You may have heard that peas, beans and other leguminous plants fix nitrogen. This means they have an innate capacity to absorb nitrogen from the air and convert it to a plant-available form in the soil. Right? Well... not quite. Shattered? I was, when I first found this out, so I thought I would finally reveal the best kept secret on nitrogen fixation.
The harsh reality is that if you sow a patch of broad beans in your vegetable patch, they are very unlikely to be fixing nitrogen in the soil. But why?
Only specific types of bacteria around plant roots have the ability to convert nitrogen to a form that is 'bio-available' to plants (ie. a form that plants can eat). To complicate things, there are many different types of nitrogen fixing bacteria, each only fixing nitrogen for a select few plants. The likelihood of having exactly the right type of nitrogen-fixing bacteria in your soil for your specific plant is very low. This likelihood is further diminished if the soil has been tilled, plowed, or brought in from elsewhere.
Let us remember that the way we traditionally grow veggies in rows is a very 'unnatural' system: they do not form the complex inter-relationships plants form in a forest ecosystem.
Most organic, regenerative, biodynamic farmers, and some permaculturalists know this and inoculate their seeds and/or soil with the correct bacteria and fungi. These days, it is possible to buy specific natural inoculums for your cover crop seeds to ensure they will be fixing nitrogen in the soil. This is something I would suggest if you are constructing a Permaculture food forest, as this will ensure your nitrogen-fixing forest layer is actually doing just that: fixing nitrogen. Green Harvest sell fantastic cover crops and green manures with corresponding inoculums.
Biodynamic agriculture does this by including specific plants in compost preparations. For example, yarrow roots secrete substances that attract a type of nitrogen-fixing bacteria called Azotobacter. When yarrow is composted and used in your garden, this bacteria will be present in your soil. All plants used and composted in biodynamic preparations accumulate specific nutrients, fungi and bacteria in their roots, leaves and flowers. These are essential elements to have in your soil to ensure a healthy food growing system.
But how do you know whether or not your plant is fixing nitrogen? Gently have a look at its roots. If your plant has little white nodules on its roots, you're on the right track. However, to make sure nitrogen fixation is occurring, break open the nodules. If they are red inside, your plant is fixing nitrogen. Hurray! In the field, small nodules can be seen 2-3 weeks after planting, depending on legume species and germination conditions. When nodules are young and not yet fixing nitrogen, they are usually white or grey inside. As nodules grow in size they gradually turn pink or reddish in color, indicating nitrogen fixation has started. The pink or red color is caused by leghemoglobin (similar to hemoglobin in blood) that controls oxygen flow to bacteria.
So there you have it. The real dirt on nitrogen fixation.