Earthworms (Lumbricus terrestris):

• live in the upper layers of soil and leaf litter
• feed on decaying organic matter, such as fallen leaves
• require moist soil conditions to facilitate gas exchange across their skin
• are highly sensitive to acidic soil conditions.

Give two abiotic factors and two biotic factors that could affect the size of an earthworm population.

Students investigated the population of earthworms in the soil of two different habitats:

• Site X: the floor of a mature deciduous woodland
• Site Y: an intensively tilled agricultural arable field

A mild, non-toxic mustard suspension, Solution M, can be poured onto the ground. This solution irritates the earthworms' skin, causing them to rise to the surface of the soil within a few minutes but does not harm them.

Plan an investigation using Solution M to compare the number of earthworms per $m^2$ in Site X and Site Y.