Question

why do you think desert soil supports the growth of cactus bushes and shrubs

Answer 1

I might suggest that your question is slightly backward. It is in fact the shrubs, herbs and cacti that are capable of enduring the harsh conditions in the soil, and the arid climate. The soils are just “there.”

It is a common saying; “nature abhors a vacuum.” Plants, including all the types you mentioned, evolved from other plants that grow under more benign conditions. But just as wildebeest herds will move into a new area to forage when conditions favor that, plants will colonize (and eventually evolve) to take advantage of the resources present within (relatively) nearby desert environments.

More directly addressing your question, the soils in desert environments are variable, but a common thread is that they are often fertile, but infertile. That seeming contradiction stems primarily from climatic inputs to the desert ecosystem. Soils may contain significant amounts of nutrients, but being so arid, those nutrients are not dissolved by water and made available to the roots of the plants. When rain does happen, it is frequently intense, but short-lived, resulting in much erosion and runoff, carrying a considerable amount of those nutrients with it, owing to the loose nature of desert soils—a considerable amount, but not all of those nutrients.

Many plants have adapted to those feast/famine conditions. They have to withstand the prolonged droughts and (sometimes) high temperatures, maybe for years, and yet be able to harvest water and nutrients from the soils when those nutrients and moisture are available. They have evolved to get by with less than plants that live in more hospitable environments.

Many cacti, for example, are capable of producing an entirely new network of roots when moisture conditions in the soil allow it, sometimes within a day. They fill up with water that can be drawn upon for photosynthesis under drier conditions. Other plants have evolved vascular systems that are “ironclad,” able to pull water from great depths without collapsing under the negative vacuum pressure. Others extend their roots widely, near the surface, and endure long periods of dormancy until the brief periods of clement weather occur, when they can stock up on water and nutrients for new growth, flowering and fruiting (some use both strategies). Annual plants choose to sit dormant as seeds, sometimes for many years, until a passing rainstorm or a wet winter brings enough moisture to enable germination, growth and flowering—perhaps in just a few weeks.

Further, desert soils are often either deficient in some nutrients, or overly abundant (e.g. alkalinity) in others. The plants that have evolved to tolerate those conditions have “learned” (evolutionarily) to turn those adversities into assets, or at least to tolerate the adverse conditions.

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