Every food item we consume has some form of cycle. Some are very small, some very large. Tomatos grown in a backyard plot, consumed by the grower, plants composted on site, regrown from seed from the previous years harvest and enriched with green manure and waste tomatos (read poop) from the grower is a simple closed cycle. The production and inputs are isolated to a backyard.
Tomatos grown in a hot house miles from the consumer might rely on petroleum derived nitrogen using imported oil & other imported chemical fertilisers. The seeds are sourced from growers many miles from the hothouse. The produce is transported via road or rail and sold in markets many miles away from the origin. The consumer eats the tomatos, whose poop gets taken away for sewerage treatment and who may compost part of the tomato. Any residual nutrient from the tomato that benefits the end consumers garden transplants the residual nutrients (from a mass of inputs) many miles from the source. The cycle of nutrients may see some returned to the earth but a long way from where they started.
There are definite issues of sustainability in these, admittedly imperfect, examples. Made me wonder about various cycles, closed and partially open, in my gardening. One closed cycle I can identify is with my lawn. The grass draws nutrients from the soil and puts them to work. I mulch my lawn and leave the clippings to degrade. The nutrients are recycled back in to the soil. Any extra nitrogen the lawn may need is supplied through clover. The deep root structure of grasses will draw up potassium and phosphorous lost through water leaching. In theory I need not bother fertilising my lawn. Using a grass catcher and transferring the grass to a compost heap actually relocated the nutrients from my lawn. In a sense I am mining my lawn for nutrients. A closed cycle would see me leave everything where it is to cycle nutrients around. In practise I do use waste coffee grounds and wood ash on my lawn but that is as much to put 'waste' to good use as any concern with a pressing need to fertilise.
I feed my dogs and cats various types of tucker. The cats shit around the garden, often on the lawn or in flower pots as they are lazy, and the dogs when they go for a walk. The inputs of food are off site however the disposal of their poop I can use on site. The cat crap is good to amend flower gardens. The dog poop goes under a hedge as a fertiliser. I can close the cycle on my own property.
Likewise with growing vegetables. I have been experimenting with seed collection from various crops. Provided I handle them correctly I can regrow from last seasons crop. Using green manures can enrich my soil to limit the amount of nutrients I need to apply, thereby minimising the amounts of nutrients that are translocated. A guy down the road has chickens. I give him some veges from time to time, he gives me the chicken poop. Aside from the food inputs for his chickens, I can complete a cycle of food on a very local level. The leaves that fall from my tree can be turned in to leaf mould and used as mulch for the vege gardens. Theoretically quite simple, even if the working out of the theory takes some learning through trial and error.
Gardening becomes quite interesting when you consider them as open and closed cycles. One way of measuring sustainability is the notion of nutrient mining. When I use prepackaged fertilisers I am mining nutrients from another part of the planet. When I use chicken poop I am mining nutrients from other soils. When my neighbour carts his lawn clippings off to the dump he is mining the nutrients of his soil.