Measuring agricultural productivity is easy, right? Kilogrammes per hectare and you're done. But that's almost the least interesting thing the land is producing, or so I thought. Then a tweet passed my way yesterday.

Calories per acre - the new metric to follow.

I saw that because someone I follow was enthusiastic.

Glad to see that @Harvest2050's buying in to calories per acre! Do we have a bandwagon yet?

And despite myself, and despite the fact that I know that actual discussion is all but impossible over there, I tried to make a case for nutrition per hectare. Predictably, I guess, that ended up with a smug pat-on-the-head putdown that sent me whizzing to the channels under my control, ready to think a little more deeply about how to measure agricultural productivity.

If you're an industrial farmer, growing grain to sell, then I guess kg per ha is a reasonable measure. You might even think kcal per ha a bit too fancy pants. And the less waste straw you have to deal with, the better. For a small-scale farmer, however, perhaps with a few animals to bed and feed, that straw is decidedly not waste. It is part of production. The weight of seed is important, but it isn't the only thing.

Now go further, and imagine that you're eating what the land produces. Maybe now kcal per ha makes a bit more sense, but only a bit more, and not only because it takes more than energy to sustain life. By that measure, half a hectare of beans and half a hectare of maize or wheat is probably less productive than a hectare of pure wheat. Cereals and pulses, however, make up for one another's amino acid deficiencies, so the total nutrition that a person could derive from that half and half hectare is greater than from a pure hectare of either. Carve out some space in that hectare for a few rows of leafy greens and what have you, and the productivity of the land, measured as "nutrition" is even higher. Allow a few animals to process the "waste" and it is higher still.

Which is why I think nutrition per hectare is the best measure of agricultural production.

Calories are, of course, part of nutrition, but by no means the most important part over the long run. We have tables of recommended daily allowances for macronutrients like Calories (or their proxies) and for micronutrients. We could calculate nutrients per Calorie for different kinds of produce. We could even try to express productivity as the percentage of the RDA for all nutrients that would be provided by some area of land. We could do lots of things more sensible -- and more difficult -- than Calories per hectare.

p.s. I want to put a marker down here for a couple of things I know are important and that I am choosing to ignore for now. One is the inputs necessary to achieve the agricultural outputs. The other is the sustainability and variance of the production over time.

P.p.s. 3 January 2016: This resulted in an episode of Eat This Podcast, How to measure what farms produce

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