Monday, July 23, 2018

Fertilizing Irises

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?

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.
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?

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 fertilizer 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.
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.


  1. Outstanding advice with clear and sensible explanation. This should be read and studied by anyone who plants an iris!

  2. This is fantastic .I have to confess to bring a consumer and practician of fertilizing irises with high phosphate fertilizers. This post is food for thought. Do you recommend an organic slow release fertilizer like Osmocote? I've started using something similar, which makes me feel a bit better about myself, but I don't know if it's actually an improvement.

    1. Thanks! Osmocote is slow release, but it is not organic. It is just a synthetic fertilizer delivered slowly over time as the coating degrades. For the advantages I describe in this post, you need organic matter in some form.

  3. Very enlightening yet very simple. I have quit using boxed fertilizer a few years ago...our soil here in East TN seems fairly good. We built on property that was once pasture, so... I have found mushroom compost, direct from Monterey Mushroom I'd black gold!!! I will share your message...thanks

  4. Thanks, Tom, for sharing your experience and knowledge. Good advice!

  5. Tom thank you for sharing this most informative post. I will take it to heart and work even harder to do things right.


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