symbiosis in the garden

I love growing covercrops, and have experimented with several over the years. I like to interplant clover in tomatoes and peppers, and have grown asparagus in a permaculture system with dutch clover. I have used field peas and austrian winter peas in a mix with oats for a summer green manure, but apart from those examples, I have not been very adventurous with legumous cover crops.

Rhizobium bacteria live in the cells of legume root nodules of and are estimated to carry out 50-70% of the world's biological nitrogen fixation.

"The nodulation process is a series of events in which rhizobia interact with the roots of legume plants to form a specialised structure called a root nodule.
The process involves complicated signals between the bacteria and the roots. In the first stages, the bacteria multiply near the root and then adhere to it. Next, the small hairs on the root's surface curl around the bacteria and they enter the root. Alternatively, the bacteria may enter directly through points on the root surface. The method of entry of the bacteria into the root depends on the type of plant. Once inside the root, the bacteria multiply within thin threads. Signals stimulate cell multiplication of both the plant's cells and the bacteria and this repeated division results in a mass of root cells containing many bacterial cells. Some of these bacteria then change into a form that is able to convert gaseous nitrogen into ammonium nitrogen (that is, they can "fix" nitrogen). These bacteria are then called bacteroids." source

In return, the microbes get high-energy carbohydrates produced by photosynthesis in the host plant. There are different species of rhizobia and they are associated with different plants: the soil bacteria that is a symbiont of clover is different than the species engaged with lupines, for example.

I have many nitrogen fixers growing wild in the pasture, and expect that soil conditions for these are better suited for my soil and climate conditions; there is vetch, red clover, white clovers and lupine for example, but no alfalfa. I expect that because these plants are thriving there is a healthy community of the soil bacteria that each of these plants require to provide the symbiotic relationship necessary to fix nitrogen in the plant. I value these natural precedents and try to take my cue from the pasture ecosystem in my garden.

Blue Lupin is good plant for nitrogen, phosphorus and deep soil
penetration. It is a very good at extracting and concentrating minerals in the soil so when the plant material decomposes it made available for the next generation of plants. Hairy and common vetch are excellent for nitrogen fixation, although they won't produce as much biomass as clover,so seeding with a mix of rye or oats will add this bulk.

White dutch clover, once established provides an excellent hardy cover for paths in the garden and will withstand foot and even tractor traffic down the beds.

Here are some great resources

Organic Ag Center; Green Manure Options
Attra Overview of Cover Crops and Green Manures