Nitrogen is one of the more essential plant nutrients. Without sufficient nitrogen level in the soil, plants’ growth will be extremely slow. That’s why the best pioneer plants that are being used in permaculture design are nitrogen fixing plants. But just because a certain plant is a nitrogen fixing plant, it doesn’t ensure that it fixes nitrogen efficiently.
That is why I decided to create a list of the best nitrogen fixing plants. And by the best, I mean only the amount of nitrogen they can fix per year.
I’ll start with 3 best nitrogen fixing trees for temperate climate.
Best nitrogen fixing tall trees for temperature climate
1. Black Alder (Alnus glutinosa)
Black alder (Alnus glutinosa) is a tall European tree that grows up to 30 m tall (100 ft.). It is cold hardy in zones 2 – 7. It is short-lived species (30 – 70 years) that thrives in low-lying damp and riparian places.
As with most nitrogen fixing plants, increased nitrogen level in the soil also increases phosphorus availability in the ground. Because of that, different alder species are often planted as a nurse crop, that will increase soil fertility for the more valuable and demanding trees like beech, spruce or fir.
How much nitrogen can Black Alder fix per year per hectare?
Black alder can fix up to 320 kg of nitrogen per hectare per year. As with most perennial nitrogen fixing plants, those rates are not achieved in the first year. Also 320kg is the highest rate recorded. If conditions are less than perfect, then nitrogen fixation will be lower. What can be expected is usually between a 100 and 200 kg (as reported by Martin Crawford from Agroforestry Research Trust).
What is the best place to plant black alder?
Black alder is somehow an unusual nitrogen fixing plant as it actually likes damp and waterlogged soil. It thrives in low-lying damp and riparian places. If the access to groundwater is not possible, it grows the best in places getting approximately 1500 mm (59″) in precipitation.
Other uses of nitrogen fixing alder in permaculture design.
Alders are pretty useful for coppicing, as they regrow from the stump.
Alders’ wood is often used for smoking fish, cheese, sausages or meats.
Alder produce fine quality wood for charcoal. (it could be used to make biochar)
Black locust (Robinia pseudoacacia)
Black locust is a tall nitrogen fixing tree (up to 52m or 170 ft. but usually about 30% shorter), native to North America, though it was introduced to Europe in the beginning of XVII century. Black locust is cold hardy in zones 3 to 8 (possibly 9).
What is the best place to grow Black locust (Robinia pseudoacacia?)
In its native range, it grows in areas between 1020mm and 1830 mm (40” to 73”) of annual precipitation but its growth is also successful in areas that receive just 500 mm (20”) of precipitation annually. It prefers soil with a pH of around 6.0 – 7.0, but it can also grow in a soil having a pH 4.6 to 8.2. Black locust growth is the best in a lime rich soil.
How much nitrogen can Black locust (Robinia pseudoacacia) fix per hectare?
Annually black locus can fix up to 220-250 kg of nitrogen per hectare.
Other uses for nitrogen fixing black locust in permaculture design:
Black locust is a honey providing plant
Black locust wood is very resistant to rot in the soil. It’s great for fence posts
Black locust wood is extremely hard and durable
Black locust is good for coppicing
Black locust is a good firewood (long and hot burning)
Empress tree, Princess tree or Foxglove tree (Paulownia tomentosa)
Paulownia is a tall, deciduous (10-25 m 33-82 ft.) nitrogen fixing tree that’s native to China. It’s cold hardy in zone 5B – 9. It’s also the fastest growing tree in the world. How cool is that for a pioneer in the genre of nitrogen fixing plants?
How much nitrogen can Paulownia tomentosa fix per hectare?
Unfortunately, I was unable to find any information about how efficient paulownia is in fixing nitrogen. But I estimate it’s very efficient, as it has nitrogen rich leaves and an impressive rate of growth, even on poor soils.
What is the best place to plant empress tree?
Paulownia doesn’t requires good soil, but the soil should be a well draining soil with a pH of 5-8.5. Full sun is required. It grows the best in balanced soil with a pH of 6.4.
Other uses for nitrogen fixing paulownia tomentosa in permaculture design:
Paulownia is especially well suited for agroforestry systems as 76% of its absorptive roots are have a depth of 40-100 cm (16-25”) and only 12% of its absorptive roots are in top 40 cm (16”) of the soil. Because of that, it doesn’t compete for nutrients with shallow rooted crops as much as the other trees do. In fact, it can help prevent fertilizers leaching into the groundwater, as it’s root will capture nutrients that weren’t utilized by crops between the rows (in alley cropping systems).
Paulownia’s leaves and branches are high in protein, so they can be a decent high protein fodder for cows, goats, sheep and horses.
Paulownia’s wood is used for making musical instruments.
Paulownia is a great tree to plant near polluted streams and ditches as it sucks up a lot of nutrients. Just keep in mind, that it will not fix much or any nitrogen if the soil is already rich in nitrogen.