(Photograph = Baja California, Alan Harper)

Introduction

Cacti can grow in very poor soils. They can grow in soils that don’t hold water well, and they can grow in soils that don’t hold nutrients well. Cacti can even grow on essentially bare lava rocks.

The Details

The giant cardon cactus (Pachycereus pringlei) is a columnar cactus in Baja California that can grow on essentially bare lava rocks. In such a situation, the plants would have access to very limited quantities of nitrogen, phosphorous, and other minerals without help.

One study tested the hypothesis that plant-associated bacteria help weather the lava rocks and free up minerals for plant growth. In fact, they determined that some of the bacteria associated with the giant cardon cactus could help fix nitrogen, and free up phosphorous, potassium, manganese, magnesium, iron, copper, and zinc from the lava in greenhouse studies.

All inoculated plants, regardless of the bacterial species, grew much better and were  more able to extract  essential inorganic minerals from the pulverized rock in which they grew than plants that had  not  been  inoculated.

More studies will be needed to determine if the bacteria actually perform useful demineralization in the field. We now know they can free up the minerals, and we know they are associated with the cactus. However, do they do so in an ongoing, ecologically significant manner? Do they really help the cactus life cycle?

Additional Reading: Growth Promotion of Cactus Seedlings on Rocks