Very interesting question Fournier .  Let me respond  to your question . Most of the studies on earthworm proliferation revealed better mass development in loamy soils compared to either sandy or clay soils.  According to one link , I could locate ( https://books.googel.co.in/books?isbn=0080869696) ,  earthworm weight of 1980 lb per acre in sandy soils , 250,000 lb per acre in loam soils and 100,000 lb per acre in clay soils have been reported  , clearing indicating the preference of medium textured soils compared to either coarse or heavy textured soils ( Enclosed PDF  for further reading ) . However , in sometimes , forest litter composition under natural ecosystem becomes more important  with respect to offering a preferential substrate to soil inhabiting earthworms ( Enclosed PDF for further reading ) , in addition to soil pH and organic carbon content of the soil. 

earthworm weight of 1980 lb per acre in sandy soils ,

250,000 lb per acre in loam soils and

100,000 lb per acre in clay soils

Dear Fournier

Earthworms are challenged when there is insufficient oxygen, organic matter, Calcium and moisture.  Moisture pH or Calcium might be your limitation.

In sandy soils low in organic matter there are difficult conditions for their sustenance.

When legumes are included in the rotation the earth worm populations are much more favored than in grass alone rotation.

Forage complete coverage with manure inclusion is the best case scenario for worms.

Check out your pH and Calcium levels as these can be used to stimulate the numbers and activities of earthworms.

Otherwise the use of manuring, legumes and plant coverage can be very helpful. I do not if my link is good but googling Hepperly Acres Earthworms should get some information on benefits and advantages of these workers.

Compost can also be used to help your organic matter especially when it stabilized and it is good for earthworms in their diets.

I do not know earth worm population in natural conditions in different soils but in cultivated soils where we provide sufficient growth conditions  like aeration,moisture ,organic matter and normal pH of soil,the soil texture may not show very dramatic differences.In clayey Vertisols  /  Vetic inceptisols  having 50-55%clay under good management  I saw normal growth and good number  of earthworms.Of course our soils have lime concretions which may  help in good drainage /water conducting properties in soil after rain. Application of compost/manure may help in providing proper aeration in Vertisoils/clayey soils.I support the views of Dr.Hepperly that growth conditions are more important than texture alone in deciding the earthworm number/ population .

The weight of  composting earthworms  is largely dependent upon the feeding substrate . Very interesting work carried out by  Chakrabarty et al ( 2008 , 15: 513-516, Aquaculture Nutrition ) , I am referring the abstract ( DOI: 10.1111/j.1305-2095.2008.00617.x) .  Three hundred adult healthy earthworms (Eudrillus euginae) ranging length and weight from 14–29 cm (mean 21.5 ± 6.8 cm) to 1.6–3.0 g (mean 2.55 ± 1.1 g), respectively, were cultured in three different substrate for 70 days. The substrates were soil alone, (control T-1), cow (Bos taurus) dung and water hyacinth (Eichhornia crassipes) (1 : 1 T-2), and partially dried neem (Azadirachta indica) leaves with kitchen waste (1 : 1 T-3). The earthworms were grown in these substrates in a wooden chamber (0.8 m × 0.5 m × 0.2 m) and these were subjected to monitoring for various growth patterns (length, biomass, number) at every fortnight (five observations) interval. The observations showed that T-2 was the best among the three treatments used. There was significant difference (P  0.05) between the mean values of survival rate of earthworm among different treatments. Hope , you find the full article very interesting .

Another PDF enclosed for further reading about how earthworms for composting multiplies on variety of substrates .

In relation to difficulties with sandy soil and earth worms. Besides making sure pH is appropriate the use of fine textured materials such as clay and silt will favor your ability to accrue soil organic matter and the activity of earth worms. Areas of encachment often accumulate fine materials in the low lying areas which can be recovered and used in composts or as soil amendments. These fine materials help the aggregation work better especially when Calcium is sufficient. Earthworms can flourish with the right conditions keep experimenting on which of these approaches might work and try them out on your own farm.

Dear Anoop and Kundan, Thanks for your answers.

The weight of worms you give for different soils seem very high to me. Assuming 2 grams/worm (which might already be high), your figures give 110 worms/m2 in sandy soils, 14000 worms/m2 in loam soils and 5600 worms in clay soils.

The recent article from Blakemore (Article Critical Decline of Earthworms from Organic Origins under In...

Article Earthworm assemblages as affected by field margin strips and...

This is also in line with the study of Field et al, you shared: they found up to 200 worms/m2.

Do you have any idea what explains this large difference between these studies and the number you presented for loam and clay soils?

Thanks!

My own experience is that transition to organic depends on the ability to increase soil organic matter. Some sandy soils have more limitation to increasing soil organic matter. Sandy soils have issues with nutrient retention and water ability. Clay soils have issues with aeration and water percolation. The lead soil having a diversity of clay silt and sand can give an optimized structure which also has great ability to increase soil organic matter. Earthworms will be greater in soils with greater soil organic matter. The retention of soil organic matter can be increased by adding materials which are high in silt and clay to give a loamier aspect. The present of earthworms is often limited by insufficient calcium the liming of soils to near neutral would be a practice which can aid earth worm activity.