The influence of agricultural soil amendments on the diversity of soil micro-organisms and disease incidence
Soil amendments and agricultural practices that are utilised to enhance crop production, can impact, either directly or indirectly, on soil microorganisms which are responsible for a whole range of processes that maintain soil health. The main aim of the present investigation was to determine the effects of these amendments and cultural practices on microbial diversity, activity and community structure. The potential impacts of these practices on the pathogenicity of soilborne pathogens such as F. oxysporum, were also evaluated. Trials conducted in soil microcosms investigated the effects of 10 soil amendments and 2 cover crops on microbial organisms. Qualitative analysis of microbial diversity, using Biolog Eco™Plates and PCR-DGGE, showed distinct differences in treated soils, compared to control treatments. Fertilizer amendments, herbicides, fungicides, cover crops and lime were all found to have a noteworthy impact on soil microbial diversity. A major factor influencing microbial diversity was a cover crop, viz. Nemat (Eruca sativa). Quantitative analysis of microbial activity using fluoroscein diacetate analysis (FDA) also showed distinct differences in treated soils compared to the controls. All treatments, except Roundup and both cover crop treatments increased microbial activity in soil compared to the control treatments. The highest microbial activity was observed in soil treated with the herbicide 2,4-D, and the lowest in the negative control treatment and soil treated with Roundup. A field study investigated the effects of Roundup, crop rotation and soil compaction on soil microbial populations. Qualitative analysis of maize fields treated with Roundup showed distinct differences between soils treated at various periods prior to analysis, and those not treated at any time. Qualitative analysis depicting microbial activity and biomass also showed distinct differences between Roundup soil and no-Roundup soil. Microbial activity seems to be immediately inhibited by the application of Roundup while microbial biomass was found to be negatively affected in the long-term. Qualitative analysis of soils subjected to rotation and compaction, also showed noteworthy differences in the microbial diversity of treated compared to non-treated soils. Both compaction and rotation was found to be a major factor influencing microbial biomass. Quantitative analysis of soils subjected to rotation and compaction, also showed distinct differences in microbial activity and biomass of the treated soils compared to the controls. Compacted soil had a lower microbial biomass and activity than non-compacted soil. Significant differences in sorghum seed germination and vegetative growth was found between microcosm soils compared to a vermiculite control treatment. Microcosm soil treated with rye grass, LAN, Roundup and mustard, significantly increased sorghum seedling germination and reduced the pathogenicity of F. oxysporum. Limed microcosm soil significantly reduced seedling germination while super phosphate (SP), benomyl, lime and mustard microcosm soil significantly improved the vegetative growth of sorghum seedlings by reducing the pathogenic effect of F. oxysporum. Results from the present study confirm the potential impact of soil amendments and cultural practices on soil microorganisms and the effects they may have towards soilborne diseases. This information may provide insights on improving soil health while helping to increase crop production by means of alternative cultural methods to control soilborne pathogens.