How Our Plants Get Plenty of Nitrogen Without Nitrogen Fertilizer


We haven’t used any nitrogen fertilizers for many years, but I think you’ll agree that there doesn’t appear to be a nitrogen deficiency in our garden. But without fertilizers, where does the nitrogen come from to replenish the nitrogen taken up by our crops? I’ll do my best to answer that question in today’s video. My approach is to work with nature’s process for recycling nitrogen, which is called the nitrogen cycle. There are two sources of nitrogen in this cycle – organic matter and the atmosphere, which is 78% nitrogen gas. I become a part of the nitrogen cycle when I mulch, make compost, and grow nitrogen fixing crops, which convert nitrogen gas in the atmosphere to a form of nitrogen that can be used by plants. Now let’s look at how the nitrogen cycle increases nitrogen in our soil, and how I help it along. The area where I probably have the biggest impact is in the decomposition of organic matter. Almost all of the plant material that’s grown in the garden, but not eaten, is returned to the soil as mulch, vermicompost, or compost. When I mulch or make compost, plant residues are broken down by bacteria and fungi, and nitrogen in the residues becomes available to other plants. Animals are part of the nitrogen cycle, too When you feed garden scraps to your chickens or rabbits, you accelerate the return of nitrogen to the soil via their urine and manure. We also bring in organic matter from outside the property. For example, plant-based food scraps are high in nitrogen and all of the scraps from food we buy at the grocery store are vermicomposted or composted and added back to the soil. Bananas, watermelon rinds, orange rinds, mango peels, used coffee grounds, and the rest all add nitrogen to the soil after they decompose. I also bring in three other external inputs to use as mulch – grass clippings from our neighbor’s untreated lawn, autumn leaves, and wood chips. You probably know that grass clippings are high in nitrogen, but it may surprise you to learn that autumn leaves and wood chips, though high in carbon, also provide a net increase in nitrogen after decomposition. Of course, none of these inputs have the kick of a high nitrogen fertilizer, but that’s okay. We’ve found that simply facilitating the nitrogen cycle provides all the nitrogen we need in our well established garden. Now let’s take a look at how nitrogen gas in the atmosphere Is fixed in the soil and made available to plants. I’ll start with some of the smaller contributing factors and work my way up to nitrogen-fixing plants. First, the enormous energy of lightning can break apart nitrogen molecules in the atmosphere, resulting in a series of reactions that bring nitric acid to the earth with rain, where it’s converted to plant available nitrates in the soil. This process is estimated to contribute about five to eight percent of the total fixed nitrogen in the nitrogen cycle. We’ve had a lot of thunderstorms this summer, so lightning is definitely making a difference in our garden. Next, there are free-living bacteria in the soil, including azotobacter, that bind nitrogen gas in the atmosphere, which isn’t accessible to plants, and release it in the form of ammonium which is. These Bacteria are unique in that they don’t have to form a symbiotic relationship with plants in order to fix nitrogen. Finally, and perhaps most importantly, rhizobia bacteria form symbiotic relationships with the roots of legumes, like these beans, to fix nitrogen in the soil. The bacteria live in small nodules on the roots of legumes, and the bacteria fix nitrogen in these nodules. Most of the nitrogen is used by the legumes themselves, but some is also shared with other plants. So, gardeners can have a big impact on the nitrogen cycle simply by growing nitrogen fixing legumes like peas, beans, lentils, and peanuts, and by growing nitrogen fixing cover crops. As I said before, most of the nitrogen fixed by legumes ends up in the legume plants themselves. So, the best way to make sure you get that nitrogen back in the soil is to return the plant residues to the soil, either by chopping and dropping them or by composting them. I also leave the plant roots in the soil. In the case of these pea plants, which I have to clear out of here soon, I’ll probably compost them rather than chop and drop them because they have some powdery mildew on them, and I don’t want to spread that around the garden. In addition, some nitrogen is shared with neighboring non-legumes via root to root transfer and through mycorrhizal networks. A 1996 study found legumes can leak or transfer 30 to 50 pounds of nitrogen per acre for use by neighboring non-legumes. This is typically considered an insignificant amount, and from the perspective of industrial agriculture, it certainly is, but in the context of the holistic approach that I’m using, I don’t consider that amount to be insignificant at all. 30 to 50 pounds of nitrogen per acre translates to as much as 0.8 pounds of nitrogen in my 700 square feet of growing space. That’s the equivalent of 8 pounds of a 10-0-0 nitrogen fertilizer. Even when I was using nitrogen fertilizers, I never used that much, and this is just one way that the nitrogen cycle fixes nitrogen. In addition, a more recent 2010 study states that the amount of nitrogen shared by legumes is likely often underestimated because of difficulties recovering and assessing roots. This study focused on rhizo-deposition in a variety of legume species. Rhizo-deposition is the transfer of nitrogen by legumes to non legumes. There are two pathways by which rhizo-deposition occurs – the decomposition of legume roots and nitrogen nodules, and the excitation of nitrogen by legume roots. Depending on the species, the study found that rhizo-deposition of nitrogen by legumes as a percentage of the total plant nitrogen varied from 4 to 71%. Even if most of the legumes I grow fall on the low end of this range, I still view rhizo-deposition as an important part of the holistic system that provides my plants all of the nitrogen they need without my having to buy any nitrogen fertilizer. Finally, one concern that people sometimes have is that their soil may not contain the rhizobia Bacteria needed to fix nitrogen. Fortunately, most soils do contain these bacteria, but if you’re in doubt, you can always purchase rhizobia inoculant and inoculate your legume seeds prior to planting. So what does all of this mean for you? One thing I want to make clear is that I’m not trying to say that you’ll never need to use nitrogen fertilizer, especially if you’re starting a new garden in poor native soil. But I do believe that over time, if you work with the nitrogen cycle and follow the methods I talk about in my videos, you’ll need less and less nitrogen fertilizer and perhaps someday you’ll need none. if you found this video helpful, please give it a thumbs up. And if you haven’t already, please subscribe for more videos on how to grow a lot of food on a little land without spending much or working harder than you have to.

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