The Three Sisters: What the Agronomy Actually Shows

What this comes down to

Corn, beans and squash grown together produce more food from the same ground than any of them grown separately. That part is measured and it holds up. The reason usually given for it, that the beans fertilize the corn, is the part that does not survive contact with the research.

The Three Sisters is a Haudenosaunee cropping system, and among the clearest voices on it is Jane Mt.Pleasant, a Haudenosaunee agronomist who spent her career at Cornell measuring what it actually does. Her numbers, and a separate root study from Penn State, tell a more interesting story than the one in the gardening books.

The yield, measured

Mt.Pleasant published a food-yield and nutrient analysis in Ethnobiology Letters in 2016. Her finding, in her own words: the Three Sisters "yields more energy and more protein than any of the crop monocultures or mixtures of monocultures planted to the same area."

The figures she gives are 12.25 million kilocalories per hectare and 349 kilograms of protein per hectare. Translated into people, that is roughly 13.4 people fed on energy per hectare per year, and 15.9 on protein.

That is not a romantic claim about old wisdom. It is a yield comparison, run against the alternatives, and the polyculture wins.

Why it works, and why it is not the beans

Here is where the common explanation goes wrong.

Beans are legumes, legumes fix nitrogen, and so the story became: the beans feed the corn. It is tidy and it is repeated nearly everywhere, including on some government pages.

The agronomy does not support it as stated. A 2014 study in Annals of Botany by Chaochun Zhang and colleagues grew the polyculture out and looked below ground, and attributed the yield advantage to something else entirely: the three species forage for nutrients at different depths and in different patterns. Maize roots stay shallower. Common bean explores the soil profile more evenly. Squash root placement shifts with phosphorus. Put together, they search more of the soil than any one of them searches alone, and all three produce denser lateral root branching in the mixture than in monoculture.

Their conclusion was that "species differences in root foraging strategies increase total soil exploration, with consequent positive effects on the growth and yield of these ancient polycultures." Root architecture, not nitrogen transfer.

Nitrogen fixation is real, and over seasons it contributes to soil fertility. But the measured in-season transfer from legume to cereal in intercropping studies is generally small, the published range is far too wide to quote as a number, and the clearest documented benefit tends to land on the following crop rather than the one standing next to it. Mt.Pleasant and Burt's own field work found maize yields were not improved by intercropping at all.

So the honest version: the corn gives the beans something to climb, the squash shades the ground and holds moisture and suppresses weeds, the three root systems divide the soil between them, and the beans build fertility over time. That is a better system than the myth, because it is the one that actually happens.

The part that is about food, not farming

The nutritional half of the system is where Mt.Pleasant's work gets genuinely elegant.

Maize is short on two essential amino acids, lysine and tryptophan. This is the kind of complementarity that shows up repeatedly once you start looking at traditional food systems rather than single ingredients, and it is one reason we keep coming back to how older preparations were actually put together. Beans are short on methionine. Maize has methionine to spare and beans have lysine to spare, so eaten together they cover each other. The FAO ratio Mt.Pleasant cites is roughly 70 percent maize to 30 percent bean to make a complete protein. Squash is not an afterthought either; the flesh complements both and the seeds carry meaningful amounts of essential amino acids.

And then there is the step that gets left out of almost every retelling. Maize in these systems was nixtamalized, soaked and cooked in an alkaline solution, which in Iroquoia often meant hardwood ashes. That is not a culinary flourish. Nixtamalized maize carries two to four times the calcium of untreated maize, it makes niacin available that is otherwise bound up, and it raises the usable levels of tryptophan, lysine, histidine, methionine and threonine.

Mt.Pleasant's argument is that bean plus nixtamalized maize together is what kept pellagra, the niacin deficiency disease that devastated maize-dependent populations elsewhere, out of these communities. Credit the beans alone and you have misread her.

How old is it

Older than most people assume for maize and squash, and younger than most people assume for the full three-crop system.

The limiting factor is the common bean. John Hart and colleagues, publishing in Antiquity in 2002, used accelerator mass spectrometry dating on beans and directly associated maize and placed the bean's arrival in the northern Eastern Woodlands in the calibrated late thirteenth century, some two to three hundred years later than had been thought. The beans then spread quickly from the Illinois River valley to southern New England.

Maize and squash in the region are considerably older. But all three in one field, in the Northeast, is a system of roughly the last seven hundred years.

The USDA's National Agricultural Library names the Seneca, the Haudenosaunee Confederacy and the Cherokee among the nations growing it, notes the Lenni Lenape as possible originators, and counts at least fifteen nations across the northeastern United States and southeastern Canada. The related milpa system runs through Mesoamerica.

One thing worth saying plainly: the Three Sisters is a cultural and spiritual framework and not only a planting method. The USDA's own page makes that point, and also notes that most of the research on it has come from scholars outside the communities that developed it. This article is an agronomy and nutrition explainer. It is not an account of what the system means to the people whose system it is.

Common questions

Do the beans really fertilize the corn?
Not in the way it is usually told. Beans fix nitrogen and build fertility over time, but the measured in-season yield advantage in this polyculture is attributed to complementary root foraging rather than to nitrogen moving from bean to corn.

What does the squash actually do?
Ground cover. The broad leaves shade the soil between the corn and beans, which suppresses weeds and conserves moisture. Interplanted crops also tend to attract fewer pests than a single-crop field.

Is corn and beans together really a complete protein?
Together they cover each other's limiting amino acids, maize being low in lysine and tryptophan and beans low in methionine. Mt.Pleasant cites an FAO ratio of about 70 percent maize to 30 percent bean.

What is nixtamalization?
Cooking maize in an alkaline solution, traditionally with wood ashes or lime. It softens the grain, raises calcium, and releases niacin and several amino acids that are otherwise unavailable.

How far back does the full system go in the Northeast?
Roughly the late thirteenth century, limited by when the common bean arrived. Maize and squash in the region are older.

Can you grow it in a home garden?
Yes, and plenty of extension programs publish layouts for it. Cornell's garden-based learning curriculum is a reasonable place to start. For more on the plants of this continent and the people who knew them, see echinacea before it was a cold remedy and wild bergamot.

Sources

Mt.Pleasant, Jane. Food Yields and Nutrient Analyses of the Three Sisters: A Haudenosaunee Cropping System. Ethnobiology Letters, vol. 7, no. 1, 2016, pp. 87 to 98. View source

Zhang C, Postma JA, York LM, Lynch JP. Root foraging elicits niche complementarity-dependent yield advantage in the ancient "three sisters" (maize/bean/squash) polyculture. Annals of Botany, vol. 114, no. 8, 2014, pp. 1719 to 1733. View source

Hart JP, Asch DL, Scarry CM, Crawford GW. The age of the common bean (Phaseolus vulgaris L.) in the northern Eastern Woodlands of North America. Antiquity, vol. 76, no. 292, 2002, pp. 377 to 383. View source

Mt.Pleasant, Jane, and Robert F. Burt. Estimating Productivity of Traditional Iroquoian Cropping Systems from Field Experiments and Historical Literature. Journal of Ethnobiology, vol. 30, no. 1, 2010, pp. 52 to 79. View source

United States Department of Agriculture, National Agricultural Library. The Three Sisters of Indigenous American Agriculture. View source

Cornell Garden-Based Learning, Cornell University. The Three Sisters: Exploring an Iroquois Garden. View source

These statements have not been evaluated by the Food and Drug Administration. This article is historical and educational and is not medical advice. Nothing here is intended to diagnose, treat, cure, or prevent any disease.