|Abdallah Alaoui and Gudrun Schwilch
|Alaoui, A. and Schwilch, G. (2016) Soil quality and agricultural management practices inventory at case study sites. iSQAPER Report 28 pp
Earthworms provide a good indicator of the biological health and condition of the soil because their population density and species are affected by soil properties and management practices. Through their burrowing, feeding, digestion and casting, earthworms have a major effect on the chemical, physical and biological properties of the soil. They shred and decompose plant residues, converting them to organic matter, and so releasing mineral nutrients. Compared with uningested soil, earthworm casts can contain 5 times as much plant available N, 3–7 times as much P, 11 times as much K, and 3 times as much Mg. They can also contain more Ca and plant-available Mo, and have a higher pH, organic matter and water content. Moreover, earthworms act as biological aerators and physical conditioners of the soil, improving: soil porosity, aeration, soil structure and the stability of soil aggregates, water retention, water infiltration, and drainage.
Use one of the two following methods.
Method 1: counting by hand
Count the earthworms by hand, sorting through the soil sample used to assess soil structure and compare with the class limits in the photograph. Earthworms vary in size and number depending on the species and the season. Therefore, for year-to-year comparisons, earthworm counts must be made at the same time of year when soil moisture and temperature levels are good. Earthworm numbers are reported as the number per 200-mm cube of soil. The class limits for earthworm numbers given in the photograph are based on the probability that only two thirds of the worms that are present will be found during a 5 minute search.
Visual scoring of earthwork counts
Method 2: extraction using mustard water
In this method you pour a solution of mustard water on the soil allowing it to percolate down. The mustard solution irritates the skin of earthworms and they come to the surface to avoid it, where they can be collected, preserved and identified.
Preparation of the mustard solution: To make the solution, mix 2 litres of water with 20 grams ground yellow mustard seed in a container. This is the same powdered yellow mustard you will find in any grocery store. You should mix up the mustard solution quite a bit to avoid its solidification on the bottom. A 2L jug of mustard solution is enough to sample a 25cm x 25 cm sample plot (see for instance Valckx et al., 2011).
- Choose a representative plot to sample.
- Sample the vegetation and remove the leaf litter in your sample area.
- Place the frame (25cm x 25 cm) on the ground.
- Slowly pour half of the jug of mustard water into the sample area.
- Over a period of 5 minutes, gather any worms that come to the surface being careful to wait until they are completely out of the ground
- After 5 minutes, pour the remaining mustard water into the sample area and again wait 5 minutes gathering any other worms that come to the surface.
- Have a collection tray to put them in until you’re done, since they can come up in rapid succession.
This technique works well for all species of earthworms but only when the earthworms are active. If it has been very dry, very hot or very cold in the week(s) prior to sampling they may not respond as very well since they may be in aestivation (earthworm version of hibernation). In contrast, if air temperatures have been moderate and it has rained recently they are likely to be active and respond well to the liquid extraction. An exception – if the soil is very compacted and/or has a poor structure (heavy clay, fields, roads, etc.) the extractant doesn’t move well through the soil and the earthworms will not respond because the liquid doesn’t reach them.
(Earthworm counts per 20cm3 of soil)
Good condition: Score 2
Number > 8
Moderate condition: Score 1
4 – 8
Poor condition: Score 0
- Alaoui, A., Helbling, A. 2006. Evaluation of soil compaction using hydrodynamic water content variation: comparison between compacted and non-compacted soil. Geoderma 134, 97–108.
- Shepherd, T.G. 2000: Visual Soil Assessment. Volume 1. Field guide for cropping and pastoral grazing on flat to rolling country. horizons.mw & Landcare Research, Palmerston North. 84p
- Valckx, J., Govers, G., Hermy, M., Muys, B. Optimizing Earthworm Sampling in Ecosystems. A. Karaca (ed.), Biology of Earthworms, Soil Biology 24, DOI 10.1007/978-3-642-14636-7_2, Springer-Verlag Berlin Heidelberg