What happens when a private householder recycles food-wastes on an industrial scale in a suburban garden?

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I happen to live in an inner-city suburb with a small garden of deep siliceous sand (https://www.researchgate.net/publication/225267192_Soils_Characteristics_of_The_Bassendean_and_Spearwood_Sands_of_the_Gnangara_Mound_Western_Australia_and_their_Controls_On_Recharge_Water_Level_Patterns_and_Solutes_of_The_Superficial_Aquifer). This is as naturally infertile as garden soil can be (https://www.agric.wa.gov.au/soil-productivity/vegetable-crop-nutrition-sandy-soils-swan-coastal-plain?page=0%2C1 and https://www.rswa.org.au/publications/journal/103/RSWA%20103%20p1-8%20Turner%20and%20Laliberte.pdf).

I also happen to live near a shopping centre where astonishingly large quantities of food are regularly discarded, mainly by a bakery/coffee shop but also by a delicatessen/supermarket.

So, for the past decade, I have been systematically fertilising my garden with the food-wastes freely available to me in my suburb.

My municipality forbids the keeping of Gallus domesticus, so I cannot feed food-wastes to fowl.

This has led me to a method of simply dumping the food-wastes into my garden instead, on the basis that:

• my soil is too infertile to produce much edible material without fertilisation,
• it would be a pity if all the local food-wastes simply went to the usual landfill (https://www.awe.gov.au/environment/protection/waste/food-waste), and
• here an opportunity to learn something new about the nutrition of plants.

The quantities of food-wastes I have been able to recycle experimentally in this way may surprise many readers: an average of about five kilograms of fresh material (most of it still edible) per day over about ten years, totalling about 20 tonnes in a garden only 50 square meters in area.

This means that I have put about 400 kilograms of food-wastes into each square meter of my garden over the decade in question.

The food-wastes recycled have consisted of:

• fresh flesh of all kinds, including fish, oysters, crustaceans, squid, poultry, pork and red meats (total about one tonne),
• dairy products of all kinds (total about one tonne),
• seeds of various kinds including 'grains' and legumes (total about five tonnes),
• pies, pasties, sausage rolls, bagels, etc. (total about four tonnes),
• greens (total about two tonnes),
• fresh fruit-pulp and dried fruits (total about two tonnes),
• processed foods of various kinds including dips, sauces, spreads, etc. (total about 0.5 tonne), and
• spent coffee-grounds (total about 4.5 tonnes, see https://www.abc.net.au/everyday/the-right-way-to-use-old-coffee-grounds-in-garden/100200830 and https://www.sgaonline.org.au/using-coffee-grounds-in-the-garden/ and https://www.thespruce.com/using-coffee-grounds-in-your-garden-2539864).

An additional total of about 20 tonnes of baked goods (loaves, rolls, muffins, cakes, etc.) has been available but I have declined to use them because I have judged their nutrient-concentration to be too small to be worth the endeavour. This bread etc. has simply gone to the usual landfill with the other garbage of the shopping centre.

My main method of recycling has been simply to bury the food-wastes in a series of pits about 30 centimeters wide by about 50 centimetres deep. I have dug on average two such pits every week over the decade in question. Each pit takes about 15 kilograms of food-wastes.

In addition to these food-wastes, I have also recycled garden-wastes from my neighbours into the same small garden, on far more than the usual domestic scale. The method in this case has been to pile the foliage, lawn-clippings, sticks, organic litter, etc. about one meter high and two meters wide, and to allow them to rot into compost over a period of months at a time. I estimate that over the decade in question I have collected more than twenty tonnes of garden-waste, producing more than one tonne of my own compost.

I have learned, with much trial and error, to minimise any problems of fly-infestation and unpleasant odours. My main techniques are:

• in the case of food-wastes, to ensure that the contents of each pit are covered with at least five centimeters of soil, and
• in the case of composting above-ground, to cover the pile with a tarpaulin so that the decomposing mass does not heat up to the degree that it would if it were fully aerated (https://seafood.oregonstate.edu/sites/agscid7/files/snic/basic-principles-of-composting-lsu.pdf).

I have been pleasantly surprised to find that a mere few centimeters of the sand naturally occurring in my garden has reliably prevented any odours from rising to the surface from the protein-rich material rotting just underground. This efficacy has at times seemed almost too good to be true: a thin blanket of soil can virtually eliminate any hygienic problems from a massive accumulation of organic matter in various stages of decay.

What all of this means is that I have converted my garden into a kind of landfill-site in its own right. This seems to be fully legal in my area, given that I have cooperated with my municipality throughout the process (my property has remained open to official inspection at all times).

In my experience, buried food-wastes take months to break down, because the supply of oxygen is progressively restricted with depth underground. This means that, given my limited area, I cannot exceed a certain rate of burial without exhuming partly-rotted material - which would risk producing problems of odour. My recycling experiment has thus been run at its natural limits.

The enrichment of my soil in organic matter has been obvious. Not only is the sand now relatively dark with humus near the surface, but the ground level of the garden has been raised by about 50 centimetres - along the lines of a 'midden'.

How have plants in and near my garden responded to this intensive fertilisation?

A short answer is: in ways that are surprisingly obscure and subtle, and of minimal short-term benefit to me as the householder.

Overall, I have seen little progress in terms of my ability to grow anything edible beyond the leafy greens of plants such as Amaranthaceae, which are too rich in oxalic acid to be useful in my diet (https://www.researchgate.net/publication/262388453_Oxalate_content_of_raw_and_cooked_silverbeet#:~:text=Silver%20beet%20leaves%20contain%20higher,lost%20during%20boiling%20.%20...).

About six years ago, after trying unsuccessfully to grow various edible plants, I settled on Ipomoea batatas (https://en.wikipedia.org/wiki/Sweet_potato) as my best prospect. Since then the sweet potato has been the main herbaceous species - other than annual 'weeds' - inhabiting my garden.

However, to my dismay the plants, though healthy-looking, perennial, and able to recolonise the ground within a few months of burial of food-wastes, have produced negligible quantities of tubers. From the viewpoint of growing my own food, my garden remains a failure, much as it was before I started fertilising it on the scale described above.

So where have all the nutrients been going?

My best guess is: mainly into the surrounding trees, and partly into the subterranean aquifer (https://www.watercorporation.com.au/Our-water/Groundwater#:~:text=Perth%27s%20groundwater%20system%20remains%20vital,4%20household%20gardens%20through%20bores.).

A mature tree, 10 meters high, of Jacaranda mimosifolia (https://en.wikipedia.org/wiki/Jacaranda_mimosifolia) stands in a neighbour's garden, with its trunk just on the other side of the fence. Until a few years ago I had a mature tree of Eucalyptus cinerea (https://en.wikipedia.org/wiki/Eucalyptus_cinerea) just within my own garden, and I retain several small individuals of Melia azedarach (https://en.wikipedia.org/wiki/Melia_azedarach) and Agonis flexuosa (https://en.wikipedia.org/wiki/Agonis_flexuosa) in the corners. There is also a tall hedge of Syzygium (https://en.wikipedia.org/wiki/Syzygium) on the fence-line, with its roots penetrating my garden.

Trees have an inherent advantage over herbaceous plants on dystrophic soils (https://pubmed.ncbi.nlm.nih.gov/28662068/). As far as I can tell, this competitive advantage is so great that no amount of 'do-it-yourself' nutrient-addition (as opposed to commercial fertilisers) makes much difference to my ability to grow edible herbaceous produce - even in the case of a species as tolerant of infertility and neglect as the sweet potato.

And all of this has taught me a sobering lesson: for all the talk of recycling and composting, we remain dependent on commercial fertilisers and - owing to economies of scale - commercial farming for our food-supply.

Part of the problem is the balance among the various nutrient elements. Commercial fertilisers are not just concentrated sources; they are formulated with the right ratios among the chemical elements (e.g. https://extension.psu.edu/how-to-calculate-a-fertilizer-ratio#:~:text=To%20determine%20a%20fertilizer%20ratio,lowest%20weight%20(0.5%20lb).&text=Thus%2C%20the%20fertilizer%20ratio%20that,is%203%2D1%2D1.) including micronutrients, such that herbaceous plants can acquire their full set of requirements simultaneously. This coordination seems difficult to emulate 'organically', at least in my experience as an extremely dedicated - albeit unconventional - gardener.

I suspect that zinc (https://extension.umn.edu/micro-and-secondary-macronutrients/zinc-crop-production#:~:text=Zinc%20(Zn)%20is%20an%20essential,sweet%20corn%2C%20and%20edible%20beans.) in particular is removed by the mycorrhizae of trees before herbaceous plants have access to it.

As far as I can tell in my situation, it is rather futile to cultivate herbaceous crops (other than leafy greens) in a small garden on deep siliceous sand, unless one raises the beds so as to prevent access to the nutrients by nearby trees. And, had I used raised beds (https://www.youtube.com/watch?v=7cPi3ixNF-Y), I would not have been able to use food-wastes on any scale in the first place.

And so we come to a discouraging reality:

although I have been by far the most ardent private recycler in my suburb, I continue to depend on sources other than my own garden for my food.

Indeed, even in the case of leafy greens my mainstay each rainy season has been the annual 'weed' Sonchus oleraceus, plucked partly in my garden but mainly from road-verges more broadly in my neighbourhood (see https://www.inaturalist.org/journal/milewski/59442-sonchus-oleraceus-undomesticated-but-perfect-for-human-consumption#).

Based on what I have learned, I am currently converting my garden to a mini-orchard of Syzygium cumini (https://en.wikipedia.org/wiki/Syzygium_cumini). I have little doubt that fruit-bearing trees will succeed where tuber-bearing herbaceous plants have failed.

I will let readers know in another five years' time.

FOOTNOTES

There are various reasons why, in my situation, I would not find soil analyses useful.

Firstly, my goal is not to grow herbaceous crops in general, or any species in particular. Indeed, I have no particular goal and instead am working on a system. If I eventually find that what works in my garden is a mini-orchard of Syzygium cumini, I will be happy even if I have failed to grow anything else. Soil tests mean little except in the context of the requirements of particular species/growth-forms.

Secondly, soil tests are most applicable to situations in which the plant species in question relies on a particular horizon of soil. Because my soil is deep, porous sand, roots explore the profile deeply and concentrations of nutrients in topsoil do not necessarily tell us much about overall availability. This is particularly important because trees, capitalising on nutrients leached to groundwater, can usurp the fertility of topsoil that would otherwise be perfect for a given shallow-rooted herbaceous species.

Thirdly, nutrients in deep coarse sand are dynamic rather than static - somewhat along the lines of a hydroponic system. Soil tests measure concentrations at a particular moment. This may approximately indicate availability in loamy soils, but in sand - where the concept of cation exchange capacity hardly applies - it is the flows of nutrients that are more important than the static concentrations.

Fourthly, even if my garden had classic loam, suitable for classic tests, and even if I could assess these tests relative to the requirements of a particular species of plant, this would be of little use to me. This is because I have no intention of correcting the soil accordingly, using specific elements or a balanced commercial fertiliser. Not only would resorting to commercial fertilisers be antithetical to my whole approach, but it would be uneconomical given that farmers can do this much more efficiently than me, based on their economies of scale.

My system is about a) somehow conserving some of the nutrient content of the trashed food that would otherwise go to landfill, and b) learning by trial and error what is practical in my situation. This is as opposed to a simple goal of 'growing my own', whatever that takes - which I see as pointless in my situation.

(Also see https://www.inaturalist.org/posts/58175-the-australian-empty-quarter-epitome-of-a-nutrient-desert.)

Posted on Μάρτιος 28, 2022 0928 ΜΜ by milewski