EFFECTS OF GLYPHOSATE AND NEEM SEED CAKE ADDITION ON MICROBIAL RESPIRATION AND UREASE ACTIVITY IN AN ULTISOL

Abstract

Application of organic amendments to soils could enhance microbial activity and reduce the inhibitory effect of herbicide. A laboratory incubation study was carried out to determine the effects of herbicide, (Force-Up with active ingredient of 480g l^-1 Glyphosate- Isopropylamine salt) and neem seed cake (NSC) on soil microbial respiration and urease activity. Surface soil sample (0-15 cm) of Iwo Soil Series (Ultisol) was amended with neem seed cake (NSC) at the rates of 0, 2.5 and 5.0 t ha^-1, respectively, and glyphosate herbicide was applied at the rates of 0 and 2.16 mg glyphosate/kg soil. There were six treatments replicated three times and arranged in a completely randomized design (CRD). The soil microbial respiration, assay of urease and pH were determined weekly starting from the 2^nd week of incubation for a period of six weeeks. The results showed that the application of NSC solely or in combination with glyphosate significantly (p<0.05) increased soil microbial respiration, with 5 t ha^-1 leading to at least, 19 and 83% increases over the treatments with 2.5 and 0 t ha^-1, respectively. While application of herbicide is mostly inhibitory, the NSC at the rates of 2.5 and 5 t ha^-1 without herbicide was better at enhancing soil urease activity. Further, NSC application improved soil pH, with higher performance at the earlier weeks of the incubation study. It was concluded that NSC application can reduce the inhibitory effect of herbicide on soil microbial activity, with the application rate of 5.0 t ha^-1 best at enhancing soil microbial respiration and urease activity.