Lumbanraja, Jamalam and Evangelou, V.P. (1990) Binary and Ternary Exchange Behavior of Potassium and Ammonium on Kentucky Subsoils. Soil Science Society American Journal, 54. pp. 698-705. ISSN 0361-5995

Nugroho, Sutopo Ghani and Kuwatsuka, Shozo (1990) -Concurrent observation of several processes of nitrogen metabolism in soil amended with organic materials. I. Effect of different organic materials on ammonification, nitrification, denitrification, and N2fixation under aerobic and anaerobic conditions. Soil Science and Plant Nutrition, 36 (2). pp. 215-224. ISSN 0038-0768

Utomo, Muhajir and Frye, W W and Blevins, R L (1990) Sustaining Soil Nitrogen for Corn Using Hairy Vetch Cover Crop. Agronomy Journal, 82 (5). pp. 979-983. ISSN 1435-0645