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Comment by beambot | Hacker News Reader

Conventional wisdom says each person requires ~1 acre (43k sq-ft) to source enough foodstuff for themselves.

With two people per 1k sq-ft apartment... you need ~100-fold improvement over the conventional wisdom to get full 100% sustainability. So let's assume you can magically get 10x gains in "land use" efficiency, and that you can stack those units 10 levels high (piping 1/10 of the 1kW/m^2 solar incidence to each level). But herein lies the rub: Urban density exceeds 2 people per 1k sq-ft of building rooftop area. So 100% rooftop sustainability is probably a no go.

Of course something is better than nothing (is it truly better than rooftop solar?), but it's worth pointing out that it can't be a panacea.

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Not all food is equal. Most of the acre (~4k m²) is needed for high energy crops like wheat or rice. It's pretty efficient to transport those into the cities. They can be stored pretty long and are pretty robust for transport. They're also pretty low price. Vegetables and berries are way more interesting as they're the opposite of all those factors.

What's wrong with growing vegetables and berries outside of cities in greenhouses?

They store shorter and lose a lot more nutrition in transit. They are more expensive to harvest but are more caloric dense in a given area. Its why raspberries are like 5 dollars a pound but bananas are 50 cents. One has a much longer shelf life.

But driving from the outskirts of a city to the inside, is just a 1 hour journey.

For caloric sufficiency, you're probably growing sweet potato or potato, at 70 Mcal/ha-day and 54 Mcal/ha-day, respectively. For a 2 Mcal/person-day diet, that's a minimum of 0.03 ha/person, or about 0.07 acre/person eating sweet potato. For potato, 0.04 ha/person, or 0.09 acre/person.

(http://www.fao.org/docrep/t0207e/T0207E04.htm)

Assuming at minimum that hydroponics provides 2.5x normal yield, and aeroponics 3.5x normal yield, one person eating exclusively aeroponically grown sweet potato would need 880 sq.ft. dedicated grow area, or 1.75x the example's (500/person) apartment floor area. If you can stack 4 levels of plants on one floor, that's still 1/3 of all building volume that would have to be occupied by growing plants.

Bear in mind that 2 Mcal/person-day is only enough to sustain children and little old ladies.

So ultralocal farming is probably not going to meet 100% of the needs of a building's residents without some extreme change in the way we design residential buildings. What you really want to grow on your roof is flavorful, nutrient-dense foods that are not quite durable enough for long-distance shipping. Then you ship the durable, bruise-resistant commodity staples from actual farms.

You eat your potatoes shipped in from the farm garnished with fresh green onion from your rooftop. You get your durum noodles from elsewhere, and eat it with sauce made from your rooftop tomatoes and herbs. Truck in the dry solid foods from far away, and grow the wet squishy foods as close as you can to your plate.

From what I recall reading some of the newer indoor hydro systems (plus LED) were getting greater than 20x increases (I think the lettuce/basil warehouse in Japan was getting over 60 heads of lettuce per square foot in comparison to traditional 3 per square foot in standardized farming). Perhaps by combining some of the different newer technologies they'll get the orders of magnitude of differences.

So why not use these techniques where land is cheap instead of on expensive urban rooftops?

Part of the thought was to also reduce transport cost. So if you have effectively the harvest and distribution localized one would reduce footprint in other ways.

That being said I'm not saying "expensive urban rooftops" are where we should be doing this. I was simply suggesting there are other changes afoot that are, for example, increasing yield. Any location will have pros on cons such as locality, cost, infrastructure, space reuse, etc.

Transport costs (in $ and CO2) are a rounding error compared to the cost of other inputs in food production.

Transportation has a large impact on the varieties and quality of food available.

Excessive handling creates waste and even drives your shopping habits; damaged fruits and vegetables don't last as long on the shelf so you have to pursue 'fresh' food regularly. Growers choose varieties based on transportation before taste or health.

It's not uncommon for someone harvesting their own root vegetables or apples to store one season's harvest for most of the year (my grandfather advocates a wooden barrel filled with sand). Almonds, and other nuts, are only harvested once a year and stored for distribution to meet demand.

In the US, modern perceived limits of 'fresh' food and expiration are greatly exaggerated, driven by the abundance of inferior product.

Seems like it'd be easier to improve upon that transportation bottleneck instead of something as drastic as restructuring the whole system and growing in urban areas.

Seems like it would be harder to re-engineer food transportation to be virtually shock-free. You'd have to revolutionize all packaging, sorting, loading and transport for each product.

Even handling when picking has an impact on shelf life; field workers aren't encouraged to be lax with their time.

As opposed to the alternative, where you walk a few steps to your garden and pick, sort, pack, transport, unpack a few minutes per day yourself.

A lot of the waste (at least from produce) comes from every step of the supply chain loosing product to buying to much or letting it sit to long. In theory, growing on a nearby roof could force people to grow only what they are going to use.

Is this 1 acre per person per year?

You don't need a new acre next year, so the units work out for 1 acre/person.

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