Stimulating Soils: An Earthworm's Approach

The infertile, granite derived ferrasols of the Ivory Coast are far from hospitable to plant growth. However, researchers there have identified native and foreign earthworm species that can survive in and improve these soils.

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.

Effects on Maize

Experiments to test the influence of four earthworm species on maize growth were conducted. The four earthworms used were:

a mixture of Chuniodrilus zielae and Stuhlmannia porifera;
Hyperiodrilus africanus (West African);
Ponotscoex corethrurus (common through-out the humid tropical zone but does not occur at Lamto); and
M. anomala (from the Lamto savannas).

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.

Effects on Guinea Grass

The second experiment sought to test the influence of C. zielae and S. porifera on P. maximum. A similar procedure was used as in the first trial, however, there were five replications. Live biomass of C. zielae and S. porifera of 0.5 and 1.0g were added to the five replication, and three seedlings were grown in each bucket.

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.

N and P Transfer

To examine how the presence of earthworms influences nitrogen transfer to plants, the microbial biomass of the soil and the earthworms were labeled with 15N, a traceable element. The microbial biomass of the soil was marked by incubating the soil for a month with 15N-labeled ammonium sulfate and glucose mixed to attain a C:N ratio of approximately 15. After one month, juvenile specimens of M. anomala were introduced into the marked soil and kept there for a month to allow adequate time for 15N to become present in the plant and earthworm biomass. P. maximum plants were grown as in the previous experiment. Three combinations were used: 1) labelled soil with unlabelled worms; 2) labelled soil without worms; 3) unlabelled soil with labelled worms.

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.

In Sum

Earthworms increased plant production in test plants in containers. This growth stimulation occurred under normal field earthworm biomass levels. C. zielae , S. porifera and M. anomala proved effective in growth stimulation. It is possible that the decline of growth stimulation with the addition of approximately 4 g of live earthworms per container is due to the formation of a poorly-permeable structure from excessive amounts of castings. Earthworms apparently do better at lower biomass levels. A clear maximum value exists between earthworm biomass and the ratio of above-ground plant biomass to root biomass. This suggests that, in the presence of earthworms, above-ground production increases proportionally more than root mass.

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.

Spain A., Lavelle P. and Mariotti A. 1992, Stimulation of plant growth by tropical earthworms. Soil Biology and Biochemistry Vol. 24, pp. 1629-1633.

Contact:

A.V. Spain
Davies Laboratory CSIRO
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