Rocks

[Eigenvector]

I was thinking about this as I dug up the winter weeds in my garden. My soil is riddled with rocks, rocks of various shapes, sizes, and construction. Nothing too obnoxious, I rid myself of the boulders long ago, but kept the walnut sized ones.

But I was thinking, I wonder how important those rocks are to my garden. Do the flowers and herbs use them to anchor themselves to the ground, do the rocks give off the important minerals that the plant needs or does the soil alone supply that, without surface rocks will the spiders have less places to live. Do the rocks provide that last shred of moisture to the roots during dry times

Question, for those who have done this for a long long time, do you find that soil that is essentially just dirt that plants grow better or worse than in soil that is natural and rocky but still of good quality?

[Phisherman]

I find rock removal improves the growth of most plants. I moved all the larger rock to the lower part of the garden to help make the garden more level and reduce runoff/erosion. In another garden I lined the border with rocks. Farmers dislike rocks in their fields (it dulls the rototiller and plow).

[Billy]

In "Collapse", Jared Diamond relates that rock provide nutrients. Soil that has geologically fresh volcanic ash, rocks left by glaciers, and rocks exposed by up welling of land masses, add nutrients to the soil. Jared Diamond points out that part of the reason that northern Australia is impoverished is that it contains the world's oldest rocks that date back 4 billion years. The result is that most of the nutrients for plants have been leached out of them. Even more rain wouldn't be sufficient to ameliorate its' soil. Where as these geological events have happened in south-west, and south-east Australia and up into Queensland. As a result, these are Australia's agricultural zones.

"Collapse" looks at a number of societies that failed and why they failed and a number of different societies that succeeded and why they succeeded. It should be at your library and isn't a difficult read at all. The chapter on Australia is particularly informative.

[David Hare-Scott]

In the short term rocks are of no real benefit. They take up space without contributing anything useful. This is because compared to the useful components of soil the surface area of rocks is tiny so there is little in the way of water, gases or soluble solids that they can hang on to. So no, they do not supply the last shred of moisture they are the first thing to dry out. All other things being equal your soil will be better off without rocks. That doesn't mean that going to huge trouble to remove them would be necessarily justified.

Neither do roots usefully hang on to rocks in most circumstances, though in restricted situations roots can go into gaps in large rocks and so anchor the plant, this is a very limited niche usually observed on wind-blasted cliffs not in gardens.

In the long term rocks may be part of the replenishing of the minerals of the soil. This happens through physical and chemical weathering which break down the rocks to useful sized particles, with enormously greater total surace area, and frees soluble minerals. This takes place over geological time and depends in part on the parent rock containing said minerals (not all do). You can simulate this to some extent by applying rock dust (of the right type) to your soil but even though the physical breakdown has been accelerated it still takes years to be effective.

If you want to understand how soil works and be able to make good decisions about soil management you will get much from investigating this question of particle size. It is a key consideration in drainage, mineral supply, aeration and microbe activity.

David

[dr-solo]