ข้อ14-EARTH-SOIL TYPE

http://www.canolacouncil.org/crop-production/canola-grower%27s-manual-contents/chapter-4-effects-of-moisture/effects-of-moisture

http://www.dummies.com/home-garden/gardening/how-to-assess-soil-composition/

http://www.krusarawut.net/wp/?tag=%E0%B8%97%E0%B8%A3%E0%B8%B1%E0%B8%9E%E0%B8%A2%E0%B8%B2%E0%B8%81%E0%B8%A3%E0%B8%94%E0%B8%B4%E0%B8%99