Nitrogen is one of the most important nutrients for growing food, and in some key ways it is
quite a bit different than all of the other nutrients and minerals that plants need to thrive.
In a lot of cases nitrogen is way too abundant, or
so much of it is used in large scale field production that it degrades the soil,
and washes into water courses or aquifers and causes lots of problems downstream.
And the large scale production of nitrogen fertiliser uses huge amounts of fossil fuels,
and increases greenhouse gas emissions quite significantly.
With the rising energy prices, and global supply disruptions, the cost of nitrogen
fertiliser is rapidly rising, and with the need to reduce emissions we need to to find ways
to radically reduce the amount of nitrogen fertiliser that we use in the longer term.
At the same time, nitrogen is often the key plant nutrient that is lacking in many
organic farms and growing spaces being managed by those of us who want to reduce or completely
avoid the use of synthetic fertilisers, or cannot afford to buy them, which in many
cases could be limiting the productivity and viability of the soil that we are working with.
So nitrogen is one of the most common things that many of us go out of our way to import and add
to our soils, or we take the time and space to use appropriate plants to fix nitrogen directly
within the soils we are working with. I have become a lot more focused on
making sure that enough nitrogen is available to the plants that we grow,
and I think it is one of the key reasons why the gardens are being more productive in recent years.
I have been exploring different sources and options for making more nitrogen
available in the different gardens that we manage,
all of them have issues and benefits, and as usual so much depends on context.
One of the key ways of getting more nitrogen into the soil, and the method that I tend
to rely on the least, is to grow plants that will fix nitrogen, or more accurately
host and feed clusters of special bacteria in their roots which will transform nitrogen gas
from the air into a form of nitrogen than can be used by plants and other organisms.
So planting peas, beans, clover and a few other plants can boost the levels of
nitrogen in the soil or at least provide the nitrogen needed for these plants.
But it is difficult to figure out how much nitrogen will become available to
other plants in the gardens, especially if we harvest the peas and beans where
most of the nitrogen that is fixed will be concentrated.
Some nitrogen will be left behind in crop residues, and some may have
been transferred to other plants though underground networks of soil biology.
Any additional boost of nitrogen is welcome in most of the gardens,
but I think to make a substantial difference it would require me to
dedicate a lot more space to nitrogen fixing plants.
It is hard to find useable details about all of this, and no doubt this would really
depend on the context, but according to the agricultural advisory body here in Ireland,
a field of red clover can fix between 100 and 200kg of nitrogen per hectare for a full year.
This would translate into 1 to 2 kg of nitrogen fixed in the 100m2 area of one of the family
scale gardens gardens that we manage, and that seems to be in the region of what a lot of
vegetable crops would need fro that same area. So I could grow a crop such as clover for a
full year, prevent it from going to seed, and incorporate all the biomass that is produced
into the soil, and, assuming that none of it is lost, it could enough to provide all of the
nitrogen needed for a crop of vegetables in the following year on the same soil.
With this approach every season half of each garden would be growing nitrogen fixing plants
and the other half producing vegetables to eat, or some
comparable form of intercropping between the two. And this is the main reason why I don’t rely on
nitrogen fixing plants for most of the family scale gardens, at least for providing most of
the nitrogen for a lot of the other plants. It is easier to import nitrogen from other
sources, and dedicate the full space of the gardens to growing vegetable plants
to eat each year, and a lot of other growers are probably in the same situation.
Of course I also produce a lot of compost, which I use to recycle crop residues back into the garden,
returning some of the nitrogen that the crops may have fixed or extracted.
And some of the compost is from kitchen scraps, and with the community composting facility that
I manage, a considerable amount of nitrogen and other nutrients will be added to the compost,
and a lot of it would have originated in other fields or growing spaces,
locally and potentially very far away.
And I add in organic material from the surrounding landscape, which also adds in nitrogen,
especially with the grass clippings from the reasonably large public space beside the gardens.
In this open space there is a reasonable amount of white clover, which I assume is fixing nitrogen,
and every time I mow it and add the material to the compost, or use as a mulch directly
in the gardens, I am gathering up more fixed nitrogen that will eventually be
available to the vegetable plants. In a way, this area that needs to
be mowed could be seen as the dedicated space for fixing nitrogen for use in the vegetable gardens,
and if I had enough space, and I could find enough material to mix in to balance the compost,
then this space could provide most of the nitrogen requirements for the vegetables.
But the vegetable growing space that I manage is probably too big in comparison,
and the requirements for compost in the gardens I manage exceeds the
amount of compost that I have been able to make. I could put in the effort to boost the harvesting,
and I could sow additional clover seeds in with the grass, but making
more compost takes more effort and time, and not everybody has enough space to do this.
So I have been importing additional supplies of nitrogen by bringing in additional compost,
and buying various concentrated forms of fertility.
In an ideal world, everyone with a vegetable garden would have a friendly organic farmer
in the neighbourhood who would deliver all of the well decomposed farm yard manure they could need,
but most people don’t live in that kind of context.
I have friends close by who have a fantastic mixed farm, but they need all of the fertility
that they produce, as they are still trying to build up the productivity of their own land.
And most of the other conventional farmers in the area rely on slurry pits to gather up all of the
wastes from their animals, so although there is an abundance of this nitrogen rich fertility around,
it is hard to access at the scale I’m working at. I have collected manure from people in the
community who keep horses, which required an afternoon of work and the loan of a trailer.
But I am concerned about the possibility of contaminants in these different supplies,
especially as there have been cases of herbicide making it through to
contaminate my vegetable gardens in the area. More recently I have been buying in large ton
bags of compost from a municipal scale compost facility, simply because it is a lot easier,
and the material is clean, easy to use and doesn’t contain any weed seeds.
I have wondered how much nitrogen I would be importing into my gardens with compost
like this which doesn’t seem very rich. Working with the typical analysis from
the supplier, I was able to calculate that a tonne bag of this compost might contain
between 3.5 and almost 9 kg of nitrogen, though I suspect it is closer to the 3kg as the carbon
to nitrogen ratio seems quite high still. And if I spread this one bag over one of
the family scale gardens, it would cover the beds to a depth of about 12mm or half an inch,
and translate to about 300kg of nitrogen per hectare, which at first seems surprisingly high.
Of course most of the nitrogen is not going to be released until the material has had a chance
to decomposed some more, and in the process that nitrogen might be released quite slowly.
And during that time some of the nitrogen might be washed away by the rains, and even more of it
might be released back into the atmosphere through the activity of some of the soil organisms,
and apparently even the most stable forms of humus that remain in the soil for quite some time,
still contain a fair amount of the nitrogen that was in the original compost material.
So I find it hard to estimate how much nitrogen would be available
to the plants this season by adding that 12mm layer of compost to the growing beds,
or how thick of a layer I would need to add to ensure the nitrogen hungry plants would
be able to access all the nitrogen they would need for the full season.
One way I deal with this uncertainty in some of the gardens is to simply add more compost,
which is quite a common approach, and over time, as the successive lavers of compost
continues to decompose, sufficient amounts of nitrogen will hopefully be released.
I have also been exploring the use of concentrated amendments that I buy in
to supplement the compost in other gardens. The supplier that I get most of my materials and
equipment from, also has a selection of different types of concentrated fertility, all of which are
seen as appropriate for organic production. This includes chicken manure pellets,
which have been the main source of concentrated fertility that I have been adding for a few years,
which contains 4% nitrogen, plus reasonable amounts of other nutrients.
If I added some compost to one of the gardens and wanted to supplement the nitrogen content with an
extra kilogram of nitrogen for the full 100m2 space, this would require a whole 25kg bag of
chicken manure pellets at the cost of about €25. Other granular fertilisers that I can buy from
the same supplier all have significantly higher concentrations of nitrogen of about 12 to 14%
This means I would need to add less, and these different types of fertility
end up being about half the cost of the chicken manure pellets to get the same 1kg of nitrogen,
so definitely a better value, especially if I don’t think I need to import the other nutrients.
There are several options for liquid feed, but using these forms of fertility would end
up costing almost twice the chicken manure pellets, to supply the same 1kg of nitrogen.
But these liquid feeds might be worth it, especially used as a foliar feed, as apparently
less would be needed to provide the same benefit to the plants as adding the nitrogen to the soil.
Of course this is only one supplier, and there are a lot of other options,
especially in other parts of the world, but if I have the money, it is a lot easier and
faster to simply order in a bag of fertility and spread it on the soil in appropriate quantities,
than it is manage the bulk of a compost pile or to dig in a nitrogen fixing green manure.
Another source of nitrogen for the garden, and one that I have been looking into a lot lately, is to
buy in materials that are usually sold as animal feed, where dividing the listed percent protein
by 6.25 will apparently give you the estimated percent of nitrogen that the feed would contain.
While this stuff is used to feed livestock and other animals,
it can be added to the soil where it is consumed by the soil organisms
which then make the nitrogen and the other nutrients available to the plants.
The recommendation for the Extensive Garden are to use seed meals, which
are hard to source here in Ireland, so I am going to experiment with using rolled oats
as a possible substitute, which are much more widely available from the local feed suppliers.
Because of the lower protein content, a lot more would need to be added to the soil to supply the
same 1kg of nitrogen, which makes the rolled oats about the same price as the chicken manure
pellets, but it will also be adding a lot of other nutrients and concentrated organic matter.
But I have been informed that the rolled oats may not release the nitrogen as quickly as the
seed meals will, which is an important factor with all of these amendments,
and with the compost, as the rate at which the nitrogen is released to the soil,
and how quickly after adding it to the soil can be important factors to consider.
I was keen to see the difference that seed meals would make and I was able to source
some rapeseed meal from elsewhere in the EU, and if I don’t include the shipping,
the cost for providing that 1kg of nitrogen is within the range of the other amendments.
Another common animal feed around here is hay, and I was able to get
a load delivered by a local farmer at €4 a bale, to use as part of a no-dig trial.
Hay can be quite variable in its protein content, but I calculated that one of the
bales that I bought might contain about one quarter of a kilogram of nitrogen,
if it has a protein content of about 10%. So, in theory, 4 bales of hay would provide
the 1kg of nitrogen, eventually, after it fully decomposes,
but I would probably want to start with a lot more or add it earlier to ensure an adequate supply of
nitrogen was released for the vegetable plants, similar to when using lower quality compost.
All of these sources of fertility have their own benefits, potential uses, issues and costs.
Growing nitrogen fixing plants locally takes more land, and effort, which could otherwise
be used for additional food production. Manure and compost and a lot of the other
material, could contain traces of compounds I might not want to import into my soils.
But even the compost I end up from my community composting system contains
a fair amount of plastic and things that I don’t want to add into the soil,
and it does require a fair amount of work to sift through it all.
Cost of all of this is definitely a factor for some people, and the amount of effort and time
that it takes to manage the bulk of so many other materials may be a key factor for other people.
Some people would prefer to avoid using anything that came from the slaughter and
processing of animals, or that have anything to do with the management of animals at all.
And there is definitely an issue with using materials to feed plants that could otherwise
be used as food for animals or even people. Having animals to eat this stuff, and then
to use the fertility they produce makes a lot of sense, but not everybody can keep animals,
or would necessarily want to keep them simply to provide more fertility for their gardens.
Some people just need the easy and fast option, while others want to be as self
sufficient as possible, and other people want to avoid anything that comes from a factory.
And of course the availability and appropriateness of any of these materials and many more
really depends on the context.
It is easy to get lost in all of this,
but I think there are two other key options that could be considered.
The first is to use conventional fertilisers that are widely available,
where the nitrogen it contains is produced through the energy and emissions intensive
Haber-Bosch process, which is the source of the vast majority of nitrogen that is used globally.
And that is probably the original source of most of the nitrogen in the materials that
I have talked about, especially with the hay and oats that are available locally.
And this raises the question whether it is better to simply buy in and use this mineral fertiliser
directly, rather than getting somebody else in some other place in the world to use the
same fertilisers, and transform it into a material that I’m more comfortable adding to my own soils.
The other key option is to close the nutrient cycles, to use our own urine or humane, as a
key component of the fertility in or food systems. A lot of people have issues with using synthetic
fertilisers in their own growing spaces and would even prefer to buy food grown without them,
and even more people would have serious problems with using the wastes from our own bodies to
grow our food, despite this being a key part of food systems for millennia.
I think that both of these sources of nitrogen can be used much more effectively and safely,
and in ways I think they are perhaps not so different, and I am interested in exploring
their use in the pot trials that I am conducting. For this growing season the nitrogen that will
be supplied to all of the plants in the gardens we are managing will come from a
variety of different sources, but most of it still comes from compost we produce and the
stuff that we buy in and have delivered. I would love to explore a lot more about
how to increase the amount of nitrogen that is fixed within the local landscape,
as I think this is an important part of the food systems as we head into an uncertain future.
But for now, there is an abundance of fertility available to small scale growers around the world,
and a lot of it is fairly convenient, and still relatively inexpensive,
especially compared to the amount of extra food that we can produce.