Nitrogen Fixing Cover Crops and Living Mulch

A list of nitrogen-fixing cover crops in Portugal and when to plant them in Portugal. Research-based on our land in Central Portugal.

• This page is updated each season as we get more experience growing these. Last update in Summer 2023
• Please read a magazine article I wrote in 2022 about how to plant cover crops and start a new garden
• Here is our youtube video on how to plant cover crops

Cover Crops Terminology

Cover crops: Annual plants such as clover that you plant on a resting field or bed to prevent soils from drying out, washing away and fixing nitrogen into the ground.

Mulch: Organic biomass that is used to cover the soil to prevent soil drying out, prevent weeds from growing, stop topsoil washing in rain and provide nutrients to the microorganisms in the soil (e.g. hay)

Living green mulch: For example, clover can be grown around plants to act as a living mulch to prevent soil drying, hold off weeds, fix nitrogen and attract pollinators and predatory insects that may eat your pests.

Green Manure: These is cuttings of cover crops that can be used in compost piles or as green mulch (see mulch above). Green mulch is higher in nitrogen than brown mulch (e.g. dry leaves, straw or hay)

Nitrogen-fixing cover crops:

Below are the ideal times to plant cover crops in Portugal that fix nitrogen in the soil. However, I am sure most of them can be planted at different times just with a lower yield or germination rate. Many other cover crops bring up nutrients (dynamic accumulators) and create biomass (lots of green mulch) that can be planted, but this list just refers to nitrogen-fixing cover crops.

English name (Latin name / Portuguese name) – There are many varieties for some of these, but we have listed at least one. We have underlined the ones we have grown successfully by planting at the time of year. The other ones we have not tried to grow yet at that time of year.

Autumn planting

1. 1. 1. Clovers (Trevo) – Naturally occurring on our land
         1. Subterranean Clover (Trifolium Subterraneum / Trevo Subterranean) – available in organic bulk seeds at the local agrological shop in Portugal. It is good as its self fertile, its bad as it does not attract so any pollinators
         2. White Clover (Trifolium repens)
         3. Red Clover (Trifolium pratense) – We have had this establish very well in our wet lower terraces
      2. Alfalfa ( Medicago sativa / Alfalfa)  – Hard to establish but a great deep-rooted perrenail. We have had this establish in very wet soils and by the roots of trees that receive issigation
      3. Perrenail Ryegrass (Lolium  / Azevém)
      4. Rye (Secale cereale / Centeio) – Lots of biomass in spring
      5. Ryegrass (Lolium  / Azevém)
      6. Oats (Avena Sativa / Aveia)
      7. yellow serradella (Ornithopus compressus / Serradela) – Naturally occurring on our land, can withstand heavy gazing
      8. Vetch (Vicia Sativa / Ervilhaca) – Naturally occurring on our land
      9. Fava Beans/Broad Beans (Vicia Faba/ Fava)
      10. Peas and Field peas (Pisum Sativum / Ervilha)
      11. Chickpeas  (Cicer arietinum / Grão de bico) – Just planted a patch of this in oct 23, lets see if it grows. usually locals plant it in spring
      12. Lupins (Lupinus / Tremoço) – easy one with nice flowers

Spring planting:

1. 1. 1. Chickpeas  (Cicer Arietinum / Grão de Bico)
      2. Clovers (Trevo) – does better in autumn
         1. Subterranean Clover (Trifolium subterraneum / Trevo Subterranean) – available in organic bulk seeds at the local agrological shop in Portugal. An advantage of this is that it is self-fertile, however, it does not attract as many pollinators as other clovers.
         2. White Clover (Trifolium Repens) – Shorter  and spreads more than red clover, so it is better to plant as a green mulch around plants
         3. Red Clover (Trifolium Pratense) – Taller than white clover so it is better as a green manure.
      3. Alfalfa ( Medicago Sativa / Alfalfa) – does better in automn
      4. Mustards (Sinapis hirta / Mostarda) 
      5. Sorghum (Sorghum Bicolor / Sorgo)
      6. Millet (Various types of millet with various types of Latin names / painço)
      7. Phacelia (Phacelia Tanacetifolia / )
      8. Field peas – do better in autumn
      9. Vetch (Vicia cracca / Ervilhaca) – does better in automn
      10. Cowpea (Vigna unguiculata/ feijão frade) There are many examples of summer beans that can be planted but this one only grows well in poor dry soils.
      11. Buckwheat (Trigo Mourisco)
      12. Yellow serradella (Ornithopus Compressus / Serradela) – Occurring naturally on our land

What to plant, when and where?

I think it is best to plant cover crops in autumn and you need to choose the application:

Perrenial Cover crops

This is also known as a permanent pasture and agriculture shops sell a pasture (Prado) mix of cover crops. They have two mixes, one for wetland and one for dryland. In Portugal, these are around 80eur for a 25kg bag.

Annual Cover Crops

I like to use clover, field peas, vetch, mustards 

How do cover crops fix nitrogen?

On the roots of these plants lives a nitrogen-fixing bacteria (rhizobium and others). These bacteria take in nitrogen from the air within the soil and the nitrogen is then available from the plant’s roots, leaves and stems. If you do not have the nitrogen-fixing bacteria in your soils you should inoculate the cover crop seeds in a purchased inoculant.  You can check to see if they are living on the roots of your cover crops by digging them out and checking for nodules on the roots.

To fix this nitrogen into the ground, the plants should be either:

1. Chopped before they go to seed and allowed to decompose in the ground, a technique called ‘chop and drop‘, then one should ideally dig the green mulch into the ground or cover it with an additional layer of brown mulch, to ensure the nitrogen isn’t released into the atmosphere but sequestered back into the soil.
2. Allowing animals to  graze it and recycle it is a less efficient way of fixing the nitrogen because some nitrogen from urine and manure will volatilize (passed off as vapour) as ammonia and is lost from the system
3. Add the green mulch to compost piles
4. Add the green mulch around plants in other areas of the land (then cover the green mulch with brown mulch)

Don’t forget to let some go to seed so you can replant in the next season.

Mulch layering

We usually use a little compost or aged manure, which is then completely covered with green mulch (you should not be able to see the compost or manure). We then completely cover this with a thick layer of hay and then a thin layer of straw (again the previous layer should not be visible). The hay has a higher nutritional value for the soil than straw but can harbour seeds from weeds so we cover it with straw which does not contain weed seeds and stays dryer so does not act as a medium for weeds to germinate on. The straw we use has grain seeds of plants which are nitrogen fixers so often a further green cover crop is grown from these seeds that can be chopped and dropped for further nitrogen fixation.

Interplanting with annuals

We have been planting clovers around our trees and annual plants as these fix nitrogen and stop weeds growing in beds. There are different theories on when to plant the cover crops around different annuals, but I would suggest doing it a few weeks after you plant annual crops such as cabbages and corn for less competition.

Below, you can see that the bed in one of our annual gardens is covered in green. In this small bed, we had summer crops of tomatoes, cabbage, peppers, brussel sprouts, leeks and onions. The broccoli leaves are still growing and being harvested for the chickens and salads. It has a variety of herbs to attract beneficial insects and to repel pests. It also has the complete area covered in a variety of clovers which we allowed to go to seed.  We have been collecting the seeds for the past few weeks. This week I will pull out all of the plants which will either go to the kitchen or to the chickens. I will cut back all of the clovers and leave the leaves in the bed (chop and drop) and dig them into the ground with a little bit of compost and cover with hay and straw. Ideally, some of the clovers will find a way to grow back from the roots and seeds to fix more nitrogen, I will then leave this bed to rest for one year. During this time I will continue to chop and drop the clovers.

Perennial nitrogen fixers

Growing annual nitrogen fixers is a quick method to get nitrogen into the soil. There are also nitrogen-fixing trees and shrubs that can do the job year after year without maintenance. These can be planted in between nitrogen demanding trees, around annual beds or in pasture fields as a lot of them also act as fodder crops providing more sustainable food for livestock as well as shade. I will write about these in a future blog. However until these are established, the quickest way is with the annual cover crops mentioned above.

References:

Click to access Cover-crops-Inoculation.pdf

Click to access FSA-2160.pdf

https://www.rodalesorganiclife.com/garden/cover-crop-basics

Click to access Ficha_tecnica_096_2001.pdf