Balanced Fertilizer and Peak Phosphorus

When I first started reading extension recommendations for plants around 15 years ago now, I read a lot about “balanced fertilizers”.  Today I still see balanced fertilizers recommended, but not nearly as frequently, which is a good thing.

At this point you may be asking yourself “what is a balanced fertilizer?”  A balanced fertilizer is one that has three numbers on its label which are the same — such as a 10-10-10 or a 14-14-14 (Nitrogen, Phosphorus and Potassium are the three nutrients indicated by
these numbers).  Although it might seem like equal numbers would mean equal amounts of these nutrients, these numbers actually indicate percent Nitrogen, percent P expressed as P2O5 (in other words, if you took all of the phosphorus present in the fertilizer, made it into P2O5 and then added it back to the fertilizer and figured out what percent of the fertilizer that made up — that would be the second number) and percent potassium expressed as K2O (basically the same as the phosphorus example you saw previously).

But here’s the problem.  The amount of phosphorus, and often potassium, that is added when you use a balanced fertilizer is typically out of line with the amount that the plant needs.  This is because fertilizers are usually applied based on the amount of nitrogen that a plant needs.  The reason that a balanced fertilizer was usually recommended was that phosphorus and potassium levels in these fertilizers is high enough that they provide all of those elements that a plant needed without being toxic to the plant.  The problem is that, while the levels of these nutrients added might not be toxic, they are in excess of what is usually needed.

OK, so we’re adding excess phosphorus and potassium, what’s the problem?  Well, for the potassium the problem isn’t usually that big a deal.  In fact, a fertilizer bag with the first and third numbers equal may be what’s called for in many cases — fruits and nuts in particular often like a higher level of potassium.  Additionally, the world has a pretty big store of potassium so we’re not likely to run out any time soon.  Phosphorus, on the other hand, is a little bit different.

Phosphorus is a bit more hard to find in large quantity than either of the other elements in a bag of fertilizer.  Because of this it is often the element that limits the growth of plants, for example algae.  When phosphorus runs off into a lake or other body of water it can allow algae to go crazy and use up all of the oxygen in the water killing fish and other creatures (actually it’s the dead algae that do this — bacteria use oxygen while breaking the algae down).  Most of you are familiar with this and know that it’s the reason why Minnesota and now Wisconsin have banned the use of phosphorus fertilizers on lawns without a soil test.

But here’s what you may not know.  Most of the phosphorus which we use for fertilizers comes from rock phosphate.  Rock phosphate is mined in only a few places around the world, Florida being one.  Just as it is a foregone conclusion that we will run out of oil someday, it is also a foregone, but lesser known, conclusion that we will run out of rock phosphate.  Estimates are that we will reach “Peak Phosphorus” (in other words maximum phosphorus production.) in 20 years or so and that we will run out in 50 – 100 years.  Here’s an interesting article on the situation

So conserve our natural resources and skip the balanced fertilizer.  When asked for a general use fertilizer recommendation I usually recommend something with a ratio of roughly 5-1-2, with a higher potassium content if you’re growing fruits or nuts.

Want an organic source of nitrogen that isn’t shipped from halfway across the world? Urine luck

There are lots of organic fertilizers out there:  Fish emulsions, corn gluten meal, guano.  Many of these fertilizers (all that I listed above with the exception of the guano) are by-products of some other industry.  Still, they need to be shipped from somewhere to somewhere to get to our garden and so they cost energy — and of course they cost us money.  But there is a high nitrogen fertilizer that you can use which doesn’t come from a long way away, and that’s pee.  Holly  mentioned using pee to help compost piles of stray a few months ago (you can find the news story on the right side of this blog), and I, for one, think it’s a great idea.  But really, pee can be used as a fertilizer without the compost.

Yesterday I was working on a project and decided to goof off a little by figuring out how much nitrogen was actually in urine.  Here’s the conclusion — Urine contains about 4,000 pats per million nitrogen.  In terms of what plants can handle, that’s a lot (which is why dogs produce “dog spots” when they pee on a small area of ground — too much nitrogen in a small area).  400 parts per million nitrogen, applied once a week in irrigation water, is what you might apply to encourage the growth of greenhouse plants.  Urine, by the way, is also relatively sterile (unless you’re dealing with a bladder infection) and so using urine is relatively safe as long as it’s used quickly.  It also conserves water because you don’t need to flush.  So, the way I figure it, you could mix 1 part urine with 9 parts water and have a really good once a week (or two weeks) fertilizer application for your flowers (I don’t know if I could bring myself to fertilize cabbage, broccoli, or tomatoes with it…).  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.

Dandelions and clover

It was fun to read all of your comments last week about your opinions on lawn care.  To follow up on it I’m going to talk a little bit about why I’m not fond of companies which apply herbicides multiple times throughout the year.  But first I think I’ll mention why I apply herbicides at all — aesthetics.  That’s it — the whole reason. Could I go the no-lawn route?  Yes, but I like having a yard to run in.  Not a huge yard, but a little yard to play tag with the kids.

What I long for though is the yard from the house that I grew up in.  Our house in southeastern PA (About an hour west of Philly and an hour east of Lancaster) was set back about 800 feet from the road and was on old agricultural land.  The area around the house was planted in grass in the mid 70s and then it was left alone.  Fertilized once the first year I think, but that’s it.  Dandelions invaded quickly as did clover.  Over the years the clover began to dominate the grass, but not to the point that the grass disappeared, and the lawn actually appeared relatively homogeneous.  Dandelions never left, but their numbers declined.  The clover grew low and the grass never shot up like it does in a heavily fertilized lawn and so mowings only happened once every two weeks or so (well, OK, sometimes more often depending on the weather and where on the lawn you were — the spot over the septic tank needed mowing every 48 hours or so).  The grass did go dormant most summers, but 800 feet from the road there wasn’t anyone to complain, and besides, the clover kept the lawn from appearing completely scorched.  The lawn looked good for well over 30 years (until my parents remodeled the house and the yard was torn up).

The typical suburbanite might not have liked this lawn, but to me this lawn looked great, and, besides, it was low maintenance.  The reason I’m bothering to tell you about this lawn though is because it illustrates so well what lawn care companies make impossible.  They say (and by “they” I mean professors like myself) that pesticides beget pesticides and fertilizers beget fertilizers, and nowhere is that as true as in a well manicured lawn.  The herbicide of choice is 2,4 D (though there are many others that are used) which lawn care companies apply multiple times over the the course of a year.  This pesticide does a great job of killing dandelions, but it also kills clover.  It rarely hurts grass unless it’s grossly over-applied.  The problem with killing clover is that this clover is the stuff that fed the grass in the house where I grew up.  Clover takes nitrogen out of the air and makes it available to grass every time the lawn is mowed (the clipped off pieces of clover degrade and the nitrogen in them feeds whatever plants are around).  Without the clover you need to add fertilizers.  So, because the lawn care company is keeping the lawn free of weeds they also need to fertilize because they’re killing all of the natural fertilizer.  Here’s the thing, the weed that most people in the suburbs like least, dandelions, is actually very sensitive to low potassium.  The lower the potassium in the soil the worse it does.  In fact, dandelions can easily be out-competed by grass and clover if potassium is low — just as happened in the yard of the house where I grew up.  But do lawn care companies pay attention to this (by using high nitrogen, low potassium fertilizers?)  What do you think?

My guess is that many of you thought that I’d cite all kinds of scary side effects of the pesticides used on lawns.  Nope.  In general I think that, if used properly, they’re pretty safe for humans (with a few notable exceptions).  I’ve spent a lot of time reviewing epidemiological and toxicology studies and I can think of many worse things.  I am somewhat fearful of what 2,4 D may do to dogs in particular — they can’t excrete this poison like we can.  Don’t think for a minute that I’m calling these poisons perfectly safe — I just think there are plenty of other better established reasons to avoid lawn care company pesticide schedules.

Getting Loaded

Spring is off to a warm and fast start here in Michigan.  March was unseasonably warm and the past week or so has seen temperature 20 degrees above average or more.  Needless to say this is pushing all of our landscape trees and shrubs.  Forsythia and saucer magnolia are in full bloom, at least two weeks ahead of schedule.  The warm weather also has us scrambling to get some research projects in the ground as well.  Today I was working with members of my lab to install a trial to look at the relationship between fertilization in the nursery and subsequent of shade trees in the landscape.  For the past two years we’ve grown Acer miyabei (‘State street’ maple) and ‘Harvest gold’ Linden trees in 25 gallon containers as part of a trial on controlled release fertilizer.  Interestingly, in the nursery we saw a significant increase in chlorophyll index and foliar nitrogen with fertilization (no surprise) but no difference in caliper or height growth (somewhat of a surprise).  This indicates that in the nursery, fertilization induced ‘luxury consumption’ or an uptake of nutrients beyond what the trees need to meet their growth requirement.  This observation provided the opportunity for our current, follow-up study.  In the forest nurseries there is a growing interest in the practice of ‘nutrient loading’ seedling trees before they are lifted.  Forest nursery managers deliberately induce luxury consumption by fertilizing late in the season.  At this time seedlings have set a hard bud and won’t grow but can take up additional nutrients.  Numerous studies, particularly by Dr, Vic Timmer and his associates at the University of Toronto have shown that nutrient loaded seedlings will outgrow standard seedlings when out-planted on reforestation sites; even though the seedlings are the same size when transplanted.  How does this apply to large-caliper shade trees?  We don’t know.  There are certainly some underlying commonalities that are intriguing.  Nutrient loaded forest seedlings have an advantage when planted on tough sites where follow-up culture is minimal – basically the seedling has to get by initially with its own energy reserves and resources.  Shade trees planted as street trees often face the same hardship; once planted they may receive little or no after-care beyond an initial watering.  Could nutrient loading provide a better internal nutrient reserve and jump start the re-establishment process for street trees like it does for the smaller forest cousins?  We should gain some insights this summer and next.

Haber and Bosch

There are people who are fascinated by plants and people who are fascinated by the science of growing plants.  While I love plants I must confess that I consider myself to reside more firmly in the latter group that the former.  I do love to see the beautiful flowers on an apple tree in the spring, but I’m more fascinated by the nitrogen, phosphorus, potassium and other elements that the tree obtains from the soil.  I like to contemplate the complex ecosystem that surrounds the tree, including the tree’s pests and the possible things that we can do to protect the tree from pests.  I love to learn about insecticides, herbicides and fungicides, and I especially love to learn about alternatives like beneficial insects that may be used to control pests.  Histories that cover fertilizers and pesticides are pleasure reading.

One of the most important things to ever happen to the world as we know it was the discovery of a process to take nitrogen from the air so that we could use it as a fertilizer.  During the 1800s we discovered that applying nitrogen to our soils made plants grow really fast.  Though this nitrogen could be supplied with manure we quickly learned that Peruvian Guano and Chilean nitrates had more concentrated nitrogen and so less needed to be added to fields to get bigger responses.  Incidentally we could also use the nitrogen from these sources for bullets and bombs.  Nitrogen is a cornerstone of most conventional explosives.  Unfortunately these sources of nitrogen did have a drawback, they are not renewable resources.  You may say — Hey, Guano’s renewable! — But you’d be stretching the truth.  You see, guano is aged manure where the nitrogen has had a chance to become concentrated.

Anyway, by the time the 20th century rolled around we had used up much of the nitrogen available from South America and so we (and by we I mean the world in general) were hurting for nitrogen — particularly Germany.  Germany had a feeling there was going to be a war and she needed a way to get nitrogen other than sailing all the way to Peru or Chile.  So she put money into research.  Pretty soon two researcher, Haber and Bosch, came up with a method to take nitrogen directly out of the air and make it into ammonia.  Once present in ammonia it could then be used to make any number of other nitrogen based compounds, from fertilizers to bombs.

Coincidentally, Haber is also known as the father of chemical weapons.  He led the German poison gas program and had a hand in developing such things as mustard gas.  He was considered to be a patriot, but, born a Jew, the rise of Hitler wasn’t good for him and he was forced to leave in the early 1930s.

The story of Haber and Bosch is absolutely fascinating, not only because of the colorful characters, but because their discovery is, arguably, the most important factor in the increase of the worlds population over the last century.  The best book that I know of on this topic is Enriching the Earth by Vacliv Smil.  It’s a great book, but it does get a little technical.  But  my dad (he’s a chemist) showed me another book yesterday that is much more entertaining and readable than Smil’s book while retaining most of the pertinent science.  It’s called The Alchemy of Air by Thomas Hager and, if your interested in fertilizers and the people who first developed them, then this is a must read.


An evolving view of plant nutrition

One of the hallmarks of science is that our view of the world evolves and changes as new evidence comes to light.  When I was a grade-schooler following the Apollo missions, for example, I knew all the planets in order from Mercury to Pluto and how many moons each one had; Jupiter was the champ with 12.  Today, Jupiter has as many as 63 moons depending on who’s counting.  And Pluto, let’s not even go there.   Likewise, our view of plant science has changed over the years.  As I’ve mentioned before, when I took introductory Botany as an undergrad thirty-some years ago we learned that there were 16 essential plant nutrient elements.  Since then we’ve learned that nickel is also essential at least for some plants.


If you took your plant science more than 10 years ago, you also learned that nitrogen is taken up from the soil as either ammonium or nitrate.  This view is now being revised due largely to evidence from the ecological literature.  Ecologists have found that in northern boreal regions where soil temperatures are cold and mineralization and nitrification rates are low, plants will take up intact amino acids from the soil (Nashholm and Persson, 2001; Kielland et al., 2006).  Recent studies have extended these observations to temperate forests (Gallet-Budynek et al., 2009) and horticultural crops. (Ge et al., 2009), indicating that a range of plants may be able to derive a portion of their N requirements directly from organic sources.


So what does this have to do with the Garden Professors and the science of landscape horticulture?  First off, this is pretty cool stuff and certainly will cause a lot of re-evaluation of some established paradigms.  From an applied perspective, fertilizing with amino acids and related organic source has some potential benefits.  In warm, well-aerated soils, nitrate-N predominates.  When plants take up nitrate, it must be reduced via nitrate reductase and nitrite reductase before it can be assimilated into amino acids.  These steps require metabolic energy.  In theory, amino acids could bypass the reduction and assimilation processes and provide a more efficient means of fertilization.  At this point, most of the scientific focus is on quantifying how much organic N can be directly taken up from the soil.  Evaluating efficiency of uptake and utilization is a couple steps down the road.  Amino acid fertilization could potentially provide another benefit over nitrate fertilization by reducing nitrate leaching.


Of course there’s a potential downside as well.  A quick Google search of ‘amino acid fertilizer’ reveals sites shilling all manner of concoctions for fertilizing plants; many of dubious value.  One of the first sites I hit proclaims that, unlike inorganic fertilizers, their product provides the 70 elements needed for plant growth.  Seventy?!  Well that’s close to seventeen.   Point is be prepared for an ever increasing barrage of claims about organic fertilizers.  There is no doubt that compost and similar organic sources can provide essential plant nutrients and effective fertilizer sources.  As always, however, be skeptical of spectacular claims and secret, proprietary ingredients and pay close attention to the cost of amino acids compared to conventional fertilizers.  There is some science here; clearly many plants have the capacity to take up amino acids directly.  Beyond that we’ve still got a lot to learn.


Gallet-Budynek, A., E. Brzostek, V.L. Rodgers, J.M. Talbot, S. Hyzy, and A.C. Finzi. 2009. Intact amino acid uptake by northern hardwood and conifer trees. Oecologia 160:129–138.


Ge, T., S. Song, P. Roberts, D.L. Jones, D. Huang, and K. Iwasaki. 2009. Amino acids as a nitrogen source for tomato seedlings: The use of dual-labeled (13C, 15N) glycine to test for direct uptake by tomato seedlings. Environmental and Experimental Botany. Volume 66: 357-361.


Kielland K, J. McFarland, and K. Olson. 2006. Amino acid uptake in deciduous and coniferous taiga ecosystems. Plant Soil 288:297–307.


T. Näsholm and J. Persson.  2001. Plant acquisition of organic nitrogen in boreal forests, Physiol. Plant 111: 419–426.

The Government In Your Yard

This year Pinellas county in Florida banned the use and sale of nitrogen and phosphorus fertilizers for lawns between June 1 and September 30.  Is that a good idea?  On the surface it seems like a great idea because it should reduce the amount of nitrogen and phosphorus which reach streams, lakes, ponds and rivers and cause algal bloom and destruction of water habitats.  On the other hand a PROPERLY fertilized lawn is less likely to have nutrient rich runoff (because of a more expansive root system.)  If this ban inadvertently stops people from properly fertilizing there is the possibility that the problem could be made worse.

I’m no fan of heavy fertilizer use by homeowners — I loath the practices of many lawn care companies which includes pesticide and fertilizer applications as many as 5 times a year — but the truth of the matter is that grass actually does a good job of grabbing nutrients that are applied to it because it has such a dense root system.  Crops like corn and wheat, on the other hand, don’t have such a dense root system.  I recently read a paper stating that, worldwide, only about 33% of the nitrogen applied to crops is actually used by those crops (this is referred to as NUE or Nutrient Use Efficiency).  A recent graduate student of mine found that the NUE for Hazelnuts is actually well below that.

My personal preference for lawns is that we start to do what was once common back in the ’50s and before — plant clover with your grass.  Believe it or not you can get an amazingly dense lawn that way.  The clover will provide much of the nutrition that the grass needs — and it’s not, as of yet, considered a noxious weed.  I also like the idea of planting leguminous trees, like black locust (I know some of you see this as a weed — it can be a nice tree too) in turf plots, reason being abscized black locust leaves have high concentrations of nitrogen — over two percent — unlike the leaves of things like maples and oaks.  Of course it’s also possible for the nutrients from clover or the leaves from black locust to end up where they shouldn’t, but because of their slow decomposition we hope that nutrients running off from these sources would be less of a problem.

Anyway, my final thought — Why couldn’t we legislate that all grass seed include some clover or that a certain number of leguminous trees be planted near turf plots rather than trying to control the use of fertilizers?

Do landscape trees need nitrogen fertilization?

I’m in Grand Rapids this week attending the Michigan Nursery and Landscape Association/Michigan Turf Foundation Great Lakes Trade EXPO.  The topic for my talk today was Landscape Tree Fertilization.  That might not sound like a subject that would generate controversy, but as with most things, there are camps emerging.  There is a rising chorus of folks that suggest that landscape trees should not be fertilized with nitrogen.  There are a couple of lines of evidence that bolster this point of view.  First, many systematic studies on the growth response of street trees or landscape trees often do not show a response.  There are numerous examples of this, for example, in Arboriculture and Urban Forestry (formerly J. of Arboriculture).  The second line of evidence for not fertilizing landscape trees relates to the relationship between tree nutrition and susceptibility to insect pests.  This argument relies on the ‘growth vs. defense’ hypothesis and suggests that fertilization promotes growth at the expense of defense compounds; essentially making fertilized trees tastier to insect pests.

So, in light of this, why do I suggest that landscape trees should receive 1-2 lbs of N per 1000 sq ft. every 2-3 years?  First, we need to understand that nitrogen is constantly lost from landscape systems.  In forests, trees take up nutrients from the soil, translocate them to leaves, shed the leaves, and the nutrients are ultimately returned to the soil in a cyclic process.  In landscapes, leaves are usually raked or blown and removed from the cycle.  Soil nitrogen is also lost due to nitrate leaching.  Additionally there are often key weaknesses in some of the papers that purport to show no response to fertilization.  For example, Ferinni and Baietto (Arb & UF 32:93-99) found no response of sweetgum trees to two levels of fertilization.  However, the data show that the control trees, which were not fertilized, had similar (and fairly high) foliar N levels as the fertilized trees.  This pattern can be found in several similar studies.  The more appropriate conclusion for these papers should be “Trees that are not nutrient deficient do not respond to fertilization”.  Similar issues pervade studies related to the growth vs. defense hypothesis.  Why would one presume that a nutrient deficient plant would be better able defend itself against insects attack than a tree that has adequate nutrition?  Ideally, fertilization decisions should be based on visual symptoms and soil and foliar samples.  Nevertheless, low rates of N from either organic or inorganic sources will make up for losses from the N cycle and maintain tree vigor.

It should be noted that the rates I’m suggesting are considerable lower than those that are found in some older extension literature, which recommend rates of N up to 6 lbs for 1000 sq ft.  As a point of comparison, Midwestern farmers apply 150-200 lbs/acre to grow a crop a corn.  The 6 lb rate for landscape trees works out to around 260 lbs per acre!


Sunday rant – the evils of chemicals

It’s days like this that I am so grateful to have this blog at my disposal!

It’s 7 am on Sunday and I’m just finishing the paper, drinking Earl Grey tea, and listening to NPR.  Liane Hansen just finished an interview with Martha Stewart, who among other things was discussing healthy eating for the new year.  She’s a proponent of organic food (as are many of us), and mentioned two reasons she doesn’t like conventionally grown produce.  The first – residual pesticides – is a legitimate concern.  But then she stated her second concern that “chemical fertilizers in the soil are taken up and stored in the plant.”

No kidding.

Plants really don’t care (excuse my anthropomorphizing) where their mineral nutrients come from.  Nitrogen in ammonium sulfate is the same element as the nitrogen in cottonseed meal.  The plant uses it for amino acids, chlorophyll, alkaloids, and many, many other compounds.

Martha’s faulty thinking falls into the “organic is safer than chemical” mindset that way too many people hold (you can read a column I wrote about this in 2001 here).  “Chemical” is not intrinsically bad and “organic” is not automatically safe.  This is an emotion-based argument and inspires fear rather than thoughtful discussion.  When someone parrots this mantra, I can’t take them seriously.

I believe that organic methods in production agriculture, ornamental landscapes, and home gardens are superior to conventional practices and support a healthy soil-microbe-plant-animal system.  I also believe that many fertilizers are misused and/or overused – but this includes both conventional and organic varieties.

Gerald Holton, a science historian at Harvard, once stated that “persons living in this modern world who do not know the basic facts that determine their very existence, functioning, and surroundings, are living in a dream world.  Such persons are, in a very real sense, not sane.”

This is the quotation that came to mind this morning.

What’s in YOUR soil? (with apologies to Capitol One)

Urban environments are always challenging for landscape plants just because they are anything but “natural.”  Temperatures are higher, water is often less available, and compacted soils have all the nourishing qualities of concrete.  The single most important thing you can do to ensure long-term success of landscape trees and shrubs is to get their roots well established in the soil.

I’m going to leave the topic of soil amendments to another day (but you can find my myth columns about them at under “Horticultural Myths”).  What I want to focus on is our propensity for fertilizing landscape trees and shrubs without really knowing why, or when, or if we should be adding any particular plant nutrient.

The smartest $13 you can spend is to have a soil analysis done before you add anything to your soil.  My favorite soil testing lab is the University of Massachusetts at Amherst.   That $13 will buy you a complete standard analysis of the available nutrients in the soil, plus a measurement of the soil’s organic matter content.  Of course, there are many other soil testing labs you can use, but UM’s Amherst lab is only providing you with information – not a sales pitch for amendments and fertilizers.

Why is this so important?  Let’s say you go to a nutritional supplement store, buy every possible supplement, and take them all.  Do you need all of these?  Probably not.  It would be smarter to talk to your doctor and find out what you’re missing, right?  It’s the same with your soils.  Don’t assume your soil needs a lot of phosphorus, even though transplant fertilizers are loaded with this element.  Non-agricultural soils often contain abundant levels of this nutrient, and too much phosphorus will hurt mycorrhizae and contribute to water pollution.  Take a look at this portion of a soil test for an organic demonstration garden:

Figure 1.  Note the high level of organic material in this soil, which contributes to the nutrient overload.

The trick to fertilizing landscape soils is understanding that landscape soils are not agricultural soils.  You’re not harvesting crops (an activity that depletes the soil of its plant nutrients).  Urban landscape soils usually have high enough levels of most nutrients to sustain plant growth.  But you’ll never know unless you have your soils tested.