Legumes fix nitrogen from the air right? Nope!

When we first moved to the Funny Farm 3 years ago I transplanted some Baptisia australis, a native perennial legume with beautiful flowers, into a couple of the vegetable beds. I got the idea after having visited Carl Jordan's Spring Valley Ecofarms where they were doing research on restoring a worn out farm using an allycropping system. In this system, perennial legumes (mimosa) were planted in widely spaced rows and vegetable crops were planted between the rows. The perennial legumes were cut to the ground several times during the growing season. The prunings were used as mulch around the vegetables where they would decay and slowly release nitrogen and other nutrients that would be used by the vegetables. Also, after pruning, the mimosa would slough off roots which would allow nitrogen stored in root nodules formed by nitrogen-fixing bacteria to be released into the soil which would then be available to the vegetables as well. The bacteria are the organisms that fix nitrogen from the air not the plants. If the bacteria are not present to colonize the roots no nitrogen is fixed.

When leguminous crops, (beans, peas, clover, alfalfa, vetch) have been colonized by nitrogen-fixing bacteria, yields are greatly improved. Once the crop has been harvested or, in the case of a cover crop, plowed into the soil, the crops planted next will benefit from the nitrogen as well. We gardeners and farmers also benefit by not having to apply additional costly nitrogen fertilizer.

We can be sure that our crops are being colonized by nitrogen-fixing bacteria by inoculating our seeds. I purchase inoculant from Johnnys Seed Co. and treat my seeds before planting or during planting. Different species of legumes are colonized by different species of bacteria. Soybeans are colonized by Bradyrhizobium japonicum; clovers and alfalfa by Sinorhizobium meliloti, beans by
Rhizobium leguminosarum biovar phaseoli. It is important that you choose the right inoculant for the crop you are growing. Johnnys sells an inoculant that has a mix of different species that will inoculate snap beans, field type peas (black eyed or southern peas), english and sugarsnap peas.

The bacteria may persist in the soil for several years. however, I inoculate my seeds every year to be sure I am getting the benefits of nitrogen fixation. A bag of inoculant is enough to treat 8 lbs. of seeds and costs about $6.00; well worth the investment in my opinion. There are native species of these bacteria but they may or may not be present and they may not be efficient nitrogen fixers. The plants can sense whether or not the bacteria can help them and will select those the best to come colonize their roots. There is much communication between the plants and the bacteria in this symbiotic, mutualistic relationship between species.

It takes about a week for root nodules to be visible and about 3 weeks for nitrogen fixation to start taking place. You can determine whether or not your crops have been colonized by carefully digging up some of the roots with a trowel and looking for nodules. If you slice a nodule open and the inside looks pink or red, nitrogen fixation is taking place. The plants and bacteria together produce a molecule that is similar to the hemoglobin in our blood which carries oxygen from the plant to the bacteria. The red color comes from the presence of iron in the molecule which also makes our blood red. Adequate supplies of iron in the soil are necessary to make this process happen.

Permaculturists often plant leguminous plants in the same hole with fruit or nut trees so that the young food producing trees can benefit from the nitrogen fixation within the roots of the legume. After several years the legume is removed or cut down allowing the food tree to grow on.

This fall I dug the Baptisia out the the beds and moved them to our orchard where they will help feed our apple trees. They were nicely colonized with large nodules of bacteria which you can see in the picture below.