How often have you read that nitrogen hungry vegetables like salad greens or the cabbage family should best be planted in a spot that has previously been planted with peas or beans? The cabbages are supposed to make use of all the nitrogen that the peas and beans have fixed.
A few explanations for inexperienced gardeners:
Nitrogen is one of the most important chemical elements for plants. If there is not enough nitrogen available in the soil they will look pale and their growth will be stunted.
Legumes - all peas and beans are legumes - are plants that work together with soil bacteria called rhizobia, to 'fix' nitrogen, meaning chemically convert the nitrogen in the air to make it availble for the plant. By living in a symbiotic relationship with the rhizobia - the rhizobia live in nodules in the plant's roots - the plant is supposed to meet its own nitrogen needs, fertilizer is not required. In addition, when the crop is harvested and the plant cut back to ground level, the root nodules should release all the valuable nitrogen for following crops.
This is great news and all permaculture designs make extensive use of native legumes to increase the nitrogen level in the soil, but there is a hook.
The relationship between the legume and the rhizobium is highly specialized. The kind of rhizobium suitable for a certain plant will usually be found naturally in the soil in the area where the plant originally developed, after all they developed together.
Well, most of our beans and peas have originated in Europe, so if you live in America or Australia don't expect the required rhizobium to automatically be present in the soil, most likely it's not, and if it is, then probably not in sufficient numbers, (the latter is even true in Europe). That means you will have to fertilize your beans and peas just like any other crop, and growing them makes no difference for the following crop at all.
The other option is to inoculate your beans and peas. Inoculant is the matching rhizobium in a liquid or powder form. It should come with instructions on how to coat the seeds with it before planting. Once present in your soil the rhizobium will happily live there for several years on its own, so if you plant beans again in the same spot a few years later they'll be just fine. This is the most cost-effective and environmentally friendly way to supply nitrogen to plants.
There's one more option, I'd like to call it the permaculture option. There are many native legumes, trees, shrubs and other kinds. Since nobody ever fertilized them they had to make do by themselves, so they developed these relationships with the rhizobia. If you grow a legume that's native to your area the required rhizobium will be present in your soil.
Use the natives as a cover crop, as an in between crop (talking both time and space here), slash or prune them regularly, get creative and find ways to incorporate them into your garden design. They don't need looking after, they just spent thousands of years adapting themselves to your particular soil and climate. As they grow they work for you, by converting the nitrogen in the air into nitrogen for your plants. No need to buy inoculant, no need to buy fertilizer, and no need to spread it. Sounds like a good deal to me.
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