EFFECTS OF NEEM FERTILIZER AND SELECTED HERBICIDES ON WEED GROWTH, SOIL ENZYME ACTIVITIES AND BIOCHEMICAL PROPERTIES IN MAIZE (ZEA MAYS L.)

Abstract

One field and one laboratory experiments were conducted over four years in a study to assess the effects of neem seed-based fertilizer and selected herbicides on weed control efficacy and soil chemical, biological and biochemical properties in maize. The field experiment involving four trials was carried out at the Experimental Station of the Department of Crop, Soil and Pest Management, Federal University of Technology, Akure (70 16’N, 150 12’E) between 2011 and 2013. Each trial involving application of Neem seed-based fertilizer (NSBF) in maize at 0, 100, 200, and 300 kg/ha and NPK at 350 kg/ha in factorial combination with 2,4-D, atrazine and glyphosate at their respective field application rates of 1.5, 3.0 and 2.0 kg a.i. /ha including a weed-free control, where weeding occurred at two weeks interval till harvest at 12 weeks after planting (WAP) and a weedy control, where no weeding occurred during the experimental period, was laid out in randomized complete block design (RCBD). The laboratory experiment, carried out in the Microbiological Laboratory of the Soil Science Department of the University of Ghana, Legon in 2014 involved the application of two rates (200 and 300 Kg/ha) of NSBF and NPK at 350 kg/ha to soil samples contaminated with 2,4-D, atrazine and glyphosate applied at the foregoing field rates in sealed plastic jars. The treatments also included a control soil sample, which was neither contaminated nor treated with fertilizer, to form a 4 x 4 factorial experiment laid out in completely randomized design (CRD), and the treated soil samples were incubated for 12 weeks. Soil sampling for data collection was done at 1, 4, 8 and 12 weeks after treatment application (WAT) to monitor in dynamics the effects of the foregoing treatment combinations on soil microbial biomass and activity of certain soil enzymes (dehydrogenase, acid phosphatase, β-glucosidase and Amidase). Soil respiration was also monitored on weekly basis from the start to the termination of the experiment. Results from the field experiment indicate that the contribution of weeding to the interaction between the two factors was higher than that from the fertilizers with regards to influencing weed suppression and maize performance at the early growth stages (4 and 6 WAP). Thereafter, the NSBF influenced maize growth and grain yield, and this was dose dependent. All the rates of the fertilizer released nutrients to the soil to influence maize performances, but only application of the neem fertilizer at 300 kg/ha caused consistent significant effects to compare favourably with NPK in promoting maize growth and yield parameters. Regardless of fertilizer application, glyphosate and atrazine treatments caused early weed suppression, which culminated in growth and yield parameters in maize that were similar to those obtained from hand-weeded plots. 2,4-D on the other hand performed very poorly with records of weed suppression and maize performance that were not significantly better than the weedy control. The interaction of the two factors to affect weed suppression and maize performance enhancement was synergistic. The NSBF at the higher rate (300 kg/ha) also reduced soil acidity and improved residual soil nutrient status compared to the control and NPK treatments. The neem fertilizer also stimulated and depressed bacterial and fungal growth respectively. Results obtained from the laboratory experiment indicate that treatments involving NSBF significantly raised microbial biomass N and C while NPK lowered these parameters at the end of incubation. MBN and MBC attained peak levels at 4WAT in the glyphosate and 2,4-D treated soils while Atrazine produced highest microbial biomass at 8WAT. All fertilizer treatments significantly increased soil respiration at 1WAT while none of the fertilizers significantly influenced CO2-C production at the end of incubation. No herbicide treatment also caused significant change in CO2-C production at 12WAT. Only NPK significantly increased dehydrogenase production among the fertilizer treatments at 12WAT, and none of the herbicides significantly altered this enzyme throughout the sampling periods. All fertilizer types stimulated acid phosphatase production at the beginning of incubation, but N300 and NPK caused significant reduction of phosphatase at the 12th week of incubation, and acid phosphatase production was also significantly influenced by only atrazine among the herbicide treatments at this time of sampling. The NSBF significantly increased β-glucosidase activity and all fertilizer treatments significantly reduced amidase activity at the end of incubation. All herbicide treatments reduced β-glucosidase activity at the start and end of incubation but production of this enzyme increased at 4 and 8 WAT. Only atrazine and glyphosate significantly influenced amidase production. It was therefore concluded that the neem seed-based fertilizer at 300 kg ha-1 can replace the conventional NPK for nutrient supply to achieve optimum growth and yield in maize. The neem seed-based fertilizer can also be recommended as bioremediation option in herbicide-contaminated soils because of the mineralized fraction, which can easily make nutrients available to biostimulate soil microbial activities, as well as the huge carbon base to provide sustainable support to the soil microbial community