In our last entry, we introduced you to earthworms and how they work and benefit the soil. In a healthy, no-till soil, approximately 1.2 million earthworms can be found per hectare. The survival of this mega soil workforce largely depends on how well we steward our agricultural lands. Before we get into discussing the silent killers of this mega workforce, here are some fun facts about these wriggly wonders:
- There are about 7000 different species of earthworms.
- The mass of all earthworms living on our planet is greater than the mass of any other terrestrial animal species.
- They are a very important form of prey for many farmland birds and predators.
- The giant African earthworm (Microchaetus rappi) can reach lengths of 1.4 m.
- Earthworms do not have eyes, but they can sense light at their front end.
- Prolonged exposure to light causes paralyses.
- Earthworms breathe through their skin.
- Earthworms can secrete calcium carbonates (lime). Calciferous glands in their gut create a buffer against acidic food and can absorb CO2 under unfavourable conditions (James, 1935).
- Earthworms eat as much as one third of their own bodyweight in one day.
Some earthworm math:
An average sized (A. caliginosa) earthworm (50 mm long, 2.5 mm diameter) weighs approximately 0.5 g. If an earthworm of this size eats one third of its bodyweight per day (0.17 g) and there are 1.2 million earthworms per hectare on average.
.........CALCULATING.........
This means that a soil workforce of 1.2 million earthworms can process 200 kg castings per hectare per day! That's 73 tons of nutrient rich excretions per hectare per year.
Silent threats:
1) Agricultural Chemicals
Although earthworms live mostly below the soil surface, the excessive use of some pesticides and fungicides on fields and orchards has detrimental effects on earthworm populations. Due to rainfall and runoff, pesticides leach deep into the soil and reach non-target areas, where the harmful effects can persist for several months. Reinecke (2005) conducted a study on the impact of organophosphate pesticides in orchards on earthworm populations in the Western Cape (South Africa) and found that worm densities were very low in orchards (22 per m2) compared to uncultivated fields at a distance from the orchards (152 per m2). Pesticides such as chlorpyrifos and azinphos methyl were found to be detrimental to earthworm populations. Metals in organic waste are not considered toxic to earthworms; however, earthworms are sensitive to copper toxicity induced by prolonged use of copper fungicides for disease control.
Herbicides do not appear to directly affect earthworms by their toxicity but can have indirect effects by altering plant cover and food supply, which in turn affects the soil microclimate at the surface.
2) Tillage
A study conducted by Malet et al. (1987) at Cedara (KwaZulu-Natal, South Africa) on different tillage practices, found that earthworm numbers per square meter declined from 90 under no-tillage to as low as 17 under conventional tillage. This reduction in earthworm numbers was believed to be because earthworm channels are blocked or destroyed by tillage actions. Malet et al. pointed out that the presence of earthworms contributes to alleviating compaction and water saturation through increased soil aeration.
3) Compaction
Compaction caused by wheel or hoof traffic can cause crushing (direct effect) and soil compaction (indirect effects such as porosity decrease, waterlogging). These are common explanations for reduction in earthworm populations. In compacted soils, earthworms need to spend more time and energy burrowing, while not making much headway in terms of distance. Soil compaction may reduce the time available to earthworms for feeding and mating and leads to lower populations as a long-term consequence.
4) Ammoniated fertilizer
Generally, moderate levels of fertilizer application tend to have positive effects on earthworm populations, as fertilizers may lead to increased plant residue quantity and quality. However, the excessive use of nitrogen-based fertilizers, especially ammoniated fertilizers, has been reported to lead to severe reductions in earthworm populations due to soil acidification caused by the conversion of ammonium to nitrate.
5) Soil moisture
Soil temperature extremes and the loss of soil moisture lead to reduced earthworm numbers. Although hot and dry climatic conditions are unavoidable, their consequences are. Soils can be protected from the elements by ensuring sufficient soil coverage with plant residue. Plant residue, cover crops, or other forms of mulch, reduce soil temperatures and keep temperatures more consistent. Mulch layers not only reduce runoff and improve infiltration, but evaporation is also reduced significantly.
Too wet conditions that lead to waterlogging are, however, also not favourable for earthworm survival. Heavy clay soils are particularly prone to anaerobic conditions, especially under high rainfall.
6) Lack of food
Residue removal through burning, baling, or intensive tillage actions, as well as fallow cropping periods, decrease the availability of food sources for earthworms and thereby reduces their numbers. Monocropping also limits earthworm abundance and species richness as the food is of a lower quality.
Practical tips for protecting and sustaining earthworms:
- Reduce tillage!
- Grow cover crops during the off-season to reduce the fallow period.
- Use hay, wood chips, or cover crops as mulch (orchards and vineyards).
- Reduce nitrogen fertilizers over time.
- Increase compost or manure applications.
- Reduce the traffic of heavy machinery and livestock during unfavourable (wet) conditions.
- Increase crop diversity through rotations with legumes.
- Keep soil pH in check with targeted lime applications.
- Do not burn crop residue.
- Apply mixed-animal crop systems. Long-term pastures are beneficial if they are practical.
As South Africans we all appreciate being hosted for a braai. The difference between a good and a great braai largely depends on how we are being treated by the host. Having enough coals, shade from the scorching sun, drinks, meat and side dishes all determine whether we want to stay or go home hungry. Just as we appreciate the efforts of the host, so we should put in all the effort to be the best hosts possible for the earthworms in our fields. It is up to us! Let's make an effort to be great hosts in the field and at the weekend braai!
