Are these fertilizer recommendations off by an order of magnitude or...?
May 9, 2016 4:46 PM   Subscribe

I got my soil tested for my new vegetable garden and am following my state extension service's recommendations for applying organic fertilizers. However, when comparing the amount they recommend versus the instructions on the packages, I'm beginning to wonder if there's a typo on the extension service's webpage...

These are their recommendations per 100 square feet. My garden is 200 square feet so I'm making a mix of twice these amounts to spread on my garden bed:
Organic Fertilizers
(Use in place of Fertilizer Recommendations on Soil Test Report.)

For optimum growth of garden plants, maintaining a high level of all the plant nutrients in the soil is desirable. This can be accomplished by adding materials such as compost, manures, and lawn clippings throughout the year.

Nitrogen: A nitrogen source should be applied each year for a successful garden. If a nitrogen-rich material such as compost or green manure (especially from legumes) has been incorporated in the garden soil (1- to 2-inch layer worked into a 6- to 8-inch depth of soil) within a few weeks before planting, then little or no further nitrogen will be required. Otherwise, incorporate any one of the following materials per 100 square feet before planting: 5 pounds of blood meal; 5 pounds of fish meal; 10 pounds of soybean-seed meal; 10 pounds of cotton-seed meal; or 15 to 25 pounds of poultry manure.

If the plant nutrients phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) indicated on the soil test report fall into the medium (M) or low (L) category, then the following materials may be added per 100 square feet to bring the nutrients back up to a high level:

Phosphorus: 10 pounds of bone meal or rock phosphate.

Potassium: 10 pounds of granite dust or green sand. Wood ash is high in potassium but should be used sparingly only on acid soils (pH less than 6.0) due to its potential to make the soil too alkaline. (General rule of thumb: If applying 10 pounds of wood ash, reduce the lime applied by 5 pounds per 100 square feet.)

Calcium: If limestone is added as recommended due to low pH, this will correct low calcium levels. If limestone is not recommended, add 10 pounds of gypsum.

Magnesium: If dolomitic limestone is recommended due to low pH, this will correct low magnesium levels. If limestone is not recommended, add 10 pounds of Epsom salts.
So my mix includes 20 pounds of bone meal, 20 pounds of granite dust, and 20 pounds of Epsom salts (our pH is 6.9), plus a bunch of well-composted hay and cow manure. We already have high calcium.

But now that I'm actually mixing the dry ingredients to put in the spreader, that seems like an awful lot of salt. And while I'm a clueless n00b at gardening, I assume that the phrase "salt the earth" in reference to destroying one's enemies is a thing for a reason. So I read the Epsom salts package and it says ONE CUP per 100 square feet for "garden startup," which would be 2 cups for my entire garden. That's a hell of a lot less than the 20 pounds recommended by the extension service.

Then I checked the bone meal package and it recommends 5 pounds per 100 square feet for "general plantings," which is half as much as extension recommends.

I don't have any packages to compare the granite dust and composted manure instructions to because my husband got them loose from a quarry and a farmer.

Again, I'm a clueless n00b, and thus I don't know how to judge who's right, the extension service or the fertilizer manufacturers. Please advise, thanks!
posted by Jacqueline to Home & Garden (19 answers total) 1 user marked this as a favorite
To set your mind a bit at ease, Epsom salts are not salts at all, but a pure mineral compound of magnesium and sulfate. So you won't be salting the earth.

I suspect that the recommended dosages from the fertilizer manufacturers are based on having an ideal composition already set up, and are being used to replace the previous year's depletion. Since you're starting from less than optimal soil you're adding larger volumes of amendments than normally would be required.
posted by valoius at 4:53 PM on May 9, 2016

Epsom salt is absolutely a salt, but not the salt you need to be concerned about, i.e. sodium chloride. In particular it lacks sodium, which is the problematic element/ion for you garden. As long as nothing is too high in sodium, don't worry too much about it.

That said, I can't tell you whether 2 cups or 20 pounds is closer to correct.
posted by Joey Buttafoucault at 5:05 PM on May 9, 2016 [1 favorite]

I'd trust the soil amendment report that was tailored to your soil samples over generic packaging - the 2 cups per 100 sq ft recommendation is for generic soil which may or may not actually need amendment.

However, if you're concerned, contact your local Master Gardener's Association or the extension folks - they'll be well-placed to help you and in general are happy to do community outreach and education. I belong to a Master Gardeners association with the county's extension office and we do this sort of thing all the time.
posted by bookdragoness at 5:15 PM on May 9, 2016 [2 favorites]

OK so now that I know that Epsom salts aren't going to "salt the earth" and make nothing grow, is there any harm from having too much magnesium via Epsom salts in the soil?

I ask because I already combined the recommended amount of Epsom salts with the other stuff (granite dust, bone meal, plus a trace minerals mix) before I got worried about the salt amount and I'm not having much luck trying to scoop just the Epsom salt layer out. I'd rather not throw everything away and start over.
posted by Jacqueline at 5:27 PM on May 9, 2016

I'd trust the soil amendment report that was tailored to your soil samples over generic packaging

They did give me a tailored recommendation for conventional fertilizer ("Apply 4 lbs (10 cups) of 5-10-10 or 2 lbs or 10-20-20 per 100 square feet"), but if you want to use organic fertilizers instead then all they tell you is to add any nutrient that tested below H (High) and refer you to the Organic Fertilizers note I quoted in my OP and let you just figure it out on your own from there. :(

So given that they don't seem interested in providing as much support for people wanting to use organic fertilizers and the huge discrepancy in the recommended amounts from the extension service vs. the Epsom salts packaging, I was worried that the Organic Fertilizers section wasn't proofread carefully enough and that either "100 square feet" was meant to be "1,000 square feet" (and thus all the nutrient recommendations were off by an order of magnitude) or that "10 pounds of Epsom salts" was supposed to be "1 pound of Epsom salts" (and thus only that one was off).

But if veteran organic gardener MeFites think that 10 pounds of Epsom salts per 100 square feet of vegetable garden is a reasonable recommendation, I'll proceed with applying the stuff I already mixed.

If it matters, I'm planting a lot more than is typical for a 200 square foot garden. My garden bed is 2 feet wide by 100 feet long and against a fence that I'm using as a trellis. So I'll be growing vertically as much as possible, don't need to leave any room for walkways, and will be planting on a hexagonal grid instead of in rows. Thus being a little over-fertilized might not be a problem, as long as it isn't so far over that it will burn my transplanted seedlings.
posted by Jacqueline at 5:56 PM on May 9, 2016

I'm a vegetable garden virgin this year too, so I don't have the right answer, but I have seen similar questions to yours at the GardenWeb vegetable garden forum. Maybe ask it there too?
posted by cecic at 6:38 PM on May 9, 2016

It does seem like a lot, and I'm not finding anywhere (including the NC soil testing info) that recommends epsom salts in that amount. For lawns I'm seeing recommendations of 30-40 lbs/acre. Epsom salt is highly soluble, though, so it's unlikely to stick around (unlike lime) because rain and watering will wash it down out of your soil into the water table. Plus, it doesn't build up and scorch plants like sodium does.

What I'd say is, since it's hard to separate, go ahead and use your mixture and just work it extra deep. If you were planning to mix down the 7-8 inches they recommend, try amending more like a foot of soil.

This is easier to do as you add soil to boxes or as you cut out whatever you're replacing with these garden plants.

Pro tip: If you pull dirt out of a hole anywhere, scoop it onto a handy tarp so you don't lose dirt as you dig. Tarps also make good areas to turn dirt and amend soil, then you can just tip it back into the hole.
posted by bookdragoness at 6:43 PM on May 9, 2016

I've been reading up more on the effects of too much magnesium / Epsom salts and have some more information that might be relevant to answering my questions:

1) Our soil's calcium level is VH (Very High) at 2539 lb/A whereas our magnesium level is just M+ (Medium) at 133 lb/A. Most websites claim that a 6:1 calcium:magnesium ratio is ideal and if that ratio is obtained by just dividing those numbers then we currently have a 19:1 calcium:magnesium ratio. So even if the 10 pounds of Epsom salts per 100 square feet of garden bed is too high, would the possible excess magnesium in my fertilizer mix actually be a good thing because it would get us closer to that ideal ratio?

2) Although our soil test didn't cover clay-vs-sand composition, I can tell just by looking at it that it's very high in clay (when I was digging the garden bed I briefly contemplated giving up gardening and taking up pottery instead since I apparently have a nearly infinite supply of free red clay). I've read that one problem of too much magnesium is that it exacerbates the problems with clay soil. So, would working in some sand help mitigate any problems caused by using my fertilizer mix?

3) Our dogs like to pee on the fence from the other side of where the garden would be. I've read that too much magnesium can interfere with nitrogen absorption but given that our dogs will be adding supplemental nitrogen throughout the growing season do you think that the possible excess magnesium might help prevent our plants from being burned by too much nitrogen?
posted by Jacqueline at 6:51 PM on May 9, 2016

Yikes, no, I think the numbers you've been given are wrong. Here's a PDF from NC State U which recommends 10 lbs per 1000 sq ft. (Sorry if the link doesn't work; I just googled 'epsom salts amendment rate 10 lbs sq ft' and it was on the first page.) Googling around I see recommendations for 10 ounces per 100 sq ft, etc. so I think you're out of luck on your pre mix. I'm trying to think of something you could add to your mix to dilute it and still be able to use it but I'm drawing a blank. You're not going to want to over apply Mg because it raises soil pH, and your pH is already a little high, which makes sense in a clay soil. Incorporating sand ... I don't know. It's a bit of a wash because if it's too fine, it won't do anything, and you have to incorporate quite a bit to see results.

Unrelated to the Epsom salts deal, I'm curious about all this. Did you bust this up from grass or was it previously under cultivation? How are you planning on working your compost and amendments into the soil and if you're using a cultivator, what depth are you cultivating to? Is there a lawn along these 100 ft or something else?

The reason I ask these things is because doing a bunch of secondary and micronutrient amendments on a narrow strip of garden is going to have limited effectiveness if there's a lawn bordering it. But you can of course successfully amend the tilth of the soil with your compost and other organic matter, and a better use for your several pounds of Epsom salts would be to use them as a foliar application during the growing season, because that way the beautiful veggies get all the benefit and the neighboring grass gets none. Plus, you can tailor your nutrient application to each vegetable's needs.
posted by bluebelle at 9:20 PM on May 9, 2016 [2 favorites]

Also -- provide your soil test provider and ag extension service with some friendly feedback about this. The soil test provider folks may not have the ability to give tailored micronutrient rate recommendations, but let them know that you're interested in organic amendment recs rather than commercial fertilizer recs. Maybe hearing that from enough people will encourage them to develop that side of things???
posted by bluebelle at 9:24 PM on May 9, 2016

Did you bust this up from grass or was it previously under cultivation? How are you planning on working your compost and amendments into the soil and if you're using a cultivator, what depth are you cultivating to? Is there a lawn along these 100 ft or something else?

It used to be lawn and there is lawn on either side. I'm borrowing my neighbor's rototiller to work all the stuff in. My plan was to mix the dry fertilizer ingredients (done), use a spreader to scatter it on my 2ft by 100ft strip, then put an even layer of composted manure on top, then rototill it all in.
posted by Jacqueline at 10:10 PM on May 9, 2016

Looking at your soil results I would just pile on the manure and let it rot down for a few weeks and maybe sprinkle it with some bone meal to add in extra Phosphorus. Your magnesium is medium level so if you have problems, then you could try and increase the magnesium level for next year, but I wouldn't worry. Plants are actually quite tough and will thrive given attention that doesn't actively kill them. 25 pounds of manure per 100 sq ft sounds like a lot, but I would easily add quadruple that to less area (and have just a few weeks ago)

What I think you could benefit from more is setting up what your crop rotation is going to be and determining where you need the manure. Mine is as follows.

Year 1 - Heavy manure then plant potatoes
Year 2 - no manure and plant root veg (beets parsnips and carrots)
Year 3 - Light manure and plant alliums (garlic onions shallots)
Year 4 - Heavy manure plant tomatoes and corn (I also topdress the corn with used coffee grounds from a coffehouse)
Year 5 - Peas/beans
Year 6 - Dig in a lot of coffee over the winter (as well as the pea and bean plants) and plant lettuces and leafy veg

then back to year 1.

So you should break your area into 6 places (of less depending on what you are growing. Since you're in virgina I would recommend squashes and zucchini as well as some cukes probably along with the peas/beans)

Once divided only manure where required and then move things along for the next year. If would look like a conveyor belt with the various veg moving along year after year. To be honest the worms will move the manure around for you anyways and you won't have to worry about it, saves on the digging.

If you post what you want to grow I can divide it into groups for you and come up with a rotation.
posted by koolkat at 5:41 AM on May 10, 2016

I called the state extension office this morning and the guy I talked to agreed that the amount of Epsom salts on their webpage seemed to be too much and he'd look into getting it fixed but he also said that the extra amount I already mixed in shouldn't hurt my soil or plants.

He didn't seem super confident in what he was saying, though, so any opinions on whether it's okay to go ahead and use what I already mixed?
posted by Jacqueline at 7:34 AM on May 10, 2016

No, I wouldn't use the mix you've already made at the full rate, with the 10 lbs of Epsom salts, on the whole garden. You could try a test strip of 10 ft and see what happens. Or if you're using a spreader where you can set the application rate, turn it down lower and apply half the amount on most of the garden. If everything grows well, then you could always apply the rest next year. The mix itself would be fine bagged up and saved until next year.

The other thing to consider is that your compost is already going to contain N, P, and micros, so adding nutrient amendments to the full recommended amounts and then adding compost as well is putting lots of nutrients into the soil.

In the fall, you can apply a light layer of compost to your soil (top dress is fine) and then next spring, do another soil test. Over the winter, the compost breaks down and will give you a much more accurate idea of what's going on in your soil after a year of compost and nutrients being applied. This first year is tough because you just have the analysis of the dirt itself, not the dirt plus the compost.

Finally, put some thought into how you're going to prevent your lawn from taking over your garden. Because you do not want to go to all this effort and have a grassy mess on your hands.
posted by bluebelle at 12:30 PM on May 10, 2016

Yeah I think I'm just going to put a partial amount down (like ~1/4th or so) and save the rest for when my mother-in-law replants parts of her lawn later this year.
posted by Jacqueline at 4:17 PM on May 10, 2016 [1 favorite]

Regarding the lawn, I've got the whole thing mulched with rotting leaves right now. I was thinking of leaving a narrow border of mulch around the garden and on the other side of the fence and then mulching around the plants where feasible. That should help, right?
posted by Jacqueline at 9:42 AM on May 12, 2016

Regarding planning a rotation, thanks for the offer but different sections get different amounts of sunlight so my rotation options are limited -- e.g., there are some sections where I'm just going to have to switch between tomatoes and peppers from year to year. So I've already planned what I'm going to plant where depending on hours of sunlight and companion planting.

Meanwhile, we're not going to live here forever and my MIL doesn't intend to continue the vegetable garden after we leave. That's why I didn't build raised beds. So in a few years it will either return to grass or be planted with rosebushes.
posted by Jacqueline at 9:49 AM on May 12, 2016

Even if you don't want raised beds it would be worth the minor expense to build one that is just one board tall. That little barrier when dug in will prevent the inevitable grass creep in from the lawn. I just dug in some boards to mark out the beds at my allotment and it made the world of difference. You would only need 204 feet of board, and I think it will be money well spent.

Actually it looks like wood is quite expensive so up to you. It looks like it would be about $100 or so. It has saved me a lot of time by not having to weed as much, but not so sure of the $100 cost. I've lived too long in the UK so I am always thinking in £ and £100 would be too much for me, thankfully boards are cheaper over here for some reason.
posted by koolkat at 1:45 AM on May 13, 2016

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