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.

9 thoughts on “Our visiting GP takes on fertilizers”

  1. And then there is the neighbor who shared our garden many years ago, before I started testing the soil. She turned the ground white with 10-10-10 one spring and we had the most lush, deep green veggie plants ever. Of course, the production was a bit slow until some of that nitrogen had been used up in vegetative growth, but the later half of the season was great. I was totally surprised that the excess fertilizer didn’t burn everything.

  2. What about the damage to the soil microbes from artifical chemicals?
    Isn’t rich compost made from a wide variety of natural ingredients the best soil enrichment?

  3. Wow, great info! It’s just never as simple as the bag would have you believe, hey? By the way, I’m a huge fan of your side rant. Metric rocks the show – way easier for calculations!

  4. . Phosphate production makes a slightly radioactive byproduct known as phosphogypsum. According to the Florida Department of Environmental Protection, there are a billion tons of phosphogypsum stacked across the state and 30 million tons are generated each year.

  5. I get really irritated with a local gardening magazine that frequently recommends that certain plants be fertilized because it is a certain month of the year. No consideration at all for whether fertilizer is actually needed or not. I only fertilize roses and potted plants and my garden looks fine, even the (small) lawn. I’m not even sure the roses need it.

  6. Linda, I’m surprised at you. A discussion of fertilizing the vegetable garden with no mention of home made compost. I thought most progressive gardeners fed their plants by feeding the soil these days. I never use store bought fertilizer in my vegie garden. Just compost and diluted urine for younger plants I want to accelerate. Urine is contraversal, I suspect as a vestige of our Victorian past, but it is safe and contains over half of the plant feeding nutrient our body excretes. Of course, it’s easier for a man to facilitate. It seems a much better environmental approach to use the materials we have around us and recycle them in our own back yards instead of sending them away where they do damage and buy products that also do damage in their manufacture and distribution.

  7. Yes, I completely forgot to mention compost (and urine!). They’re great, but it is important to know that extra phosphorous, from any source (including compost), is still extra phosphorous. My garden has at least 5% organic matter and lots of phosphorous already. Adding enough compost to meet my veggies’ nitrogen needs probably adds more P than they need.

    In an effort to write about supplying what plants need, I didn’t include what all those fertilizers that I have were made of. One of them has some ammonium sulfate, but the rest are so-called ‘organic’. In terms of what goes on biologically within the soil, that matters.

    My point wasn’t that compost isn’t good, or that YOU shouldn’t use compost on YOUR garden. We use our own compost, but our pile isn’t big enough to meet our garden’s needs. Rather, I just wanted to point out that recommendations on the bag, or ‘balanced’ fertilizers, isn’t necessarily what your garden or your veggies need. And yes, I really should start a pee bale, my kids would love it.

  8. Charlie, actually I thought that was a very good explanation on how to methodically use commercial fertilizers, but a “pee bale” ? I guess you mean peeing on straw to create human stable waste. I find a gallon gas container works well. It’s sturdy and unlikely to be confused for a bottle of apple cider. My wife hides mine when we have guests over.

    Sorry Linda for blaming you on the political incorrectness of neglecting the recycling of table scraps.

  9. A little late comment, but to those promoting compost as fertilizer, check with your local extension or master gardener offices — compost is not rich in fertilizer and is not considered as such; though it is important for tilth and other factors. More to the point, unless compost is put down in the fall, what nutrients are there are NOT available to the plants until microbes break them down into a form the plant can use; sometimes this happens after the period the nutrients are needed for the plants. Both compost and fertilizer are important, and more to the point the author was correct — know and use what you need based on testing, not from a bag, wives myth, or online forum

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