Look on My Compost ye Mighty

Subtropical gardeners in Australia suffer greatly from our mostly European cultural heritage. Those in the tropics at least can’t delude themselves that all the lovely pictures and techniques in the glossy gardening books are worth trying. Instead many of us go through a drawn out process of trying and failing to bring those temperate ideals to life. Those of us who persist long enough eventually figure out different ways of approaching things. One crucial ideal we have inherited is the idea of “building soil” and that deserves closer attention.

In colder climates organic matter breaks down relatively slowly. In really cold climates this doesn’t matter as much since not so much organic matter is produced since growth rates are quite slow, but in the temperate zones ample biomass is produced then decomposes slowly. In these climates the nutrients locked up in the organic matter can be released by all sorts of composting techniques, providing a more soluble form for fast growing vegetables. Organic matter in the soil also breaks down relatively slowly, allowing the elbow deep black and friable beds we see in posh gardening shows to be a reality. The gardener has a fairly easy time adding more organic matter than breaks down every year, gradually accumulating stabilised humus that improves soil texture, making it easier to dig and for roots to penetrate, and retaining water. This crumbly soil is easy to dig, rapidly incorporating more compost and manure each season, and forcing oxygen into the soil to spur further breakdown of organic matter and nutrient release. In the most extreme example of this ecotype temperate wetlands accumulate so much organic matter in the form of peat that minerals become the limiting ingredient to plant growth.

The problems arise when we try to recreate this in warmer climates. The common shortcut is to buy bags of black and crumbly composted pine bark/chicken manure that at least looks the part but is barely even adequate for potted houseplants let alone growing vegetables. The main problem is in the subtropics organic matter breaks down so rapidly that the amounts needed to make a dint in the soil structure are impractical and uneconomic (especially if you have to buy and import manure or mushroom compost). Making your own compost is time consuming, and requires prodigious inputs if you want to grow more than a tiny garden. Digging the organic matter into the soil creates a short lived desirable effect of fluffing up the bed, but as the organic matter completely breaks down the soil slumps back to end up with even worse structure than it had to begin. The introduction of oxygen causes what little stable organic matter was present to break down. Luckily there are alternative approaches that work better in warm climates.

One is mulching (covered in more detail in a previous post). Here the organic matter is barely broken down so takes longer to completely decay. Putting it on the soil surface provides an energy source for soil microbes that make nutrients and water available to the plants, and to soil animals that help build soil structure more elegant and durable than a human smashing clods with a shovel could ever achieve (think of the difference between a building with well-engineered stone arches versus a pile of rubble). Mulch is still an unsatisfactory way of sustaining soil life, relying on stripping other spaces of their photosynthetic output in order to support weaker plants elsewhere, with a whole lot of human labour and/or machinery in the process. The best relationships with the soil ecosystem are not with dead mulch but between living plants with their direct microbial partners. Just as some plants pair up with bacteria to fix nitrogen from the air, all plants cooperate with a wide range of specific soil bugs to deliver water, phosphorus and everything else they need. When you pour imported nitrogen on a legume the plant determines it has plenty and doesn’t bother forming a relationship with its nitrogen fixing microbes. When you irrigate a garden the soil microbiome changes dramatically as plants stop providing energy to their normal microbial partners. Growing plants that are well adapted to the basic soil that you already have is the best strategy in warmer climates, and doing it in a way that fosters a strong relationship with the soil ecosystem. Intercropping unrelated species seems to boost productivity in these systems since different plants form relationships with different microbes.

These kind of systems are more in line with tropical rainforests. Instead of accumulating a store of organic matter and nutrients, the soil in a rainforest under a thin layer of mulch is very low in organic matter and nutrients. Instead all the nutrients are stored in the living biomass that towers above. As soon as any nutrients are released by decay the extensive root systems absorb it and turn it back to life. This can be imitated in a warm climate garden by focusing on feeding the crops and not the soil. Some people make manure or weed teas, though I find these to be too smelly and bothersome to distribute on any scale, and go against my goal of zero irrigation. Instead I simply top dress my beds with uncomposted goat manure. I normally place it down the middle of my beds so I can direct sow seeds on the edge of the manure to ensure they don’t get too much nutrient when first germinating, and to avoid the work of moving the manure aside to ensure the seeds are sowed in the underlying mineral substrate. The manure acts as a kind of mulch, maybe 2-3 inches deep when first applied, that reduces the number and strength of weeds in the middle of the beds. For long lived or hungry crops I will often add a second layer once they are established, usually spurring a flush of strong growth, even when the soil is quite dry thanks to the strong microbial connections.

It is worth pointing out here that soil that is hard to dig with a shovel is not necessarily hard for plants to grow through. Clay soil that has experienced significant animal activity will be riddled with tunnels of various sizes that assist root growth. Plant roots often follow channels created by old roots as they die as well, a good reason to avoid pulling out old crops and even weeds if you can get away with simply beheading them instead. “Improving” soil texture is mostly a matter of human convenience, with digging tubers and sowing seeds much easier in soil with good friability and tilth. One way to achieve this is with accumulation of stabilised organic matter as outlined above. The problem with more intensive growing spaces is that the addition of nitrogen rich inputs such as manure acts as a catalyst for organic matter breaking down by spurring microbial growth that needs to consume both. Adding nitrogen rich urine to soil in warm climates can quickly render it devoid of organic matter. It is interesting to contrast the soil in my low fertility goat paddocks after a few years of cobblers pegs growing and disintegrating, with 7% organic matter and remarkably good texture, versus my manure enriched vegetable garden that is still highly prone to clods (though the crops grow pretty well since by not digging I have allowed the manure to spark high levels of tunnelling).

One final tool is left in my arsenal for developing soil texture. The only form of carbon that is stable in warm climate soils even if nitrogen levels are high is charcoal. This is popularly known as bio-char now but there isn’t much of a difference. This form of carbon is highly porous, allowing it to absorb nutrients like a sponge and host microbes, and as it is incorporated into the soil it prevents clods of clay sticking together, improving friability. Like all good things people are prone to taking it too far. Biochar, like ash, is highly alkaline so adding too much at once can make life very difficult for acid loving crops. That means you need to add it gradually over a few years, allowing the alkaline minerals to be absorbed by the soil each time before adding more. This means I can simply top dress it along with my manure and wait for the soil life to incorporate it and break it down into smaller particles. Tomatoes, capsicums and eggplants seem to love an extra thick layer. Putting the charcoal below the manure means it can absorb nutrients leaching from the manure, acting as a buffer before they move into the soil. The incorporation process takes 3-5 years but leaves me with darker soil that doesn’t form clods as readily. Once our fire ban is lifted I will do a post on how I make my own charcoal without any fancy contraptions.

There are as many ways to grow as there are gardeners, but every space has its own unique quirks and potential. Mastering growing requires responding to these unique challenges that are right in front of us and this often involves letting go of dreams that belong in other places. I now see friable and dark soil as a lucky side effect of successful growing rather than a precondition before I can start.

Unimproved clay in the vegetable garden- still good enough for a potato crop with a little manure and charcoal.

A clod of clay showing a hint of the metropolis for soil invertebrates within.

Unirrigated potatoes growing on a thin film of goat manure under great clods of clay.