Nitrogen in the form of nitrates is critical for healthy plant growth. Therefore it is important that you maintain nitrate levels in your soil. You can replace soil nitrogen using legumes.
Legume root nodules add nitrates to soil
All legumes have small nodules on their roots that contain many specialised bacteria that convert atmospheric nitrogen (not used by plants) into nitrites. These nitrites are then converted into nitrates, which are used by plants. This is an adaptation that legumes have developed that enable them to thrive in soils that are deficient in nitrates. Heavy nitrate feeders such as corn and leafy vegetables have difficulty growing in these soils.
For hundreds of years, farmers and gardeners have taken advantage of this by growing a crop of legumes to replenish soil nitrates prior to growing a crop of heavy nitrate feeders. This is a good practice, but it needs to be noted that they ploughed or dug the legume crop into the soil before it reached maturity and started to flower and produce seeds.
The Legume Myth!
So what does this mean for the backyard organic vegetable gardener? It is a myth that by growing a legume crop of beans or peas, enjoying the harvest and then digging the remaining plants back into the ground will replenish nitrates in the soil. Some benefit will be gained, but nowhere near as much as you may have thought.
The reason for this is because the legume uses the nitrates they produce for their own benefit. They use the nitrates produced for their own growth, flowering and seed development.
When you harvest the beans or peas, you are removing from that little part of the ecosystem the nitrogen that was fixed by the bacteria.
Therefore, you can definitely replace soil nitrogen using legumes. But, if your purpose for growing legumes is nitrate replenishment in the soil, consider growing a legume that you then dig into the soil before it starts to flower and produce seeds. This way you will maximise the nitrogen replenishment into the soil rather than removing it to your dinner plate.
I am an educator and passionate gardener and traveler. Throughout my adult life, gardening has been my passion, therapy, drive and source of purpose. Even as a child I had an intrinsic interest in plants and a desire to understand what makes them grow.
I distinctly remember the moment this began - my family was on one of our regular road trips from Hervey Bay; Australia. We were driving past a field of sugar cane. Dad pulled the car over and we cut a couple of sugar cane stems and brought them home for a treat. To be honest, I didn’t really like the taste, but I did want to try and grow it; and that is exactly what I did. It was then that my fascination, interest and passion for gardening and understanding plants began.
Fast forward a few years and I studied biological sciences and began what would be a 36 year career as a Biology educator. From this, I don’t only love gardening, but I also love helping others learn about gardening. I am also always looking for new ways to develop my own gardening knowledge. I like to think I am truly a life-long learner.
Fundamental to my beliefs about education is that learning is often best done as a part of a community - learning from others, and helping others to learn. It is this type of community that I hope iCultivate will be for its members - a community of gardeners, keen to share their gardening knowledge and wanting to learn about new ways to garden - a community built on the love of gardening.
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