fertilizer – The Garden Professors™

To Fertilize, or Not to Fertilize, that is the question

You see a bright shiny package at the garden center saying that it can help you have the most bountiful garden ever, the greenest lawn in the neighborhood, your plants will have miraculous growth, or it will supply every element on earth to make sure that your plants are living their best life. It’s got what plants crave….It’s got electrolytes! You reach out to grab that package and ……. Woah! Pump the brakes! Do you know if your plants even need to be fertilized? Are you just falling for that shiny marketing, or do your plants really need added fertility to grow?

It turns out that many gardeners add fertilizer out of habit or because a shiny package or advertisement told them they needed to do it. The fact is, though, that you may or may not need to add fertilizer to get your plants to grow healthy. It is actually more likely than not that the level of nutrients in soil is perfectly adequate for healthy plant growth. And guess what, there really is a way to know what plants crave…or at least are lacking: A soil test.

We here at the Garden Professors (and those of us who work in extension) often get questions or hear comments about gardeners adding fertilizer or random household chemicals and items to their plants and soils with no idea what they do or even supply. They’ll throw on the high powered 10-10-10, the water soluble fertilizer, rusty nails, or even (shudder) the oft mentioned Epsom salts because it is just what they’ve been told to do.

A few months ago, my GP colleague Jim Downer talked about why to amend soil– focusing mainly on organic material and a little bit of fertility. In this article, I’m going to share some how and what: what plants need in terms of nutrients, how to determine what nutrients you need to add, what you can use for increasing fertility (conventional and organic), and how to calculate how much fertilizer to add.

What plants really need

Plants have a number of essential plant nutrients that they need from the environment in order to properly grow and function. Hydrogen, carbon and oxygen are all important, but are not something that gardeners have to supply since they are taken in by the plant in the form of water and carbon dioxide (unless you forget to water your plants, like I sometimes do — but death will occur from dehydration well before lack of hydrogen).

There are six soil macronutrients, which means that they are used in larger amounts by the plants. These include nitrogen, phosphorous, and potassium, which form the basis of most common fertilizers that have those magic three numbers on them (example: 10-10-10). Those three numbers indicate that the fertilizer contains that percentage of the elemental nutrient in it. For this example, the fertilizer contains 10 percent nitrogen, 10 percent phosphorous, and 10 percent potassium. The other three soil macronutrients are magnesium, sulfur, and calcium. Depending on your location, your soil may be abundant or deficient in these nutrients, especially magnesium and calcium. Sulfur is commonly released during decomposition of organic matter, so it is usually present in sufficient amounts when soil is amended with (or naturally contains) organic matter.

If a soil is deficient in a nutrient that a plant requires it is usually a macronutrient since plants use them at higher levels. However, deficiency is still unlikely in most soils unless there is a high volume of growth and removal, such as in vegetable gardens and annual beds (or if you’re growing acres of field crops like they do here in Nebraska). These are also the nutrients that are most common on soil tests, since they are the ones that are used the most by plants.

Soil micronutrients are needed in much smaller amounts. Those nutrients are boron, copper, chlorine, manganese, molybdenum, and zinc (remember the periodic table?). These are also usually supplied from organic matter or from the parent soil material so deficiency is even less likely than for macronutrients. Tests for these aren’t usually part of a basic soil test, so if you suspect you might have a deficiency you might have to get a specialized test. There are some basic physiological signs of deficiency that plants might exhibit in response to specific deficiencies, but their similarity to other conditions make it an imprecise tool for diagnosing a deficiency.

Compost is a good source of nutrients, especially micronutrients (as we’ll read later). Using compost alone may be sufficient for many gardens, such as perennial beds. However, higher turnover and higher need areas like vegetable gardens may need supplemental fertilization beyond compost. That’s where the soil test comes in.

What’s on the menu….interpreting soil test results

If you’ve had your soil tested by a lab (which is recommended, since it is much more precise than those DIY test kits), you’ll get results back that give you the level of nutrients in your soil and usually recommendations for how much of each nutrient you need to add to the soil for basic plant health. This is a general recommendation that is common for most plants, which is generally sufficient for average growth. If the test says that the nutrient levels are normal, you don’t have to add anything….I repeat….YOU DON’T HAVE TO ADD ANYTHING. If it says you need one nutrient of the other you’ll need to add it to your garden or around the plant. As we’ve said before, disturbing the soil as little as possible is best, so if you’re using a fertilizer product aim for one that you can broadcast on top of the soil or is water soluble. This goes for compost as well – try to apply it to the top of the soil and it will incorporate over time.

Your soil test results will usually tell you to add nutrients in pounds per a certain square footage. In the example pictured, there’s a recommendation of 3.44 lbs of Nitrogen per 1000 square feet. That number is for the actual nitrogen, and since different nutrient sources have different amounts of nitrogen you’re going to have to do some math to figure out how much fertilizer you need per 1000 square feet and then multiply that by how many thousands of square feet you have.

I’ll note here that soil labs do not usually test for nitrogen due to the variable nature of nitrogen in the soil and the lack of affordable or reliable tests. Nitrogen fluctuates widely over a short period of time and is not as persistent in the soil as other elements due to plant take-up, microbial action, and weather conditions. Nitrogen recommendations are usually made based on the crop indicated for the test and may be informed by the levels of other nutrients.

Let’s say that I’m using an organic fertilizer product I purchased at the garden center and the nutrient analysis is 4-3-3 (these numbers are standard for organic nutrient sources, which have lower nutrient levels than conventional fertilizers). That means that for every 100 lbs of that product, 4lbs are nitrogen, 3 are phosphorous, and 3 are potassium. My (hypothetical) garden is 10ft by 20ft, which is 200 square feet.

So we divide 200 by 1000 to get .2, which represents that my area is 20% of the area listed on the recommendation. If my garden were 3500 square feet, then that number would be 3.5.

Next, multiply the Nitrogen recommendation of 3.44 lbs by .2. This give me 0.688. This tells me that I need .688 lbs of nitrogen to amend my 200 square feet.

So I just need to weigh out .688 lbs of the fertilizer, right? Nope – we have to account for the fact that my fertilizer is only 4% nitrogen- only 4 lbs out the 100 lb bag. We can estimate amounts by figuring out how much nitrogen is in smaller amounts of the fertilizer. Since we know that 100lbs has 4lbs of N, then 50lbs has 2lbs of N, and 25lbs has 1lb of N. If I want to get a more precise amount of fertilizer poundage to get my .688 lbs of N, then we divide the pounds of N needed by the decimal percentage of N in the fertilizer. So that would be .688 / .04, which gives us 17.2 lbs of fertilizer.

Now, considering that the bagged product that I bought is $25 for 8lbs, I may want to reconsider using it for this application…unless I enjoy throwing my pearls before swine or I’m fertilizing my money tree.

If you do the math, you’ll note that this fertilizer will add more than the recommended amount of phosphorus and potassium. You’ll either need to decide if that is acceptable or if you need to find another source of nutrients.

If you’re not using a prepared fertilizer product but rather an organic source of nutrients, you can still calculate how much to add to get to the recommended amount. The following are some good lists of nutrient ranges of organic materials:

https://extension.psu.edu/using-organic-nutrient-sources

https://vegetableguide.usu.edu/production/soil-nutrient-water-management/organic-nutrient-sources

A note about pH

Another thing your soil test will tell you is the pH of the soil. In general, plants prefer a soil pH just on the acidic side of neutral (between 6.0 and 7.0). There are plants that prefer different pH levels – such as blueberries and azaleas and their need for a more acidic soil between 4.5 and 5.2. Changes in pH affect the availability of nutrients to plant by affecting ionic bonds of the elements. For the most part, the nutrients are more available at that neutral pH. You’ll note that iron is more available at lower pH levels, which is why those acid-loving plants grow better at lower pHs – they’re heavier iron feeders.

If your pH is extreme in one way or the other, you’ll either need to find plants that thrive at that level or adjust the pH if that isn’t possible. To raise pH in acidic soils the most common method is application of lime. To lower pH, you’ll need something high in sulfur. For more information, visit https://articles.extension.org/pages/13064/soil-ph-modification .

Having a philosophical moment in the garden

NON-GMO FERTILIZER?

http://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1057077340&topicorder=3&maxto=14 — Image courtesy of Plant & Soil Sciences eLibraryPRO at UNL

I was asked by Dr. Linda Chalker-Scott to look up some information in order to answer a recent comment and question on a previous post.

Paraphrased, the question is, “… are there any verifiable “organic” fertilizers that can be guaranteed to be made from 100 percent non-GMO sources.”

First off, let me state up front that the whole “Non-GMO” labeling effort is pure marketing. There is no evidence to suggest products that come from genetically engineered crops are any different than crops made from other plant breeding methods. The body of evidence in fact suggests they are as safe as their conventional counterparts, and have some excellent benefits to farmers and consumers from an economic and environmental standpoint.

Having gotten that disclosure out of the way, and assuming that price is not a factor, it turned out to be an interesting question to answer.

“USDA Certified Organic” fertilizers would be problematic, since there are exceptions to the organic standards, which allow manures fed GE crops to be used.

Similarly, oilseed meals like cottonseed, soybean meal, etc. also can be certified organic, even though they come from genetically engineered crops.

ALFALFA MEAL

One possible alternative in that category is alfalfa meal, since genetically engineered alfalfa is currently grown on only 13.5% of alfalfa acreage, whereas in the case of cottonseed, soybean, sugar beet, and corn products, the rate of adoption of genetically engineered crops is well over 90% of U.S. acreage.

Only about 13.5 percent of harvested U.S. alfalfa acreage is genetically modified, compared to more than 90 percent of corn, soybeans, cotton, canola and sugar beets acres, according to a new USDA report that cites 2013 farmer surveys.

It appears likely the percentage of genetically engineered alfalfa will continue rising, though: Roughly one-third of newly seeded acreage planted that year was of a biotech variety resistant to glyphosate herbicides, USDA said.

Farmers have been slower to adopt genetically engineered alfalfa partly because it’s a perennial crop that stays in the ground for roughly five years, said Dan Putnam, an alfalfa extension specialist at the University of California-Davis.

It would be incumbent upon the buyer to ask, however, if the alfalfa meal came from a grower who does not use genetically engineered alfalfa, and whether or not the supplier of the alfalfa meal guarantees that.

MANURES FROM LIVESTOCK FED ONLY ORGANIC FORAGE

https://commons.wikimedia.org/wiki/File:Hestemøj.jpg — Manure, a field in Randers in Denmark. Image courtesy of Malene Thyssen at Wikimedia Commons

“Demeter USA” … the private certifying entity that guarantees “Biodyamic” preparations does require that any manures come from livestock fed only “USDA Certified Organic” feed. So manures that carry that seal should satisfy the question.

As an aside, here is Dr. Linda Chalker-Scott’s literature review of “Biodynamics” and why that certification has little science to recommend it.

Further, finding the product would be difficult, since it is primarily produced on-site at certified Biodyamic farms, and used there.

SEAWEED FERTILIZERS

https://commons.wikimedia.org/wiki/File:KelpforestI2500ppx.JPG — An underwater shot of a kelp forest. Image courtesy of Wikimedia Commons

Next products that might qualify are seaweed, or kelp products. There are no genetically engineered seaweed/kelp products I’m aware of. However, there are real concerns about the sustainability of harvesting seaweed and kelp from the wild.

Dr. Linda Chalker-Scott wrote about them here:

The ecological impacts of increased seaweed harvesting are currently under investigation and the possibility of significant ecosystem damage is real.

There is however, some interesting research and startup companies that are farming seaweed and kelp for a variety of potential uses.

I can’t however, find any products available for the home gardener that are sourced from this effort. Still early.

So, when it comes to seaweed/kelp products, you’ll have to (again) ask a reputable supplier to answer the “sustainable” question.

BAT GUANO

https://en.wikipedia.org/wiki/Bat_Cave_mine — Bat Cave Mine Image courtesy of Wikimedia Commons

In a similar vein, “Bat guano” products would also qualify as “non-genetically engineered”, but the sustainability question also comes into play. How is it harvested?

I can’t deny that it’s a great fertilizer, but if you want to use an organic fertilizer why not at least consider one that is renewable instead of one that is from a limited resource and which may cause harm to a unique ecological system?

SEAFOOD BY-PRODUCTS

Fertilizers made from by-products of the seafood and fish industries, assuming they don’t come from aquaculture farms, since the livestock feed for those operations could be sourced from genetically engineered crops, do have a history.

Two links (there may be others, but these seem sufficient for now), a comprehensive review of products (including fertilizers) from the Alaska seafood industry, put together by Oregon State University …

Fish Fertilizer Product Descriptions

Fertilizers are characterized by their Nitrogen-Phosphorous-Potassium content (N-P-K). Therefore all fish material will have some fertilizer value since fish contain protein which is Nitrogen,
the bone contains Phosphorous and the flesh and bone contain Potassium. Generally, fish products are re-allocated to fertilizer use for any number of reasons including quality too poor for feeding, volume too small to convert to fishmeal and oil, and an available agricultural market in the vicinity of the waste material.

And a similar document put together by Michigan State University about the use of fish by-products for other uses.

In an effort to help the Michigan fish processing industry find better solutions to handle fish processing waste materials, a project was initiated to determine the viability of composting fish waste.

CHILEAN NITRATE

https://commons.wikimedia.org/wiki/File:Dusičnan_sodný.JPG — Mined Sodium Nitrate (NaNO3) Image courtesy of Wikimedia commons

There is a mineral product called Chilean Nitrate or Nitrate of Soda that is mined from a desert in northern Chile that is allowable for use under the standards for organic production in the U.S. However, it is not allowed for use under Canadian, or international organic standards, and a change to prevent its use under U.S. standards is still pending. Up until 2012, this was the wording for its use.

Sodium nitrate, also known as chilean nitrate, cannot account for more than 20 percent of the N requirements of organic crops in the United States.

Its use is also prohibited by the International Federation of Organic Agriculture Movements (IFOAM) and most other standards for organic production outside the United States.

After 2012, the 20% restriction was dropped in the U.S.

The expiration of the current notation will effectively mean that sodium nitrate may be used in organic crop production without a specific restriction on the amount used: however, producers must continue to comply with all requirements of the soil fertility and crop nutrient management practice standard.

Although the National Organic Standards Board (NOSB) recommended that sodium nitrate become a completely prohibited nonsynthetic substance, the NOP has not issued rule-making to carry out this recommendation as of yet.

FEATHER MEAL

A by-product of the poultry industry. Is it from poultry fed only non-GMO feed?

LOCALLY PRODUCED

The final piece of the puzzle can be found (only partly in jest) in Dr. Jeff Gillman’s post about a cheap, locally available source of Nitrogen.

You’d be saving yourself the cost of fertilizer, saving the environmental cost of shipping the fertilizer you might otherwise purchase, saving water, and you’d have something unique to tell your gardening friends about. Win – win situation as far as I’m concerned.

In summary, I don’t buy into any of the fear-based marketing of products that come from genetic engineering. There may be (at this time) sources of alfalfa meal that do not come from genetically engineered sources.

Biodynamic manures certified by Demeter USA require that the animals be fed only “USDA Certified Organic” feed, but will be difficult to come by. Seaweed/Kelp and Bat guano products would qualify, but have major sustainability questions about them. Lots of potential with seafood/fish by products, and finally … a personal possible solution.

Many thanks to Emanuel Farrow, a consultant to both conventional and organic farmers, who helped point me in the right direction and provided important fact checking expertise for this post.

Out of the bottle and into the bag

Last week I was having lunch with my mom at our favorite nearby nursery/café. After failing to resist the grilled cheese sandwich (3 cheeses! And buttery panini bread!), we walked off lunch in the garden supply part of the nursery. Normally I’m on my best behavior when I’m shopping with my mom (i.e. I don’t take photos of things I’m going to take to task on the blog). But like the 3-cheese grilled sandwich I was unable to resist the bags of biodynamic compost.

Biodynamic compost is now available at garden centers

Long-time readers of the blog may remember my earlier column and post on biodynamics. Since I wrote the original column over 10 years ago I’ve watched biodynamic marketing move from boutique wines to coffee, tea, tomato sauce…and now to garden products. Really expensive garden products, as in $19.99 for one cubic foot of compost.

An "untapped source of power and majesty" makes this compost different. — An “untapped source of power and majesty” makes this compost different.

What makes this bag of compost worth $19.99? One has to assume it’s the biodynamic preparations used to treat the compost. They’re referred to in the label under “concentrations of yarrow” and so on. Do these preparations make a difference? The label suggests it might be to restore the soil’s vitality. Is there validity to this claim?

It's doubtful that all of these ingredients are locally available. And why are so many materials needed? — It’s doubtful that all of these ingredients are locally available. And why are so many materials needed?

In 2013 I published a review of the scientific literature on biodynamics, specifically looking at whether biodynamic preparations have a measurable impact on anything they’re applied to. In a nutshell, the answer is no. (Though this article is behind a paywall, I can send a pdf to you by email if you’d like to read it.)

Don’t let packaging and magical words sway you. Compost made with local materials like bark or agricultural wastes and certified by the US Composting Council is reasonably priced and sustainable.

What about fall fertilization?

Posted by Bert Cregg
We had a question on the Facebook site regarding fall fertilization of landscape plants. Fertilization in general, and fall fertilization in particular, is a complex topic and needs a little more room for explanation than the Facebook discussion allows.

As a general rule, most landscape trees and shrubs can maintain acceptable growth and appearance without fertilization. There are a couple of reasons for this. As Linda noted in the Facebook discussion, woody plants are fairly efficient at internal nutrient recycling. I’ve done a couple of studies where we sampled leaves of hardwood trees during the season and then re-sampled right after senescence and about 50% of leaf nitrogen is re-absorbed by trees before they fall. Conifers are even more efficient at conserving nutrients than hardwoods since they typically only lose 1/4th of their needles (or less) each year. In addition, many landscape trees are able to utilize fertilizer that is applied to surrounding turf. On the flip-side, nutrients that occur in litterfall are removed from the nutrient cycle in many suburban landscapes and this may eventually contribute to deficiencies.

Bottomline, landscape fertilization should be based on need; which can be assessed based on soil sampling, foliar sampling, or visible symptoms. At least two of the three methods should be employed to make a diagnosis. Each method has drawbacks and visible symptoms are usually the least useful since many nutrient deficiencies have similar symptoms or the symptoms may not be nutrient-related at all. In our area the only nutrient problems I am comfortable diagnosing based on visible symptoms are iron chlorosis in pin oaks and manganese deficiencies in red maples, both of which are induced by alkaline soils, not a lack of those particular elements.

So assuming we’ve established that fertilization is needed, what about fall fertilization? There are a couple of arguments that are usually brought forth for fall fertilization. One is that trees can absorb nutrients during the fall and then use them for spring growth. This is generally true provided that soils are warm enough to allow continued root growth and absorption. Another argument is that fall-applied fertilizer that is not taken up by roots in the fall be will available for uptake when soils warm again in the spring. A third, and less scientific reason, is that fall is often a slow time for arborists and landscape companies and fall fertilization is an easy service to add to their sales program.

There are a couple of objections that are usually raised to fall fertilization. One is that nutrients will leach through the soil over winter before they can be absorbed. This is one of those ‘it depends’ scenarios. If a nitrate-based fertilizer source is used, this is possible since negatively-charged nitrate anions won’t bind to negatively-charged cation exchange sites in the soil. If the nutrient source is urea or ammonium-based, the amount lost will be dependent on temperature since this will drive the conversion from ammonium, which can bind to cation exchange sites, to leachable nitrate.

The other usual objection to fertilizing trees in the fall is that it will reduce cold hardiness. There is no clear evidence to support this, however. Harold Pellett and John Carter at the University of Minnesota compiled dozens of studies on the effects fertilizer on plant cold hardiness (Horticultural Reviews 3:144-171). For conifers and temperature hardwoods they found no clear trend across studies, except that fertilizing with potassium improved cold hardiness is most cases (see table). The common perception that fall fertilization, especially with N, will increase cold damage probably stems from studies of fertilization of turf, which had negative impacts in 26 out of 29 studies cited by Pellett and Carter.

In summary, landscape trees and shrubs should be fertilized only where there is a demonstrated need. Fall is a good time to fertilize provided you avoid nitrate-N sources that will be prone to leaching.

Phosphorus and Big Macs

Minnesota, and I were cruising through old pictures and files and getting all sentimental about the cool stuff we used to do. A lot of it was never published just because after we were done with one thing we were just too damn excited to move on to the next. Anyway, one of the neatest experiments that we never wrote up was a phosphorus experiment. Here’s what it looked like to the casual observer.

Now let me explain the neat part to you a little. Inside those boxes, underneath three of the six plants in each container, are vials set up like this – three vials per plant (the black tubes provide air to the vials).

Each plant had one root placed into each of the three vials – one vial contained 1 ppm phosphorus, one vial contained 10 ppm phosphorus, and one vial contained 30 ppm phosphorus. The tub itself was also filled with one of these three solutions (1, 10, or 30 ppm phosphorus) as seen below.

At the end of the experiment we weighed the roots filling each vial, as well as weighing all of the roots from each plant. Here’s what we found for the individual vials.

As you can see, more phosphorus in a vial meant that the plant would devote more energy to growing roots there – but also notice that the 10 ppm solution has the greatest mass of roots overall. Here’s what we saw when we looked at the total size of all of the roots from plants for the different solutions.

As you can see, the roots from the plants in the 10 ppm solution are the largest (shoots showed the same trend). So here’s the way I see it (this is the Big Mac part). I love Big Macs. If I see a McDonald’s I want to go in there – I gravitate towards McDonald’s to get Big Macs. But too many Big Macs aren’t good for me. They might even stunt my growth! It’s the same for phosphorus. Roots do grow towards phosphorus (this isn’t technically correct, but it works for my analogy so I’m sticking with it!), but that doesn’t mean that a tremendous amount of phosphorus is actually good for them. In fact, it might even stunt their growth! This could be for a variety of reasons, but most likely because the phosphorus would interfere with the uptake of other elements.

Good Stuff

Boy oh boy, what a fun day! People yelling at me from the left and from the right. But hey, I didn’t start doing what I do to make everyone happy. With that said….Nah, I don’t feel like attacking anyone today. Instead, let’s look at a good renewable fertilizer: Cotton seed meal. It’s got a reasonably good ratio of nitrogen to phosphorus and potassium — slow release of course. Basically a waste product given a meaningful purpose. And look at the label — no mycorrhizae or other gimmicks. Just pure, unadulterated, cotton seed meal. This is what I want on my garden.

Does fertilization increase insect herbivores?

Always fun when you find a research paper that confirms what you’ve suspected all along. I ran across a paper last week in the Annals of Applied Biology entitled ‘Fertilisers and insect herbivores: a meta-analysis’ (Butler et al. 2012. Ann Appl Biol 161: 223–233). I’m interested in the topic because in recent years a dogma has emerged that if you fertilize a landscape tree it will be immediately devoured by insects. In this study the authors conducted a meta-analysis (basically a compilation of studies on a given topic and then combining and analyzing the aggregated results) and looked at dozens of studies of the response of insect herbivores to fertilization to answer the question, does fertilization increase insect damage? The answer was absolutely no surprise to me: It depends.

What does it depend on? First, what type of insect. Secondly, what kind of fertilizer. For example, fertilizing with nitrogen greatly increases populations of sucking insects. This makes sense when you stop to think that aphids and other sucking insects have to consume a lot of phloem sap –which is essentially sugar water – in order to get sufficient nutrients. Nitrogen fertilization did not significantly increase populations of chewing insects, however. This could be related to off-setting effects of improved nutritional quality of leaves versus increased presence of defense compounds or leaf toughness. For other fertilizer elements Butler et al. found that phosphorus decreased insect populations in 2/3^rd of the studies (14 out of 21) and that potassium decreased insects in 7 out of 10 cases. As with nitrogen only, complete fertilizers (NPK) tended to increase insect populations, especially for sucking insects.

I should hasten to point out some limitations of the study as it relates to tree fertilization. First, of course, is the British spelling of fertilizer. Second, the study mainly dealt with fertilization in agronomic crops, not trees. Lastly, the authors only included studies on insect adults. In many cases insect larvae, not adults, are the most damaging life stage, especially for insects that affect trees. Nevertheless, the study highlights the difficulty of making generalizations when discussing host stress and insect interactions. In addition to type of insect and type of fertilizer, we could have added nutritional status of the plant before fertilization to the ‘It depends’ list. My rule of thumb is that trees shouldn’t be fertilized unless a problems is noted by visible symptoms, a soil test, and/or a foliar test – and preferably by more than one of these.

Bottom line: Before you buy into the notion that fertilizing a tree is going to increase insect problems make sure you know what type of pest you’re dealing with, what type of fertilizer and the current nutrient status of the tree.

A Challenge

As I was looking over the label on a bag of fertilizer this morning I was reminded of the time, a few years ago, when a friend of mine and I went to a local K-mart and decided to see what the people in the gardening section knew. We started small—we went over to a bag of fertilizer and my friend asked what the three numbers on the bag meant. Now, as most gardeners know, those numbers indicate the amount of nitrogen, phosphorus and potassium in the fertilizer. Unfortunately the guy we asked told us that those numbers were actually a computer code…We never did find out exactly what this computer code was for. I have no idea why the guy couldn’t just say “I don’t know”. We had intended to ask more questions, but both of us were too stupefied to continue.

So I have a challenge for all of you this weekend—I’m curious to see who takes it up—go to a box store, or a garden center—your choice—and ask them what the three numbers on the bag of fertilizer are for. You can list responses in the comments section below—or feel free to e-mail me directly at gillm003@umn.edu if the answers are too embarrassing!

Pop quiz time!

It’s the start of new semester. Best way to get student’s attention is with a pop quiz right off the bat! So in that vein, we’ll cross things up and give a quiz on Monday instead Friday. Relax; to make things a little easier we’ll make this one a matching exercise.

Here goes. At our recent Christmas tree conference in Austria, a colleague of mine at Oregon State University, Chal Landgren, presented the results of a study to look at the effectiveness of foliar fertilization on Nordmann fir. Trees were grown in 15 gallon containers and assigned to one of four groups:

1) control: no fertilizer

2) soil applied controlled release fertilizer

3) foliar nitrogen fertilizer

4) soil applied fertilizer + foliar feed

Since Chal has yet to publish this I need to be a little careful with details but all fertilizers were commercially available products labeled and marketed for this purpose and were applied at manufacturers’ suggested rates and intervals.

At the end of the growing season, the trees were sampled for needle nitrogen content. As a point of reference a needle nitrogen content of 1.5 – 1.6 % is usually deemed adequate for this species.

For your quiz: match the treatments listed above to the nitrogen concentrations below:

a) 1.14%

b) 1.91%

c) 0.98%

d) 1.70%

Answer and discussion tomorrow…

Our visiting GP takes on fertilizers

Like many readers of this blog, I’m like a kid in a candy store where plants are sold. I try to justify the extra cost of a large annual pot instead of a scrawny 4-pack, or I imagine I’ll find room for that lime green Heuchera and my wife will learn to love it. But unless I keep my blinders on and stick to the shopping list, I’ll probably leave with a fertilizer. This year, I’ve purchased 12-0-0, 5-6-6, sulfur, and some 5-1-1 liquid. Those go with my 6-9-0, 11-2-2, 9-0-5, 2-3-1, and 4-6-4. I can explain why I ‘need’ each one. I have a decent idea what my soil is like because I’ve had it tested (though I’m due for another test). But I’ve always questioned how those bags of fertilizer can know exactly what my garden needs. The rates listed on the bag imply they’re universal under all circumstances and will give great results if the directions are followed. Is that true? And at what cost?

For example, 2 of the bags are listed as ‘lawn’ fertilizers (the veggie garden doesn’t care about that though). But if I apply these to my lawn at the rate listed and 4 times per year, I’m adding 3-4 pounds of nitrogen per 1000 ft². That’s a reasonable rate if I irrigate and bag my clippings, but I don’t do either. Therefore, I only need ~1 pound of nitrogen, not 3 or 4 (see this publication for more info). I just saved myself some money by disobeying the bag. That extra nitrogen isn’t useful for making MY lawn healthy.

One of my fertilizers is labeled ‘tomato’. If I do exactly as the bag tells me for tomatoes, I would be applying the equivalent of 400 pounds of nitrogen and almost 500 pounds of phosphate per acre. So what? Well if I look at a guide for how to grow tomatoes commercially, I’d notice that the recommended nitrogen rate is 100 to 120 pounds per acre, and phosphate is 0 to 240 pounds per acre. Yes, those are commercial guidelines, but they shouldn’t be too far off from garden recommendations. And of course, recommendations should always be based on soil tests. But 4 times the N and 2 to infinitely more times the amount of phosphate than is required? That’s likely a waste of money at least. And yes, those recommended guidelines are real: you CAN grow food without adding phosphate or potassium-containing fertilizers. If the plants you’re growing don’t need much and your soil has plenty, you don’t need to add any.

Say I’ve got an acre of onions (Fig. 1; not quite an acre). One of the bags of fertilizers, were I to follow its instructions for fertilizing ‘vegetables’, tells me that I should add 100 pounds of nitrogen and 120 pounds of phosphate and potash at planting (per acre), followed by half that partway through the season (next to the row). The commercial production guidelines tell me that the nitrogen rate is similar to what the bag of ‘vegetable’ fertilizer says, but I actually need about 7 times less phosphate and potash (based on my soil test results; I have quite a bit of P and K already in my clay-loam soil). I don’t want to add stuff my soil doesn’t need, so I use my shelf full of bags, a scale for weighing pounds of fertilizer per cup, and some math to come up with a custom fertilizer regime that suits my soil and the onion’s needs (see Table 1, and remember that the numbers are for MY soil, not necessarily yours).

One problem with using extra fertilizer may be in the extra cost (wasting nutrient the plant won’t use), but that depends on what fertilizer it is and how much it costs. Another problem may not be immediately apparent, and that is nutrient deficiencies. Too much phosphorus can cause zinc deficiencies, for example. Excesses of some nutrients can create greater chances for pest and disease problems. One big problem with using too much is the potential for these extra nutrients to go where they shouldn’t be, like in groundwater, rivers, lakes, and streams. And as Jeff has mentioned, phosphorus fertilizers won’t be around (cheaply) forever.

Do the work of figuring out what kind of soil you have and what’s in it, what your fruits and veggies need, and what kinds of fertilizers can do the job for you. Heck, you can even organize your fertilizers based on “cost per pound of nitrogen” to see where the best bang for your nitrogen buck will be. But none of us are THAT obsessed about our fertilizers, right?…. [$ per bag / (pounds per bag * (% nitrogen/100))].

As a reminder, the numbers on your fertilizers are percent nitrogen, phosphorous (as ‘phosphate’, P₂O₅), and potassium (as ‘potash’, K₂O). One cup is 16 tablespoons, and an acre is has length of one furlong (660 feet) and width of one chain (66 feet), or 43,560 square feet. Side rant: metric rocks.