And so to questions of soil fertility…
My training as a horticulturist led me to believe that my capability as a gardener would be measured by the extent I was aware of, and could provide, the optimum conditions for all plants I grew. Optimum performance was the goal.
All this is changing.
There’s a growing school of thought amongst cutting edge planting designers that stress can be worked to the designer’s advantage, and even that it can be actually beneficial to build controlled stress-factors into a planting system.
It has been clearly demonstrated, for instance, when working with perennials in a naturalistic perennial planting, that mild and/or controlled stress factors can stabilise a planting by curtailing the dominance of individually competitive species. This dominance is an almost inevitable outcome for long-lived perennial plantings, but planting designers have found that minimising either water or nutrient levels can create what amounts to a more level playing field of conditions, thus maximising the amount of time between corrections needed when one or two species start to dominate a planting.
The consequence is a kind of calculated ‘underperformance’ of plants in a scheme, in which the goal is acceptable, rather than optimal, performance, in the interest of planting longevity.
While this ‘stabilisation’ effect has only been demonstrated (so far, to my knowledge) in perennial plantings, woody plants from drier climates, and particularly Mediterranean climates, often grow exceptionally fast (and often very sparse and leggy) in well-fed conditions, but then either fall to fungal diseases prematurely, or simply reach the term of their natural lives much sooner than plants grown ‘hard’, with minimal soil nutrition.
There’s whole books (or Ph.D theses) in this thinking, and it’s not my intention to do anything here but to introduce an idea that will either undermine a lot of your assumptions, or confirm your suspicions, or possibly make your brain hurt.
To summarise, when gardening for dry conditions, it may well be beneficial to minimise soil nutrition. In some locations this is being achieved by the removal of top soil, and stripping back to subsoil. In some other designs, the whole planted area is being spread with at least 200mm of sand or gravel, and plants planted straight into this skeletal substrate. Both lead to sustainability questions for me. I’m yet to form any opinion in opposition, and I’m totally intrigued by both of these approaches, but there’s questions to be answered. At bare minimum, if you’re planning a dry garden, or at least a garden using drought-tolerant or dry-loving plants, think twice, thrice, or more before you add any nutrition to your soil whatsoever.