Impact of Cover Crops and Organic Amendments On Soil Microbial Activity (SMA) and Tomato Yield Under Organic Production Systems

Soil fertility under organic production systems is largely managed through use of cover crops and organic amendments, mainly compost. Apart from adding nutrients, one of the key processes facilitated by cover crops and organic amendments is stimulation of SMA. Soil microorganisms are directly implicated in the breakdown of complex nutrient molecules/forms which can then be readily taken up by plants. Therefore, SMA parameters (biomass, respiration, metabolic quotient etc.) have the potential for use as diagnostic indicators of soil quality. This study seeks to better understand how cover crops and compost application affect SMA under organic tomato production system.  Cover crop treatments were cereal rye (Secale cereale L.) alone and a combination of rye and hairy vetch (Vicia villosa Roth.).  Each cover crop treatment was used either alone or in combination with compost in a split-plot arrangement. Cereal rye was drilled at the rate of 78 kg·ha^-1 in rye only treatment and 34 kg·ha^-1 in rye-vetch treatment. Hairy vetch was seeded at the rate of 25 kg·ha^-1. Compost was applied at the rate of 20 t·ha^-1. In 2007, highest soil microbial biomass (SMB, 231.4 µg-C/g soil) was recorded for rye-compost treatment. This can be explained by the dense root system and total biomass production of cereal rye. There was a positive relationship between root biomass and SMB. Addition of compost significantly increased SMB for rye treatment however; such an increase was not found for rye-vetch treatment. Soil respiration (µg CO₂ evolved/g of soil) was also highest for the rye-compost treatment. Rye-vetch treatment, without compost, had the lowest soil respiration value (140.3 µg CO₂/g of soil). Soil metabolic quotients for treatments ranged from 0.74 to 1.09 but were not statistically significant. Similar trends were observed in 2008. The effect of compost on marketable yield of tomato was significant in both years. Rye-vetch-compost treatment produced highest marketable yields (5.2 kg fruit/plant) in 2008. There wasn’t a marked difference in yield between cover crop treatments (rye alone and rye-vetch) indicating that both approaches can be adopted; however, the decision will largely depend on prevailing seed prices. Clearly, there is a positive relationship between compost application, SMB and marketable yields. Enhancement of soil quality through incorporation of cover crops and compost is a viable option for organic systems as it not only enhances soil organic matter but simultaneously increase SMA. Such soils would remain healthy and contribute towards sustainable future yields.