Balanced fertilizers are usually out of balance

I’m in the midst of grading papers for my nursery management class, and something that I’m running across is an incredible number of papers where the students are recommending balanced fertilizers.  Why are they doing that?  Or maybe an even better question is, what is a balanced fertilizer?  A balanced fertilizer is a fertilizer which has three numbers which are about the same, like a 10-10-10.  The problem with balanced fertilizers is that they are much higher in phosphorus than what most plants need — at least in relation to the amount of nitrogen and potassium which plants need.  Especially here in Minnesota, where there is usually plenty of phosphorus in the ground, this extra phosphorus serves no purpose except to pollute waterways.  We have got to break the cycle of just assuming that a balanced fertilizer is the way to go.  I get to see a lot of soil tests from old agricultural fields where balanced fertilizers were used for years and years.  Usually 10-10-10.  What I usually see — with very few exceptions — are phosphorus and potassium levels which are either very high or off the charts entirely.  Phosphorus and potassium don’t move readily in the soil while nitrogen does, so every year that you add 10-10-10 in the appropriate amount for your plants needs for nitrogen you’re adding too much phosphorus and potassium.  Any extra nitrogen which you add will move through your soil, but P and K will build up year after year (and some will run-off into gutters and drains).  So what do I recommend?  I like a ratio of about 5-1-2 or 5-1-3 for an N-P-K ratio in a general use fertilizer.

22 thoughts on “Balanced fertilizers are usually out of balance”

  1. It can’t come as any surprise that your students are recommending such horticutural practices because their professors have instructed them to do do. Maybe the corrective measure would be to have the instructors rethink their horticultural approach and direct their students accordingly.

  2. Would you please shed some scientific advise on reusing potting soil. There are a tremendous amount of folk lore on this subject but it can be confusing. Thanks
    Love your blog!

  3. Jeff, thanks so much for this post — it’s really, really useful, and puts the question of how much/what to fertilize with in the broader context where it belongs.

  4. Jeff’s singing my song here. Not only do many nonagricultural soils not need phosphate, but excessive phosphates inhibits mycorrhizae. It’s just a bad practice all around to add nutrients without knowing baseline amounts.

  5. This is useful information. Is there a a “Myth of Balanced Fertilizer” fact sheet summarizing the info out there somewhere?

  6. Since there are many studies documenting the movement of nitrogen in our waterways, is it reasonable to assume that farmers are applying the correct amount of nitrogen and excess phosphorus and potassium? Perhaps the bigger problem is over fertilization.

  7. Great post!

    When searching ‘balanced fertilizers’ the first link I found to balanced fertilizers from a university was an earlier post by Jeff on this blog.

    We could make an FAQ or two on the subject. 1)What is a balanced fertilizer? 2)How do I use balanced fertilizers? –something to that effect?

    Now the other problem is that a product that is not balanced is often hard to find at the store. If you are shopping at any place that is not specifically gardening/horticulturally focused it can be hard to find the right product. I’ve been frustrated by this for years.

  8. Good advice, and worth repeating. The first time I heard this was 22 years ago, 1988, from a county extension agent here in Georgia.

  9. Foy, yes, it is often the case that the correct amount of N is applied with too much P and K. Deirdre, yes, balanced fertilizers work, it’s just that they aren’t particularly environmentally responsible.

  10. One of the fundamental problems in dealing with nutrition is that we tend to focus on the ‘supply’ side. I think a better question is what is the demand? How much N (or P or K or Mg) does the crop actually take up? The
    19e0
    n we can look at uptake efficiency and application efficiency to get to the optimum application. We’ve got a ways to go but we’re starting to make some inroads.

  11. Jeff, I’m surprised that you suggest a remedy that still seems like shooting in the dark albeit probably wasting fewer bullets.

    Your suggested NPK
    ratio is another blanket recommendation when it seems that professionals should be basing nutritional management on as much information as economically practical.

    I would expect fertilizer applications to be based partially on a basic acid wash soil test to get some sense of what’s already there. I wonder why you wouldn’t automatically recommend this approach.

    Ultimately, it can be difficult to obtain fertilizers with the balance that might be best- at least for landscapers.

    I get my fertilizers custom mixed from an agricultural supplier but for many landscape contractors this option is not available. Even commercial suppliers like John Deere usually offer only a choice between a “balanced” fertilizer and lawn fertilizers containing mostly urea and only a sliver of P and K.

  12. Hi Alan — You’re absolutely right. I should have recommend a soil test prior to recommending a fertilizer. Not to excuse myself in any way, but the reality is that despite the pleas of extension specialists across the country, my experience is that about 10% of homeowners get soil tests done, so sometimes “wasting fewer bullets” is the best you can hope for.

  13. In my mind soil tests are just one piece of the puzzle. I happen to have an Ohio State Christmas tree bulletin on my desk “Soil characterizes, soil and foliar nutrient levels in plantations of four Christmas tree species in Ohio” by Jim Brown. As part of the study he correlated soil and foliar nutrient levels of four species (Fraser fir, Douglas-fir, Canaan fir and blue spruce) across a range of sites. For two species foliar P and soil P were not related, for one species they were positively correlated, and for one species they were negatively correlated. Foliar K levels were correlated with soil K for only one of the 4 species, otherwise there was no relationship.

    When I get involved in nutrient diagnostic/fertilizer recommendation scenarios I want to see: 1) site history and visible symptoms; 2) soil test; and 3 foliar samples. Over the years I’ve found soil tests alone are of limited value unless a specific item (usually pH) is really out of whack.

  14. Jeff, if your advice was directed at non-professionals I understand completely.

    In many states and/or counties the P issue is being dealt with legislatively and you’re beginning to see P taken out of lawn fertilizers entirely.

    When I was in hort-school excess K was not taken as seriously as plants will take up a lot of the excess without apparent harm as I remember it. Maybe you can refresh my memory on that.

    Also we learned about mychorizal relationships that assure that most plants in most soils will receive adequate P regardless of low levels in the soil.

    Bert, of course commercial producers of agricultural commodities rely on leaf samples as well as soil analysis and that leaf analysis is specific to varieties (let alone species) as far as desired levels of NPK. Even when you put leaf and soil analysis together you aren’t provided with an absolutely clear road map.

    Extension agents tend to serve a lot more home owners and landscape professionals then farmers these days and this forum seems to be aimed primarily at the former two.

  15. I don’t usually recommend fertilizer for perennials or woody plants to consumers. In most cases they aren’t needed. When I do recommend them, I try to stay with organic fertilizers and I like to use things like corn gluten or blood meal, no or low P and k. At the same time, I have a real problem with municipalities banning all fertilizers containing P, as they’re neccessary for container gardening, especially when using a soilless mix. I have customers that drive 1 1/2 hours to come buy miracle gro and osmocote from me because they can’t purchase them where they live. Good for me I guess, but it certainly hurts the garden centers in that town when the customers also purchase most of their plants for the season on those trips.

  16. What should I use in my pots? I don’t use chemical fertilizers. My “balanced” fertilizer is from organic sources.

    I wish one could subscribe to responses on this site.

  17. Jeff
    This is a bit of an eye opener. I have an allotment and have always used balanced fertilizers recommend by “experts”. Best I can get to your recommended NPK is Pelleted Chicken Manure which is 4,2.5,2.5. If I made up my own using Sulphate of Ammonia N 23%, Superphosphate P 18% and Sulphate of Potash K 48%,would I mix using the above % and your ratio by weight. All the above are available at the allotment shop on site.

  18. Terry, I think chicken manure is a great choice — it is higher in P than I’d like — but it’s such a good fertilizer that I’d stick with it. I tend to stay away from sulfate of ammonia — it can be dangerous stuff.

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