A Guide to Vegetable Legumes

Longtime Maine farmer and homesteader Will Bonsall shares his knowledge and experience with different vegetable legumes.

| August 2016

Vegetable Legumes

Vegetable legume plants—beans and peas—are able to host bacteria that converts atmospheric nitrogen (N2) into nitrate (NO3). The legume plants then use the converted nitrate to build proteins.

Photo by Dani Vincek/Fotolia

In Will Bonsall's Essential Guide to Radical, Self-Reliant Gardening, author Will Bonsall maintains that to achieve real wealth we first need to understand the economy of the land, to realize that things that might make sense economically don't always make sense ecologically, and vice versa. The marketplace distorts our values, and our modern dependence on petroleum in particular presents a serious barrier to creating a truly sustainable agriculture. Bonsall draws upon the fertility of on-farm plant materials: compost, green manures, perennial grasses, and forest products like leaves and ramial wood chips. And he grows and harvests a diversity of crops from both cultivated and perennial plants: vegetables, grains, pulses, oilseeds, fruits and nuts―even uncommon but useful permaculture plants like groundnut. In a friendly, almost conversational way, Bonsall imparts a wealth of knowledge drawn from his more than forty years of farming experience.

Vegetable Legumes

Vegetable legume plants—beans and peas—are distinguished by their tendency to host certain soil bacteria that have the magical ability to convert atmospheric nitrogen (N2) into nitrate (NO3). Legume plants can use the nitrate to build proteins and stuff. Those plant proteins (which I tend to think of as dietary nitrogen) in turn feed us; indeed some of their amino acids are not adequately found in grasses or other plant foods. I think it is only too cool that we benefit from this major source of protein from plants that do not even make the protein themselves, but rely in turn on yet another level of life. We may be a predator of the legumes, but their relationship with the bacteria seems somewhat more cooperative. The rhizobacterial (that’s techno-speak for “root-cooties”) colonies are at first glance parasitic—they feed on surplus sugars in the root sap—but they pay very well for their keep by performing that nitrogen trick. Both parties seem well content with the arrangement, so who are we to naysay?

In order to make this miracle appear out of thin air, the bean, or rather the bacteria, must have access to that air, and since they cannot climb up out of the soil to get it, the air must be brought to them. Therefore abundant organic matter in the soil is critical, not so much because of any particular ingredient in it, but for its role in allowing air to enter the soil and remain. If, however, you reduce the humus level and add lots of nitrogen fertilizer, you will gain nothing and lose a lot. Legumes in general do not appreciate being spoon-fed; they prefer to make their own.

Despite their importance in the nitrogen/nitrate/protein connexion, not all of the vegetable legumes can be properly thought of as protein sources. In the case of green beans and snow peas, the pod is the food part, and that part, I suspect, is not much higher in protein than, say, a pepper. As I understand it, it is in the mature dried seed—the pea or bean or lentil—that the concentrated protein is found. But we also eat the full-sized but immature seed of many legumes, such as table peas, shelled beans, runner beans, favas, and vegetable soybeans (aka edamame). These legumes are still among the most nutritionally dense vegetables. They may have less protein than their mature forms, but that protein is much more hydrolyzed (chemically bound up with water) and thus much easier to digest and assimilate.

The combination of comparative richness with easy digestibility makes most of the legume veggies uniquely suitable for making dips and spreads resembling guacamole.

The Old World legume crops—peas, chickpeas, and favas—are extremely ancient domesticates and all very cold-hardy. Like all legumes, they enjoy lots of alkaline minerals: potassium, calcium, and magnesium. However, because I plant my vegetable legumes in among other vegetables that I feed with high-mineral compost, the legumes get all the minerals they need.

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