Part 6 · Capstone: Design Your Own Subsistence Farm
Calorie Accounting
Turn your crop plan into a single number, the people your site feeds, and name the macronutrient that limits it.
A plan is a wish until it produces a number. The sixth deliverable is the calorie ledger: the arithmetic that turns your crop plan into one hard figure, the number of people your site can feed, and the name of the macronutrient that stops it feeding more.
What you produce
A ledger with one row per crop and three summary lines: people supported on calories, people supported on protein, people supported on fat. The design feeds as many people as the smallest of the three, because a diet that meets two requirements and misses the third does not feed anyone. The last line of the ledger names the limiting macronutrient. If that line says calories, you have not done the arithmetic, because calories are almost never the binding constraint on a small mixed plot. Fat is.
The exact format
Per crop:
| Crop | Area (sq ft) | Gross (lb) | Seed reserve | Storage loss | Edible (lb) | kcal | protein (g) | fat (g) |
Then the totals, and then the division. This is the arithmetic you built in Lesson 310.
The edible weight is the gross yield less the seed you keep back and less what storage takes. From the engine: a dried grain keeps about 3 percent to loss in store, a root crop about 15 in a root cellar, a nut about 5 dried. Peas keep back about 10 percent of the harvest as seed, potatoes about 12, hazelnut about 5. Multiply the edible pounds by the crop's energy and macronutrient density to get its contribution, then sum every crop.
The requirement is the model's adequate standard, per adult per day: 2,200 kcal, 55 grams of protein, 50 grams of fat. Per year that is 803,000 kcal, 20,075 grams of protein, and 18,250 grams of fat. The number of people supported on each is the total you grew divided by the annual requirement. The design's carrying capacity is the smallest of the three.
Why fat is the number that binds
The protein column and the fat column do not behave alike. A legume block fills protein quickly, because beans and peas carry 96 to 136 grams per pound. Fat is slow, because oil crops and nuts are land-hungry: olive returns the lowest calories per labour hour of any staple, and hazelnut and sunflower carry their fat at a cost in area that no grain comes close to. This is why the worked example below lands on fat as the limiting macronutrient, and why the whole-system rule is never to evaluate a plot on calories alone.
Worked example at the 10,000 square foot scale
Take the model engine's standard one-hectare mix, wheat 0.4, fava 0.3, olive 0.1, potato 0.2, at the adequate standard. The engine returns 10.5 people on calories, 18.7 on protein, and 3.7 on fat, so the hectare feeds three people and change, limited by fat.
Our bench is 10,000 square feet. One hectare is 107,639 square feet, so our bench is 0.093 of it. Divide every figure:
- calories: 10.5 × 0.093 = 0.98 people
- protein: 18.7 × 0.093 = 1.74 people
- fat: 3.7 × 0.093 = 0.34 people
The ledger says the site can feed one person on calories, and a third of a person on fat. The third of a person is the true number. One adult needs 18,250 grams of fat a year. At 0.34 people the plot delivers about 6,200 grams, a shortfall of about 12,000 grams.
Now close it. A hazelnut block on loam yields 35 pounds per 1,000 square feet, carries 170 grams of fat per pound, and keeps back 5 percent for seed and 5 to storage. That is 5.4 grams of fat per square foot a year. To find 12,000 grams you need about 2,200 square feet of hazelnut, which bears nothing for four years. Sunflower, at 40 pounds per 1,000 square feet and 100 grams of fat per pound, gives 3.6 grams per square foot and needs about 3,300 feet, and it bears in the first year. Either way the fix is the same: about a fifth of the site given to oil, held until the trees come on.
For scale, the reference farm fed one person, in fact a little more, on 7,965 square feet, because its olive trees carried the fat. Our 10,000 feet, at the same density, would feed 1.26 people. The generic model mix needs 0.66 acre per person; Pelman did it on 0.18. The difference between those two numbers is not soil and not luck. It is design.
The test
The ledger passes when it produces one number for your own site and names the limiting macronutrient. It fails if the summary line reports only calories, because a caloric surplus with a fat gap is a household that eats enough and still starves. It fails if any crop appears without its seed reserve and its storage loss, because those two subtractions are the difference between the harvest on the stalk and the food in the room. And it fails if you do not show your working, because a number no one can check is a number no one should believe.
Exercise
Produce your own site number
Take your crop plan from Lesson 605. For each crop, get the gross yield, subtract the seed reserve and the storage loss, and multiply by the calorie, protein, and fat density. Sum the columns. Divide by 803,000, 20,075, and 18,250. Write the three numbers and circle the smallest. That circled number is your design's carrying capacity, and the column it stands in is your limiting macronutrient.
Fieldwork
Weigh a week of food
Weigh every food that comes into the kitchen for a week and every scrap that leaves it, in pounds, on a kitchen scale. Record the weight, the crop, and the meal in a notebook at the end of each day. Then weigh one cup of each of your staples, grain, bean, and oil, to check the density figures against what you actually eat. Bring back a filled one-week table of weights and the three checked densities.
Failure mode
The failure mode is the calorie-only ledger. It flatters the design, because grain piles up calories fast, and it hides the fat gap until the first winter, when the household is eating grain and beans and cannot keep warm. Report all three columns or none.
