How is our soil doing for earthworms?

Earthworm survey of soil quality 30.3.19

In early February and March 2018 Dr Jackie Stroud, a Natural Environment Research Council Soil Security Fellow at Rothamsted Research, led and co-ordinated a project to study the worms in farm soils. A total of 126 farmers took part. Participating farmers volunteered to dig 10 pits, each 20 cm x 20 cm x 20 cm, in one field. They counted the number of adult worms in the sample. Adults are identified as those having a saddle on their bodies. The total number of worms were counted, then the juveniles returned to the soil. An identification guide allowed them to allocate any sightings to one of the three main types of earthworm. The adults were then split into small surface red ones (epigeic), small or medium pale worms which were grey, pink or a darker green (endogeic) or larger pencil sized worms which were heavily pigmented red or black (anecic).

Each of these worm groups has a different function. The epigeic surface worms breakdown surface litter and are a good source of food for native birds such as thrushes and blackbirds. The endogeic topsoil worms mix soil and mobilise nutrients for plant uptake and so support plant productivity. The anecic, deep burrowing large worms are the drainage worms which can form 2 metre vertical burrows which help with water infiltration and deep plant burrowing.

On Saturday 30 March 2019 we conducted the same experiment over our ten plots, with a few teams adding a second count. Here are all our results

Earthworm Sampling at HCF 30.3.19

Plot	Total worms	Epigeic worms	Endogeic worms	Anecic worms	Grown last year	Manure added in autumn?	Compost added in autumn	Roto- Last year	Roto - this year	Plastic over winter
2	14	0	3	0	Brassicas	No	No	No	No	No
4	15	5	3	1	Fennel	Yes	No	No	No	No
6	7	0	6	1	Parsnips	Yes	No	No	No	No
8	30	2	5	0	Broad beans	Yes	No	No	No	No
10	7	0	6	1	Potatoes	No	No	Yes	No	Yes
12	0	0	0	0	Onions	Yes	No	No	No	Yes
14	31	0	4	5	Potatoes	Yes	No	No	Yes	Yes
16	4	0	2	0	Onions	No	No	No	No	Yes
18	16	1	4	0	Potatoes	Yes	No	Yes	Yes	Yes
20	13	1	1	0	Carrots	Yes	No	Yes	No	Cardboard mulch
Total	141	9	37	9
No of plots with worm	9	4	9	5
Fruit	3	0	3	1	Rhubarb	Yes	No	No	No	No
1	15	0	4	0	Squash	No	No	No	No	No
11	2	0	0	0	Potatoes	Yes	No	No	No	Yes
3	13	1	8	0	Sweetcorn	Yes	No	No	No	Sweetcorn mulch

How do our results compare with Dr Stroud's research for 126 farms? Dr Stroud analysed the results she obtained from farmers on 5 counts:

(a) Total number of soil pits with ≥1 earthworm (juveniles or any adults below),

She found that the average field had 9 earthworms per spadeful,

We found an average of 14.1 worms per count

(b) Total number of soil pits with ≥1 adult epigeic surface earthworm.

She found that 21% had no sightings of these worms and 42% of fields had very few. These worms were significantly less likely to be found in fields that had been tilled (ploughed or dug). Low numbers in a field suggest a lack of surface litter which earthworms can pull into the soil.

We found that 6/10 of our plots had no epigeic worms and a further 3/10 had very few.

(c) Total number of soil pits with ≥1 adult endogeic topsoil earthworms.

She found that 67% of fields had good presence of these worms.

We found that 9/10 of our plots had >1 adult endogeic worm. Endogeic topsoil worms mix soil and mobilise nutrients for plant uptake and so help to raise crop productivity.

(d) Total number of soil pits with ≥1 adult anecic deep burrower worm.

She found 16% of these fields had none of these worms and a further 23% had 1 or 2 out of the 10 samples that were examined that had these deep burrowing worms.

We found that 6/10 plots had no anecic worms and another 3/10 plots had just one anecic worm.

The lack of anecic worms was of concern to Dr Srroud because they are 'drainage worms' with vertical burrows that aid water infiltration and help stop fields getting waterlogged. The deep-burrowing worms have slow reproduction rates so recovery in their populations could take a decade under changed management practices. Deep burrowers may not always be in the topsoil, so it is important to look for pencil sized vertical burrows at the bottom of the hole of a pile of straw or stones on the surface (a midden) which overlies a vertical burrow. If these are observed then we recorded that an anecic worm has been observed.

(e) Total number of soil pits with high numbers (≥ 16 earthworms per pit, ≥400 earthworms per m²) of earthworms (total number including all juveniles and adults). She found that one in 10 fields had more than 16 worms per spade. Top fields had around 27-30 worms per sample.

We had an average of 14.1 worms per pit which gives us an estimated 353 worms per square metre. Only two of our plots reached “top field” numbers of worms.

Dr Stroud concluded that 42% of fields had sub-optimal numbers of worms (defined as <10% presence for at least one ecological group) and may be "overworked" leading to absence or scarcity of surface dwelling and deep-burrowing worms. Only 15% of fields had a good presence of all three groups. Her results indicate that tillage is likely to reduce the numbers of surface and deep burrowers first. This may help explain the alarming decline of the song thrush which often feeds in fields on these worms. Topsoil worms are generally least affected by over-cultivation.

What do you think we should conclude about our results? Clearly they are just a snapshot, giving us a baseline for further monitoring. Our results may have been affected by dry weather the previous week, but we can expect the worm numbers in the top 20cms to fall further as the weather warms up. Should we do more repeats on the plots surveyed? Should we do studies on the other plots? Should we be thinking of changing our soil management practices in the light of these results?

I should be really grateful if you would write your comments, observations and thoughts below, so that we can have a dialogue about how we are caring for the soil at HCF.

Refs
J L Stroud Soil health pilot study in England.: Outcomes from an on-farm earthworm survey. 2019  https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0203909    Viewed online March 2019