Sorry for the long gap between posts.  I’ve mostly been off touring around national parks and permaculture places in northern New South Wales and southern central and southeastern Queensland with my daughter.  Some great walks in spectacular country, and some interesting comparisons between different permaculture demonstration sites.

Not long after we got back I discovered the great blog Living at Gully Grove via a guest post that its author, Chris, wrote on Farmer Liz’s Eight Acres blog about how their family uses permaculture.

When I clicked across to Living at Gully Grove, I found to my delight that, like us, they live in the hilly margins of the Lockyer Valley and clearly face some of the same issues.  It’s always great to find someone who lives in your region who is willing to share their knowledge.

That happy discovery led to an exchange of views via comments on Chris’ blog and the start of a “conversation” about sandstone soils.  As you may know if you’ve been following this blog, our place is steep sandstone country, with many rock outcrops and soil that is made up of sandstone in various stages of decomposition, and generally not more than 60-70 cm deep.  Even that shallow layer of “soil” often has at least half its volume made up of gravel and small rocks.

What passes for soil on a sandstone ridge

Early on we recognised that one of our main food production challenges was going to be the need to create suitable soil.  To that end we have done all kinds of things, including sieving the rocks and gravel out of huge quantities of the native “soil”, green manuring, mulching, terracing, etc.

So when Chris said:

Have you ever let a garden bed go (ran out of time to keep up to it) and noticed the good soil revert to something like dry potting mix?

The hardest challenge for us hasn’t been building the soil, but rather maintaining it. I notice where we have swales, the soil doesn’t need much of our attention, except where it crosses a sandstone shelf. It only takes a season of hot dry weather, to cook any good soils we don’t maintain.

I knew exactly what she meant.

For us, the challenge hasn’t only been to create good quality soil (and I can’t claim to have satisfactorily cracked that one), but to keep it in good condition.  An apparently well prepared garden bed can start the growing season with lovely fluffy, moist soil that holds together exactly right when squeezed in the hand and produces a good crop.  Then, unless it is constantly maintained, a few months later it is dry, loose and apparently lifeless.  Chris’ description of “dry potting mix” soil hits the nail on the head, particularly if one interprets it as the crap bagged potting mix that supermarkets and garden supply places sell. The plant material in these mixes is generally at best only partly broken down, and there is no evidence that there is, or ever was, life in them.

I’m no expert on soil processes, but I suspect that the coarse material in our “dry potting mix” soils is compost “residue” that has not been broken down. This is probably because (a) the soil was not sufficiently healthy initially, and in particular did not contain sufficient humic matter and soil organisms; (b) when we let the bed go there isn’t sufficient ongoing moisture in the system for biological processes to continue creating and maintaining humic matter; (c) if there isn’t a continuous cover of thick (but air and water permeable) mulch then soil temperatures rise and water content decreases; and, on a sandstone base, there is likely to be significant leaching of nutrients when major rainfall events occur. I have to say though that I have had this problem in some beds that I was actively managing, not just in ones that I’d been ignoring for a few months, but that may have been due to the leaching mentioned in the last point above.

Does the above explanation seem to match your experience / observations?

My way of tackling this problem is still evolving, but it includes:
# keeping a fluffy straw mulch cover on the soil that allows air and water to penetrate, and at the same time significantly reduces drying and insulates from overheating;
# adding green manure to the soil and digging it in. This isn’t the usual “green crop dug in” approach, but a mix of moist and drier (but still living) plant material put through the chipper / mulcher sufficient to make a 25-50mm layer on the surface, then watering it and digging it in;
# adding sieved compost “fines” (containing the humic material) to the soil;
# adding dry horse manure that has been put through the chipper / mulcher to the surface layer.  Processing it this way produces a fine, light fluffy material that holds moisture and gives the soil a great “texture”; and
# to the extent that our water supply allows (we have only tank water), keeping the soil moist, even when the area is not in production.

Good compost of course contains humic material, but can also contain a lot of woody material if you use coarse chip mulch as part of your carbon source.  This is part of the reason it’s a good idea to sieve your compost and put the finer, humic, fraction into the soil, reserving the coarser material for mulch or for feeding the next batch of compost. Unless you already have a healthy soil, there’s not a lot of point in incorporating coarse, only partly broken down, compost material into it.

If you want a good guide to how soil “works” and how to maintain its health, the best book I’ve come across is Toby Hemenway’s Gaia’s Garden (Chapter 4: Bringing Soil to Life). This is by far the best and most practical permaculture text I know. The other good source for soil matters is, perhaps surprisingly, Harvey Ussery’s The Small-scale Poultry Flock (pages 137-144 for soil matters). We have a pretty comprehensive permaculture / organic library but these are the two books I go to first when I have a question, and I seldom need to go past them.