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Excess phosphorus in the soil

Phosphorus cycle

The last thorny soil issue I want to cover is phosphorus.  Steve Solomon's worksheets aim for an equal amount of phosphorus and potassium in the soil, while Michael Phillips thinks more phosphorus is better and recommends building orchard soil to have twice as much phosphorus as potassium.  On the other hand, Harvey Ussery explained that excessively high phosphorus levels cause a decline in mycorrhizal fungi, which means that plants actually have a harder time finding enough phosphorus and can experience a deficiency.  In a worst case scenario, extra phosphorus can even wash into streams and cause eutrophication.  Fertilizing with manure (especially chicken manure) boosts phosophorus levels in the soil quickly, so the question is --- should I be concerned about phosphorus excesses?

Location lbs P2O5/acre Deficit or excess P
Powerline pasture 56 −237
CP3 and CP4 225 −209
CP5 117 −196
Forest aisles 269 −127
Blueberries 345 unsure
Front berries 778 −21
Forest garden 1136 180
Back garden 1599 354
Front garden 2029 487
Mule garden 2246 551

Using Solomon's recommended phosphorus levels, it's clear that our native soil is low on phosphorus, but that my preferred horse manure fertilizer has increased levels within the vegetable garden beyond the recommended range.  Of course, if I was using Phillips' goal instead of Solomon's, the vegetable garden would actually be considered deficient in phosphorus like the rest of our homestead.

Solomon backs up Phillips by writing that excess phosphorus is seldom a problem since it will actually increase the nutrient density of your produce.  In fact, in a perfect world, Solomon believes our soil would have 2,000 to 5,000 pounds of phosphorus per acre, at which level enough of the mineral would naturally be released each year to feed our crops with no additional applications of fertilizer.  Based on these figures, I suspect we can carry on with my heavy manuring for another five or so years before phosphorus levels in our vegetable garden exceed biologically healthy amounts.

On the other hand, my readings suggest that excess phosphorus can be an issue in the blueberry patch, where the extra phosphorus can cause some of the same deficiency symptoms you'll notice from high pH or high calcium levels.  I doubt that we're anywhere near that level yet, but it might be worth focusing on building acidic, low phosphorus, low calcium compost for that patch for the future.  And, one of these days, I'll probably have to escape my reliance on off-farm manure and bring our homestead into more of a closed loop.  Stay tuned for a lunchtime series based on How to Grow More Vegetables on that very topic soon.

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This post is part of our The Intelligent Gardener lunchtime series.  Read all of the entries:

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About us: Anna Hess and Mark Hamilton spent over a decade living self-sufficiently in the mountains of Virginia before moving north to start over from scratch in the foothills of Ohio. They've experimented with permaculture, no-till gardening, trailersteading, home-based microbusinesses and much more, writing about their adventures in both blogs and books.

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Anna, have you worked though what using urine in the fertiliser mix would do to Steve Solomon's formulas for restoring nutrient balance. My thinking was that it could remove the need for the seed cake for Nitrogen and remove the need for Phosphorous, but was wondering whether you had run the numbers on this.
Comment by Jeremy Sun Feb 3 15:05:25 2013

Jeremy --- Excellent question. I've been meaning to write more about urine, but I'm actually not very scientific about it, so I don't have much to say yet. (I mostly just fill buckets with pee and pour them on woody mulches around perennials, moving to a new spot each time.) I think you're definitely right that diluted urine could replace seed meals in Solomon's system, but I haven't done the math to see how much you should apply.

Urine is lower in phosphorus --- NPK is about 11-1-2.5 --- which means that providing the deficit of 200 lbs/acre in our less-managed areas would require about 20,000 pounds of pee per acre. That's not terrible, though, since I fill up a five gallon bucket with pee about once a week, which is about 80 pounds per week or 4,160 pounds per year. So, if I played my cards right, I could eliminate the phosphorus deficit for a fifth of an acre every year.

(Assuming I did my math right --- my head's a little sleepy right now...)

Comment by anna Sun Feb 3 16:39:31 2013

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