Amnat Suwanarit, Piya Chalermglin, and Jarong Rungchaung. “Equations for Calculation N-Fertilizer Rates for maize from Soil Analysis”. Agriculture and Natural Resources, vol. 29, no. 3, Sept. 1995, pp. 345-57, https://li01.tci-thaijo.org/index.php/anres/article/view/241336.