poultry to be sold to Dumaguete, and at the onset of American rule, the island served as that town’s main supplier of chicken and eggs.

All of this cultivation severely depleted Siquijor’s already poor soils (see Siquijor under Spanish Rule: Economy) of their nutrients. Constant tilling badly eroded the soil, and farmers often planted three crops of corn per year on the same land, reducing the size of each consecutive harvest and leaving the soil unable to withstand drought. Because the farmers always needed to be planting corn to feed their families, crop rotation would not have worked. Nor would have fertilizers, as Siquijor did not have any and lacked the means to buy or produce them. Opening up new land to agriculture was not a possibility either, as it had already all been taken.

Siquijor’s only major non-agricultural industry was manganese, which is typically used in fertilizers and steel production. Following the discovery of a massive manganese deposit in the highlands of Larena, Enrique Villanueva, and Maria by a German prospector in 1937, Lt. Gov. Jumawan and his two brothers–Jose and Maximo–quickly bought the mineral rights. Their fully operational mine, owned by Mine Factors Inc., brought windfalls for both themselves and the island, extracting nearly 30,000 tons in 1938 alone. 

In fact, solely because of the Jumawans’ endeavors, Siquijor became the largest source of low-grade manganese in the Philippines and one of the main suppliers of manganese in general during the American period, surpassed only by Busuanga. Larena, too, benefited massively. Technicians, skilled laborers, and American businessmen all converged at this bustling boomtown.

Fishing was also common, although Siquijodnons only practiced it for their subsistence and, unlike the other industries, did not grow it to commercial scales. Nor was much attention paid to it. Governor-General Leonard Wood saw that himself when he visited Lazi in 1922 and observed that fishermen were allowed to dynamite the water, recklessly depleting the local fish stocks. The townsfolk on shore had no fresh fish and were forced to subsist on imported canned salmon.