by Tom Waters
When I first began growing irises in the 1970s, the standard
advice (and it was not new advice even then) was “fertilize with superphosphate
and/or a balanced fertilizer low in nitrogen, such as 5-10-10, in spring before
bloom and again in fall”. The advice was repeated everywhere, without reference
to climate or soil. This was the heyday of the use of synthetic chemicals in
the garden. Every problem, major or minor, had a solution that came out of a
bag or cardboard box.
Much has been learned since then that should put a damper of
our enthusiasm for synthetic fertilizers. Sadly, however, that knowledge seems
to have not permeated very much into the culture of iris enthusiasts. Almost
daily, I read the same advice I heard decades ago repeated on Facebook and
other discussion fora, still without qualification or any evidence of caution
or indeed reflection. People don’t even seem to care whether their soil
actually needs phosphorus; they just
follow the advice without question.
Have you ever wondered how plant life has flourished on
Earth for more than 400 million years before there were factories to synthesize
superphosphate? Have you ever wondered how the great gardeners of Victorian
England managed to grow irises without plastic bags of fertilizer granules?
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| A walk in a nearby forest. Funny, these trees have gotten awfully large without superphosphate each spring and fall. |
In nature, the nutrients essential to plant growth are
perpetually recycled. All plant and animal tissues contain nitrogren and
phosphorus and the other essential elements, and as these tissues decompose,
soil microbes process them through stages until the nutrients are once again
accessible to the roots of growing plants. Recently, we have become more and
more aware of the complex ecosystem of bacteria, fungi, and small plants and
animals that exists in healthy soil, and the role they play in sustaining the
larger plants and animals that live above ground. Plants on Earth have evolved
in conjunction with soil life to make the most use of the natural processes by
which nutrients are recycled. Have you heard of mycrorrhizal fungi? These soil
fungi exist in symbiosis with plant roots, extracting and processing soil
nutrients for the plants in exchange for carbohydrate food which the plant
produces by photosynthesis. They can increase the nutrients available to plants more than a hundredfold. This is but one example of the complex interaction between
plants and the soil life that supports them. Soil organisms provide many other
benefits to plants, such as reducing susceptibility to pathogens.
If nutrients were not recycled through living soil
ecosystems in this way, every spot of Earth would become completely barren of
life in a short period of time.
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| This handful of soil contains billions of microorganisms - more microscopic living creatures than there are human beings on Earth. |
Feed the Soil, Not the Plants
Synthetic fertilizers bypass this natural process of
nutrient recycling in the soil, essentially giving the plants a direct
injection of specific nutrients, such as nitrogen or phosphorus. This can be
very effective in producing growth, especially if the amount of nutrients
available naturally through the soil is small. The agricultural revolution of
the twentieth century was made possible in large part by supplying additional
nitrogen and phosphorus in this way, thus increasing agricultural yields even
in poor soils. But is there no “down side”?
The first potential problem with synthetic fertilizer use is
that it affects everything in the soil, not just the plants you grow. All the
soil microorganisms now find themselves in a radically different chemical
environment, one they are not evolved to deal with. The additional nutrients
can cause a population explosion in the microorganisms, which then devour every
bit of organic matter in the soil. With the organic matter (their food source)
gone, the microorganisms die off, leaving a soil without organic matter and
without much life. The plants you grow have gotten their quick boost of
nitrogen or phosphorus, and you can pat yourself on the back at how big and
green they have become, but beneath your feet the web of life that supports
them has been damaged or destroyed. As Tony Avent of Plant Delights Nursery
explained, “If all you ate were Snickers bars, would you get larger? Absolutely! No question! You would get dramatically larger. But would you be healthy? That is the difference.”
Once the soil ecosystem has been damaged or destroyed, the synthetic fertilizer “boost” becomes an
addiction. Without a healthy soil ecosystem, the plants now need the regular application of
synthetic nitrogen and phosphorus to provide what the soil would otherwise
provide naturally. The garden is now essentially an experiment in hydroponics,
with the soil merely anchoring the plants in place as you wash solutions of
chemicals past their roots.
Organic gardening uses an approach that seeks to enhance the
natural nutrient cycling process, rather than bypass and cripple it. By
building your soil with compost or other organic matter, the soil life builds
up in a sustainable way. The organic matter not only provides the nutrients
needed by the plants and the soil life, but also provides that soil life with the
carbon-rich organic matter that is its food source. You thus secure not just
the short-term benefit of a nutrient injection, but the long term benefits of
healthy, living soil.
Nutrient Pollution
That might be enough to make a thoughtful person reconsider
reliance on synthetic fertilizer. But there is more. Waterways in the US and
indeed all over the world are being destroyed by synthetic fertilizer use,
through a process called eutrophication.
When excess phosphorous or nitrogen applied to farms, lawns,
and gardens makes its way into streams and lakes, the nutrients create a
population explosion of algae that quickly consume available food and and block
sunlight, depriving the water of oxygen and choking out the other water life.
(Sound familiar? It is not dissimilar to what happens to the soil life when you
saturate them with nutrients.) About half our lakes now suffer from
eutrophication. The situation has become so severe than eleven states have
enacted bans on phosphorus fertilizers. These bans all have various exceptions,
so you may not be restricted from spreading superphosphate on your irises,
depending on where you live. But it should give one pause for thought. If the
environmental damage caused by phosphate fertilizers is becoming so severe that
legislatures are trying to stop it, do we really need to be adding to the
problem in our home gardens? [For an excellent summary of the history of phosphate fertilizer and the problems it causes currently, check out
this episode of the Gastropod podcast.]
Climate Change and Sustainability
Fossils fuels are essential to the production of synthetic
fertilizers, nitrogren and phosphorus fertilizers both. We now know that the
Earth is plummeting rapidly toward higher global temperatures, faster than ever
before in the geologic record, and faster than life can adapt. The Permian
extinction, which eradicated 90% of life on Earth, was triggered by a global
temperature increase of only about 5 degrees Celsius. We need to think of a
better way of meeting our agricultural and horticultural needs, very soon.
The nitrogen for synthetic fertilizers comes from the
atmosphere, but the phosphorus must be mined and extracted from minerals. This
is a finite resource, and it is already under stress. We need to return to the
natural process of recycling the phosphorus that is already incorporated in
plant and animal tissues, rather than extracting the last reserves from the
ground and poisoning our lakes with excess run-off.
Fertilizer Advice for the 21st Century
So if the advice from fifty years ago is so problematic,
what is one to do? Here is how I answer the question of how to fertilize irises.
1. Build your soil. Add lots of organic matter. Compost is
the form closest to what the plants can use, but even partially decomposed
organic matter will benefit the soil. Not only will you be providing nutrients
and encouraging your soil life, but you will be improving the soil’s structure,
too. Soil with organic matter mixed in holds both air and water better, and has
improved texture. This is a win all around. Keep this up.
2. Observe your plants. If you’ve been building your soil
for several years, chances are your plants will be healthy and getting what
they need. You’ll have Earthworms and insects enjoying your soil too. Now go
have a lemonade. Most gardeners will never have to proceed to the following
steps.
3. Identify the problem. If there is a problem, figure out
what it is. If your plants still seem sickly or fail to thrive, have your soil
tested. Don’t just guess and dump things on your soil, because an advertisement
or someone on the internet says a particular product will work miracles. This can do more harm than good.
4. Research solutions. If your soil really is deficient in a
particular nutrient, despite all your soil building work, investigate the
options for addressing the deficiency. There are organic sources for most
nutrients, and those are to be preferred.
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| Yes, they do bloom. This bed was planted eight years ago. No synthetic fertilizers have ever been used. |
And Finally…
These are messages that not everyone is receptive to. I
understand. I started gardening in a time when following that old advice just
meant you were a good gardener. Now, it has come under criticism, and some of
those criticisms seem to carry moralizing overtones. I, like many other
advocates of organic gardening methods, have a certain passion about the
subject. But that does not mean I expect everyone who reads this to have some
kind of religious conversion and abandon their evil ways. Rather, my goal is
more modest. I’d just like to see all iris growers study a bit. Learn a bit
about soil. Learn a bit about fertilizer pollution. Learn a bit about organic
methods. Then follow up with making a few new choices you are comfortable with,
and try them out. Then see what your irises have to say.
