Earthworms do wonders for the soil, improving soil structure, nutrient content, infiltration and crop yields in trials with a traditional low input rotation. The catch is that most species have limited success in surviving outside their own environment. Scientists at the French Institute for Scientific Research for Cooperation in Development (ORSTOM) are trying to identify species that can be introduced to new environments. They have found three earthworms which show promise.
ORSTOM has completed the first of a two stage MacroFauna project. The project sought to 1) identify native and wandering species of endogeic earthworms which can survive in cultivated soils with low-input annual crops; 2) quantify earthworms' demographic parameters and their short-term effect on soil texture, soil organic matter and nutrient release; and 3) evaluate the effects of earthworms on plant production and soil fertility parameters.
Their research has identified earthworm species that can withstand disturbances and serve as soil enhancer.
Field experiments were conducted in small, 0.28 to 1.5 m2 experimental units, distributed within 400 to 600 m2 plots and divided into blocks. The units were put into three different residue management systems and randomly treated with, or without, worms. The systems were 1) no incorporation of crop residues; 2) incorporation of stubble mulch; and 3) stubble mulch + legume green manure.
Three types of earthworms were chosen for their tolerance of varying environmental conditions and the type of agricultural practices applied to the area:
- Native species from Lamto, Ivory Coast
with long gestation (15-42 months) and low fecundity (1.3-10.7 cocoons/adult/yr);
- Pontoscolex corethrurus and Polypheretima elongata with short gestation times (3-4 months) and high fecundity rates (35-100 cocoons/adult/yr); and
- Native species from India with intermediate demographic profiles.
Earthworms released significant amounts of mineral nitrogen and assimilable phosphorus in their casts, relative to the mineralization rates in the original soil and the earthworm species. The highest concentration of mineral-N (1095mg/kg) was found in casts of P. corethrurus in an andosol of Martinique. The lowest concentration (24mg/kg) was measured in casts of Millsonia anomala in a ferric soil in Ivory Coast.
In the sandy soil at Lamto, Ivory Coast, mineral-N levels dropped to control levels after eight days. In an ultisol at Yurimaguas, Peru, mineral-N concentrations were significantly higher in casts than in a control soil after 16 days of incubation. Based on these results, the annual production of mineral-N by P. corethrurus in a tropical pasture was estimated at 50-100kg/ha. Assimilable phosphorus was 50% greater in fresh casts of P. elongata fed with a vertisol than in the control (non-ingested soil). In an ultisol at Yurimaguas, fresh casts of P. corethrurus were 4 times higher than in the control.
To test their effects on soil fertility in the field, small amounts of selected earthworm species were introduced in low input cropping systems at Lamto, Ivory Coast, La Mancha, Mexico and Yurimaguas, Peru. After 5 cropping cycles at Lamto, total biomass of the native M. anomala was limited to a few grams fresh weight/m2 due to low soil nutritive quality. At Yurimaguas, earthworm biomass was sustained at a higher level (40g fresh weight/m2, with peak values of >80g/m2) at harvests two and three in treatments receiving green manure. At the fifth harvest, biomass was significantly lower in the "no organic input" treatment than in treatments with crop residues and green manure. Results were based on a critical earthworm biomass level, estimated at 30-40g fresh weight/m2 of soil.
Earthworms significantly improved soil structure. At Lamto and Yurimaguas, soil macroaggregates and bulk density increased, infiltration rates decreased slightly and N-depletion was delayed. At Lamto, more coarse organic matter was found in treatments with earthworms. Microbial activities were modified, while N-mineralization rates were generally higher during the early phases. At the fifth harvest, less nitrate was released in soil from "with earthworm" treatments but microbial biomass was 10-40% greater.
At Yurimaguas, grain production was significantly higher with average increases of 145% in a continuous maize crop fertilized after the 3rd harvest (-5% to +350% depending on the cropping cycle). In the traditional low input rotation the grain yield increased 36%. At Lamto, soil fertility could not sustain a sizeable earthworm population. Increases in grain production were relatively low (10-20%) and limited to the first harvest. Slight increases of plant production (up to 30%) were observed at La Mancha with P. corethrurus biomass at 21-27g in treatments with organic inputs and 10-12g in treatments without mulch.
Wandering species clearly are advantageous for agricultural systems. Their tolerance for a wide range of environmental conditions and fast population turnovers make them suitable for colonization. P. corethrurust is the most commonly found earthworm, but P. elongata is another widely distributed species with high potential for use.
It is too early to know how sustainable introduced earthworm systems are, but for farmers who abandon their cropland after two or three cropping cycles (e.g. at Yurimaguas), the introduction of earthworms that increase production at that stage may be a good technique. The key research issue is to identify practices that maintain earthworm biomass levels.
Based on preliminary results, research on earthworm production techniques needs to address four critical issues:
- Production of large quantities of earthworms for colonization;
- Spatial and temporal colonization patterns of earthworms in arable land;
- Specific responses of plant species to different earthworm species, grown in different soil types;
- Improving earthworm activities with low quality organic wastes not normally used in farming systems (sawdust, coir, coffee wastes, etc.)
This study demonstrates the potential for earthworm colonization on arable lands and the need for further studies.
Summary Report 1994: MacroFauna Project, Institute Fran¨ais de Recherche Scientifique pour le Dˇlevoppement en Coopˇration (ORSTOM).
Patrick Lavelle, ORSTOM
Centre de Bondy, 72
93143 Bondy Cedex, FRANCE
Tel: 33 1 48 02 55 01
Fax: 33 1 48 47 30 88