This group of soils is similar to that of soil group 17 in characteristics and physiographic position. However, soil reaction of group 18 is increased by increasing in depth. pH value of the top soil is 5.0 - 6.0 , and that of sub-soil is 6.0 - 7.5. Soils in this group are Chon Buri (Cb), Chai Ya (Cya), Kok Samrong (Ksr) and Khao Yoi (Kyo) series.

Problems: Massive structure, low fertility, strong acid in some places, lack of water in dry season and water logging in rainy season.

Management for rice: Prepare and mix crop residue into moist soils and leave for 3-4 weeks. If managing by green manure, it should be the mixture of 4-6 Kg/rai of 50-70 days Sesbinia rostrata and 16-16-8 chemical fertilizer or LDD liquid organic fertilizer in the soils and leave for 1-2 weeks.. Top dressing again with 16-20-0 chemical fertilizer, 35-45 days after transplanting. Farm ponds are recommended to preserve water to be used in dry season or second rice crop or short season upland crops after first rice. If vegetables or legume will be planted, furrows would be constructed with application of 2-3 ton/rai compose fertilizer or animal manure with chemical or liquid LDD fertilizer.

Management for vegetable or orchard: Raised bed to 6-8 meters wide with 1.0-1.5 meters drainage canal. Apply 2-3 ton/rai compose or animal manure with chemical fertilizer or LDD liquid organic fertilizer. Apply 20-35 kg of compose or animal manure into 50*50*50 meters pit to grow orchard. Apply compose fertilizer or animal manure with chemical fertilizer or LDD liquid organic fertilizer at vegetative growth, before and after harvesting. Farm ponds and water management are recommended to preserve water to be used in dry season.