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The soil sample I took last fall from my vegetable garden came back from the lab with this result:

                                                Optimum Range
Calcium, Mehlich 3 (Ca)     2063     (ppm)  H   800 - 1200   
Magnesium, Mehlich 3 (Mg)   416      (ppm)  H   60 - 120     
Potassium, Mehlich 3 (K)    573      (ppm)  VH  170 - 280    
Phosphorus, Mehlich 3 (P)   248      (ppm)  VH  30 - 50      

Where "VH" means "very high" and H means "high".

Fortunately the pH is 6.5, which is about where I want it. However, all the lime and manure I've added over the past few years has kind of thrown things out of whack...

According to this handy chart:

  • Too much potassium "Causes N[itrogen] deficiency in plant and may affect the uptake of other positive ions such as Mg and Ca".
  • Excess phosphorus "shows up as micronutrient deficiency of Zn, Fe, or Co".

Obviously, I don't need to add fertilizer except for nitrogen.

Is this something I should try to balance out? How?

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Add more unenriched soil? Grow some plants that thrive on high-K/P soil? –  Alex Feinman Jun 16 '11 at 18:21

1 Answer 1

up vote 11 down vote accepted
+50

This is an interesting question. I have studied the chart you mention and, besides all the deficiency/ toxicity symptoms that it states may result from an excess of these nutrients, I would add that in the long term (1) excess calcium can increase the pore space in the soil to as much as 50%, as a result of which the soil dries out too quickly, water uptake is severely reduced and this leads to disease and/or low yields, and (2) excess potassium can contribute to the proliferation of pathogenic organisms and bacteria.

If your soil is rich enough in humus, you can avoid adding any more manure or standard fertilizer, and it should slowly normalize. For nitrogen, I would use a product such as blood meal (12% nitrogen, but only 1% phosphorus and 0.6% potassium). This is ideal for nitrogen-hungry greens, although perhaps not for cabbages, as it produces a mildly acidic soil reaction; nor, of course, for beans and peas, which have root nodules which fix atmospheric nitrogen.

Unfortunately, in soils like yours, where the phosphorus level is between 150 to 350 ppm, it can take from 3 to 5 years for the phosphorus balance to return to normal.

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Thanks for the detailed answer. I am using blood for N, and I think I have enough humus. Interesting info about the calcium; I had to add a lot of lime to get the pH a reasonable level (native soil is/was 5.0). –  bstpierre Jun 17 '11 at 13:14

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