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Excessive Alkalinity - Symptoms

Page history last edited by Philip Small 13 years, 10 months ago

Plant Symptoms of Excessive Soil Alkalinity. Visible symptoms of nutrient deficiencies can be most informative in establishing that soil pH has become a problem worth dealing with. Because of a cool, moist spring 2008 season, iron chlorosis was the first clue that I had induced elevated soil pH.  Unlike the more common nitrogen chlorosis, iron chlorosis affects new growth first, turning it pale green, then yellow-green, and in extreme cases, to almost white. Leaves showing iron chlorosis often retain green veins. Even in mild cases where yellowing is slight, growth is noticibly reduced. Iron chlorosis usually clears up when soil warms up.


Another visible symptom of elevated soil pH is phosphorus deficiency.  This is a more persistent effect than iron chlorosis.  Plant development is slow, growth is stunted with very limited root growth. Many plants develop dark green leaves with purplish or reddish hues in the leaves and petioles. 


Other nutrient deficiency symptoms associated with high soil pH are yellow mottling on young leaves (manganese deficiency) and rosetted new growth.  Both boron and zinc deficiency can cause rosetted new growth. Boron deficiency can also cause the plant to become a dark green.  Copper can be deficient in high pH soils: new shoots won't open, the whole plant is pale colored and young leaves are thin and yellow.


Responding to Biochar-induced Excessive Soil Alkalinity.  If it looks as if biochar-induced high soil pH is a concern in your garden, you might consider simply waiting and watching: the caustic (ie alkaline) contituents in ash are reactive, that is, they are not persistent.  If your soil has a high buffering capacity, associated with high clay, high calcium, and/or high organic matter content then you should see soil pH moderate with time.  One dormant season may be sufficient for soil to achieve a balanced level of acidity.  Otherwise, there are several steps you can take to mitigate biochar's lime effect:

  • Use a reduced alkalinity feedstock for your biochar. Little has been published in this area, however, biochar derived from bamboo and pine-needles is purported to have an acidifying effect on alkaline soil.  Mulga (Acacia species native to the Australia bush country) charcoal is purported to have a pH of 6.0.
  • Use low-temperature, high bio-oil condensate biochar. This includes biochar where bio-oil condensates (example: wood vinegar) are recovered from the wood gas and returned it to the charcoal.
  • An optional way to make a quick change to soil pH is to water the plant several times with a solution of 2 tablespoons vinegar to a gallon of water. This is a handy way to adjust pH in containers indoors and out. (Source)
  • Water processing of the charcoal can eliminate liming characteristic of charcoal: the alkaline constituents of charcoal are soluble. The downside is that ash-based nutrients (especially Ca, K, and S) are also removed.
  • Increase applied soil organic matter. Thoroughly matured compost is preferred but may require large volumes.  Supplementing compost with acid sphagnum peat can be especially effective in reducing alkalinity. Mixing peat 1:1 (volumetric) will neutralize a 1% calcium carbonate equivalent soil. (Source)  Peat applied at 2.5 lbs per square yard is capable of reducing pH by 1.0 unit in some soils.  
  • Something to consider once organic matter is where you want it:  You can apply an acid-effect fertilizer, an approach which is more effective in combination with applied organic matter. Examples of acid-effect fertilizer are ammonium sulphate, urea, or an ammonium phosphate.
  • As a last resort, and in moderation, you can apply sulfur.  This is an approach that requires time, moisture, warmth and microbial activity to effect pH.  This makes it a candidate to mitigate ash-based alkalinity both in soil and in the compost.  To be safe, use flowers of sulfur formulated specificically for garden soil use.  To reduce soil pH by 1.0 unit, apply 1.2 oz per square yard on sandy soils, or 3.6 oz per square yard on other soil types.  Elevated soil sulfur is known to raise the hotness of peppers and onions. If you like your onions, leeks, shallots and garlic to be mild, you might want to shy away from this one.


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