How to Increase the Rate of Biological Nitrogen Fixation
This short article is a chapter from my book “Fertilizer for Free: How to make the most from Biological Nitrogen Fixation”.
One important question permaculture designers should ask themselves:
Is there anything you can do to increase the rate of biological nitrogen fixation?
The benefits of having more nitrogen rich organic matter in the soil are myriad. Starting with:
- higher general productivity
- richer and more diverse soil life
- more available phosphorus
- higher availability of various other nutrients
- higher capacity to hold nutrients
Fortunately there’s actually quite a lot you can do to make sure your plants are growing the fastest and they fix the most nitrogen! Here are nine methods you can use in your permaculture design to make sure your nitrogen-fixing plants are giving you and the whole ecosystem more benefits.
Method 1 — choose the right species
The first method you should use if you want to increase biological nitrogen fixation is choosing the right species of nitrogen-fixing plant for your climate. This step is very important, as some species fix nitrogen more efficiently than others. Good candidates for efficient nitrogen-fixing plants in a temperate climate are:
- ground cover: lupines, cowpea, fava bean, vetch, clover, alfalfa (on good soil)
- tall trees: black alder, black locust, empress tree
- shrubs and short trees: Autumn olive, gumi, Siberian pea shrub, Russian olive, sea berry
Method 2 — right amount of plants
Usually the amounts of nitrogen fixed per hectare (or acre) are expressed by how much nitrogen certain plants fix if they grow in monoculture. That means that the whole area is covered in certain types of nitrogen-fixing plants.
Young trees and shrubs take up much less space than mature trees and shrubs. So if you want your soil to get a lot of nitrogen you should have the entire area or almost the whole area covered in nitrogen-fixing plants.
Planting a few hundred little trees per acre or per hectare will not give you a lot of nitrogen for a few years, until the canopy covers more area.
One way to ensure you are going to fix a lot of nitrogen from year one in agroforestry or forest gardening settings is to plant nitrogen-fixing trees or shrubs and also plant nitrogen-fixing cover crops.
Method 3 — inoculate your plants with the right nitrogen fixing bacteria
If the field in which you want to plant your forest garden or establish a perennial pasture has not grown any legumes in the last three years, you should inoculate your seeds or seedlings. Some nitrogen-fixing plants require different species of bacteria than the other, so you need to get an inoculant specific to the species you are going to plant. If you can’t find the right legume inoculant you might use some soil from the ground around a healthy nitrogenous plant of the particular species.
To check if a certain plant is inoculated you need to dig out the plant’s root and check for bacteria nodules. If it has nodules it means there are at least some nitrogen-fixing bacteria. That’s good enough, so you can take a few shovels of that soil to add to the "nitrogen fixation enhancing compost" you will be using later. If you plant bare root seedlings you will see if there any nitrogen-fixing nodules or not on the seedlings you are planting, so you can evaluate whether or not you need legume inoculant.
Sea-berry — nitrogen fixing plant proven to fix up to 180kg of nitrogen in poor, sandy soil
Method 4 — right pH and calcium level
Nitrogen-fixing bacteria need a high calcium level to work efficiently. Nitrogen-fixing plants for temperate climates grow the best in soil with a pH of 6.4 with adequate level of calcium. Your soil should have minimum of 1500 lbs. of calcium per acre in the top 7" of your soil (1500 kg per hectare in the top 18 cm).
Just because your nitrogen-fixing plants are growing doesn’t necessarily mean they are fixing nitrogen for themselves, or even creating a surplus for other parts of the ecosystem! If you want optimum nitrogen fixation rates you need to have sufficient calcium in the soil.
If the soil pH is low, consider adding some lime. If liming your soil is not possible add finely ground limestone to the planting holes (if you are planting trees or shrubs).
If your pH is high, but the calcium level is low (for example because your soil has a lot of magnesium or sodium) consider adding calcium sulfate dihydrate (CaSO4•2H2O). Don’t worry, it is just gypsum! It will provide your soil with calcium without raising pH.
Method 5 — adequate phosphorus level
I highly recommend checking the phosphorus level of the soil. If the available phosphorus is low, consider adding some source of phosphorus to the soil. If you are planning to add soft rock phosphate or any other (natural) source of phosphorus, add it to the compost or manure and incorporate it into the field. If you are planting shrubs or trees you might want to add some compost or manure to the planting hole. In the future, higher nitrogen levels will help to make more phosphorus (and other minerals) available for the plants, but by making sure there is available phosphorus at the beginning it will speed up the whole process. I recommend making sure you have at least 200lbs/acre or 200kg/ha of phosphorus in the top 7" of your soil (top 18cm). If rock phosphate is not available and your soil needs it, add some diammonium phosphate (DAP). If financial constraints do not allow you to add the full amount of soft rock phosphate or DAP, apply as much as you can afford. Even a dose of just 20 kg DAP per hectare (20 lbs. per acre) can make a difference to your young nitrogen fixing plants. With time and larger sized plants, and with mycorrihiza fungi inoculation and efficient nitrogen fixation, they will be able to make phosphorus that’s already in the soil available. But that’s in the future; your plants need phosphorus now to make the process efficient. So think about it as an investment.
Method 6 — proper soil molybdenum level
Molybdenum is one of the essential nutrients plants require for growth. It is critical for nitrogen fixation. A low soil molybdenum level means low nitrogen fixation level….
Molybdenum is easily available for plants around a pH of 6.4. It’s more difficult for plants to get molybdenum in low pH soils (below pH 5.5). Quite often sandy, acidic soils are deficient in molybdenum. Make sure you have at least 1ppm (2kg per hectare or 2 lbs per acre) of molybdenum in your soil for optimum biological nitrogen fixation. If your soil levels are lower you can use the following methods to make sure your pioneer plants have enough molybdenum for optimum nitrogen fixation:
- add 3 oz. per acre (90 grams per acre or 180 grams per hectare) of sodium molybdate as foliar spray
- use 1 oz. for 1 acre (60 g for 1 hectare). Use either molybdenum trioxide or ammonium molybdate as a seed treatment. You can mix your seed treatment with the legume inoculant, but do it just before you are going to sow your seeds.
Bill Mollison recommended using molybdenum as fertilizer (if necessary) in Permaculture – A Designers’ Manual (page: 194, 197).
If you don’t want to test your soil for molybdenum, you can plant nitrogen-fixing plants anyway. If you recently added lime, or if you have pH around 6.4, chances are you will not need to add this essential micronutrient.
Method 7 — add some rock dust
Add some rock dust to the soil. It contains micronutrients and trace minerals. They are proven to enhance the growth of trees and shrubs, including the nitrogen fixing ones. You can use basalt rock dust, granite rock dust, azomite, or simply rock dust from a local quarry. If you are planting trees, add a handful of rock dust to the pit you are planting your nitrogen fixing trees or shrubs in, to make sure your plants are going to have a healthy start. You can also add some rock dust to the "nitrogen fixation enhancing compost".
Method 8 — add some beneficial microbes to your soil
This method of increasing biological nitrogen fixation is based on making sure you add a broad range of beneficial microbes and fungi. They are important especially for the long term success of a sustainable ecosystem, as they make nutrients more available to the plants, especially phosphorus.
This will increase both the speed and growth of nitrogen-fixing shrubs, trees and herbaceous plants.
You need them especially if your are planting your nitrogen-fixing trees on land that was used for conventional agriculture. There are many commercial products on the market that contain beneficial organisms. You can also make some actively aerated compost tea yourself.
Method 9 — don’t use pesticides and help your soil to get rid of pesticides residues
A lot of pesticides have a bad influence on soil biology, including nitrogen-fixing bacteria. For example glyphosate (the active ingredient in Monsanto’s Roundup herbicide) is very detrimental to nitrogen-fixing bacteria. On top of that glyphosate ties up many trace elements in the soil — mainly: manganese, iron and zinc — that might slow down the growth of your plants if the level of those elements is low. This effect is especially predominant on sandy soils. The solution is simply to not use pesticides, especially not the conventional ones. If necessary (as indicated by your soil test) add required amounts of manganese, iron and zinc. It is especially useful on sandy, acidic or very acidic soils. To some degree it will help to overcome limitations of your current soil conditions.
On the other hand a clay soil might contain more residues of pesticides that were used up to a few decades ago, some of which were already banned 30 years ago! To combat this, increase organic matter and soil activity and hope that some microbes and fungi will eventually decompose those harmful ingredients. All previous methods will also expedite this process.
How do you make nitrogen fixation enhancing compost?
Get a or create a pile of rich and mature compost. Add required amount of rock phosphate and rock dust. For every 10 parts of compost you can add:
- 1 part of rock phosphate
- 1 part of rock dust
- required amount of beneficial microbes (mycorrihiza fungi, actively aerated compost tea, etc.)
- 1 part of topsoil from a place where nitrogen fixing plants of the species you want to grow are already growing (and fixing nitrogen).
Mix it all together and spread it on top of your soil. Recommended rate: minimum 1 ton per acre (2 tons per hectare). If you are planting trees or shrubs you might want to put one shovelful of this compost in the soil pit where you will be planting your trees.
Do you have any other tips about helping nitrogen fixing plants to thrive and provide you with more biologically fixed nitrogen for free?
If you like this article, please consider buying my ebook: Fertilizer for Free: How to make the most from Biological Nitrogen Fixation.