To connect this series with my regular gardening a bit, let’s talk about fertilizing the fields and garden beds in a self-sustainability scenario.

A bit of the topic was already mentioned here and there in previous posts, but there is definitely more to it than I have written so far.

First, let’s assume that the solid sludge from the sewage cleaning facility is recycled back into the environment via composting instead of being taken out to be treated elsewhere. Dealing with shit is not pleasant, but it is an essential part of agriculture. Second, all the wood ash from heating would need to be recycled back into the coppice and the fields. Third, all other waste – manure from the animals, inedible biological leftovers etc. would have to be composted. Building up enough organic material in the soil fairly quickly should not be a problem.

Yet still, some outside inputs might be necessary from time to time.

Depending on local geology, calcium could be a problem in two different ways.

If the bedrock is too rich in it, then the problem might be that the groundwater cannot be used for watering the plot too much, because it would alter the soil chemistry. And some plants (like blueberries) could not be grown.

With geology close to my real-life garden, however, calcium inputs would be necessary, at least in the beginning. Once good levels in the soil and the environment overall are established, it might not be necessary anymore. Hens will happily eat crushed eggshells and crushed bones, thus recycling the calcium and making it bioavailable fairly quickly.

The second nutrient that could be a problem is sulphur. An anaerobic septic tank can convert some organic sulphur into elemental sulphur, and thus reduce its bioavailability to the grass in the subsequent gravel bed. And it would also be depleted by burning the wood from the coppice. Some sulfur would return with rainwater, but it might be needed to supplement some plants that need more, like garlic and onions.

Nitrogen is also lost in all stages. Whenever a stink occurs, nitrogen is lost (and sulphur too). In an ideal scenario, the legumes should fix enough of it to keep the growing going more or less indefinitely with proper crop rotation. Still, occasionally it might be advisable to add some to boost the harvest, especially at the start, before a buffer in the pantry is accumulated to account for dips and outright crop failures.

AFAIK, potassium and phosphorus are not depleted by offgassing during decomposition or burning, so they might need to be added only in the beginning to jumpstart the system. Once that is done, they will be recycled through the sewage cleaning and ash nearly perfectly.

A 100% self sufficiency is possible in this regard, but having the ability to get some inputs from outside reduces the reliance on nature’s lottery a bit.