9.5 percent of Thailand's GDP. Loei province had an economic output of 54.769 billion baht (US$1.77 billion). This equates to a GPP per capita value of 100,796 baht (US$3,251), almost double than for Nong Bua Lamphu province, lowest in the ranking. Khon Kaen province had an economic output of 211.192 billion baht (US$6.8 billion). This equates to a GPP per capita value of 122,950 baht (US$3,966), nearly 66 percent more than for Maha Sarakham province, next in the rank-list and double then for Yasothon province, lowest in the ranking. Nakhon Ratchasima province had an economic output of 295.511 billion baht (US$9.5 billion). This amounts to a per capita GPP value of 115,517 baht (US$3,726), 56 percent more than for Maha Sarakham province, next in the rank-list. Agriculture Agriculture remains the largest sector of the economy, but its importance is decreasing. Its share of GRP fell from 45 percent in 1980 to 33.2 percent in 1990, and to 27.1 percent in 2013. During the 1990s, the agricultural sector grew at 3.8 percent per annum, compared to 9.7 percent for the service sector. Although 76 percent of the population engages in some form of agriculture, only 46 percent of the workforce is formally employed in the sector, reflecting the fact that for many it is a part-time occupation. Of the region's 155,400 km2, only 92,204 km2 is cultivated, and only 7,770 km2 of that is irrigated. There are a number of reasons for this. Saline contamination makes a third of the land unfit for cultivation. Although average rainfall is not much less than in other parts of the country, its unpredictability often causes droughts or flooding. The latter is worsened by the generally flat terrain of the plateau, while both are exacerbated by the sandy soil, which is poor at retaining water. Deforestation reduced forest cover from 25 percent in 1975 to 14 percent in 1995, again inhibiting water retention. Attempts to improve the agricultural sector through irrigation and diversification have caused unforeseen environmental damage. Increased irrigation has raised the water table in some areas, bringing salt to the surface and contaminating the soil. The same effect has also been observed following forest clearance and the planting of cereal crops, cassava, and sugar cane. Irrigation has also proved somewhat ineffective in increasing agricultural production. Workers commonly work in agriculture during the wet season and turn to non-farm activities during the dry season.