The effects of some grazed tropical grass-legume mixtures and nitrogen fertilized grass on total soil nitrogen, organic carbon, and subsequent yields of Sorghum vulgaris

Abstract

Mixtures of some tropical legumes and Paspalum plicatulum (Michx) cv. Hartley, and stands of P. plicatulum were fertilized with urea at 100 and 200 lb nitrogen an acre a year, and were intermittently grazed by cattle over a period of four years. Soil analyses for organic carbon and for total soil nitrogen in the fourth year of the pasture phase revealed large treatment effects in three of the five replicates. These three replicates which were on a podsolic soil were lower in fertility than the remaining two on a latosolic soil. Soil nitrogen at the 0-3 inch depth in the high nitrogen treatment, and in two Phaseolus atropurpureus D.C. treatments was significantly higher than the control (P<0.05). Organic carbon at the 0-3 inch depth was significantly higher than the control (P<0.05) in the nitrogen treatments and in one of the P. atropurpureus treatments. For both soil nitrogen and organic carbon the Lotononis bainesii Bak. treatment did not differ from the control. There was no significant difference between treatments for soil nitrogen or organic carbon at the 3-6 inch depth though trends were similar to those at 0-3 inches. Organic carbon and nitrogen were closely correlated for all treatments at both depths, and there were no significant differences in the C : N ratio in any treatment. Yields of sorghum grown as a test crop after the pastures were significantly correlated with soil nitrogen values in the three low fertility replicates. A high correlation (r = +0.976) also existed between yields of nitrogen obtained in the pasture phase and test crop yields of nitrogen for all treatments except L. bainesii. Reasons for the apparent lack of improvement in soil nitrogen and carbon on the higher fertility replicates and for the poor test crop yields following L. bainesii are discussed.