[dice]

Up here in the maritime Pacific Northwest, we have a lot of winter
rain. Nutrients that are mobile in the soil, like nitrogen, potassium,
and, to a lesser extent, calcium (probably others that I have not
researched, too), can leach down below the root zone.

But we also have verticillium in places, and compostable materials
like manures, leaf molds, grass clippings, wood chips, shredded
arborist debris, and so on are often best allowed to leach nutrients
into the soil over the winter to feed competing organisms. Depends
on whether your topsoil is sitting on sandy soils (river valleys, fast
nutrient leaching) or clay subsoil (higher elevations, where nutrients
leach through the subsoil more slowly and the topsoil layer is thinner)
which way is best for a particular garden or field.

In places with tropical rainfall (hard rains of whatever
duration that drop a lot of precipitation in a short time),
leaching of nutrients below the typical root zone of crops
is perhaps even more of a problem than it is up here,
and maybe that can happen with any kind of subsoil
under the topsoil. In the Amazon and other tropical
rainforests, when they clear cut land for timber, the soil
can become a "wet desert", where the nutrients in
the top layer of soil are leached out very quickly.
Without prompt planting of new trees, such land can take
a century to recover on its own from the logging operation,
because annual rainfall washes away nutrients faster
than casual accumulations of organic matter from weeds
and undergrowth can restore it.

Hence the tarp, perhaps.