Scientists at the Station d'Ecologie Lamto in the Ivory Coast are trying to assess the ability of the tropical earthworm to stimulate plant growth. They have found increases in the growth of maize and guinea grass (Panicum maximum ) upon the introduction of select earthworm species. The results of their research demonstrate the positive effects earthworms can have on the soil and plants, and highlight the importance in selecting species suited to given environmental conditions.
Studies were conducted at the Station d'Ecologie Lamto in the Ivory Coast. The average annual rainfall is 1228mm, and average daily temperature ranges from 25.7oC in August to 28.8oC in February. The soil is classified as a ferralsol (7.5% clay, 14% silt, 29.4% fine sand, 46% coarse sand).
Experiments were conducted to asses the influence of four earthworm species on maize growth, the response P. maximum to eudrilid earthworms and the effects of the earthworm, Millsonia anomala on P. maximum in the transfer of nitrogen and microbial biomass.
Within three days of planting maize, the biomass of the five earthworm species were added to 10-liter plastic containers containing 2mm of sieved soil from the top 10cm of the profile. The containers were furnished with holes covered with a fine mesh to allow free drainage and prevent the entry or exit of earthworms and plant roots. The evaluation, 12 weeks in duration, was randomly conducted with four replications.
Maize plants survived poorly in the nitrogen- and phosphorus-deficient soil. Neither P. corethrurus nor H. africanus survived, and the biomass of C. zielae and S. porifera either remained constant or declined throughout the experiment. The biomass of M. anomala declined similarly, although where both plants and earthworms survived to maturity, M. anomala biomass increased slightly.
The presence of earthworms led to a significant increase in the final biomass of P. maximum compared with the control. The mean above-ground biomass of P. maximum at harvest was 3.75g for the control, compared to 6.98g with the 0.5g earthworms and 11.77g with the 1.0g earthworms. Between the two treatments with 0.5 and 1.0g earthworms, there was no significant difference in the final biomass of P. maximum. But, the earthworms' biomass increased substantially; to an average of 1.45g in the 0.5g treatment and 2.03g in the 1.0g treatment.
Total biomass with 15N-labeled M. anomala was significantly higher than the corresponding treatment in the previous experiments with M. anomala . The reason is unclear. Higher P. maximum biomass in the two remaining treatments may have been due to the N added in labeling the microbial biomass. In the two treatments with earthworms, the worms grew actively throughout the experiment. In the treatment labeled earthworms and unlabelled microbial biomass, concentrations of 15N in the foliage and roots were only marginally higher than in the unlabelled soil. Where the microbial biomass alone was labeled, both the earthworms and plant parts had significant amounts of 15N.
Earthworms facilitate the transfer of N and P from soil microbial biomass to the plant and, in the case of M. anomala, elevate microbial biomass levels in the roots. The casts of anecic earthworms and M. anomala appear to increase phosphate availability.
Poor maize growth in the infertile Lamto soil suggests that the stimulatory effects of earthworms did not meet the nutrient requirements of the high-performing variety. The reasons for the failure of P. corethrurus and H. africanus to survive in the maize growth experiments is unknown. P. corethrurus does not occur in the Lamto area and H. africanus is found locally only in coconut plantations where higher organic matter levels are found. Even M. anomala, which is adapted to these soils, failed to flourish in the absence of productive plants. M. anomala coped better with the infertile soils in the presence of P. maximum. This emphasizes the need to carefully select earthworm species based on their tolerance of soil and environmental conditions if they are to increase soil productivity.
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