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.

This post is part of our The Intelligent Gardener lunchtime series.  Read all of the entries: The Intelligent Gardener Why soils need to be remineralized Soil cations How to fill out Solomon's soil analysis worksheets Using Solomon's worksheets to determine soil additives Do wood and straw raise soil potassium levels? Can you have too much organic matter in your soil? Clay, humus, and CEC Managing blueberry soil micronutrients Excess phosphorus in the soil

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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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