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Soil pH

Page history last edited by Philip Small 15 years, 6 months ago

Continued from 5.01


The ideal garden soil pH is 6.0 to 7.0.  Phosphorus becomes available through biological transformation. Available phosphorus that is not taken up by plants and soil microbes is subject to geochemical fixation.  The degree of fixation is regulated to a large extent by soil pH.  Phosphorus is least available at high and low soil pH.  At soil pH above 7.2 to 8.5, phosphorus fixes as insoluble calcium phosphates. At soil pHs below about 5.5, iron and aluminum phosphates form, reducing phosphorus availability. Phosphorus availability is greatest between 6.0 and 7.0. For this reason, more than any other, 6.0 to 7.0 is considered the ideal soil pH range for most garden plants.


Liming Effect of Biochar. 

Biochar tends to have a liming effect: it tends to increase a neutral or acidic soil pH to a more alkaline pH. From 1.06:


In the opinion of this author the single most important quality of charcoal to the gardener is the ability to lower acidity, also termed liming capacity or effective neutralizing power.  This is easily measured in an agricultural laboratory as calcium carbonate equivalent (CCE). If you are growing acid-loving plants you will want a charcoal with negligible CCE, and purportedly this is true of Mulga (Acacia) wood, bamboo, and pine needle derived charcoal.  If you are combating low soil pH and aluminum toxicity you will want a charcoal with substantial CCE.  Oak and maple hardwood charcoal appear to have substantial CCE.  Apparently Amazonian hardwood derived charcoal shares this characteristic. Raising soil pH has been identified as biochar's most important contribution to influencing soil quality in the context of Terra Preta. (Source)


The ash component (as opposed to the black carbon component) of biochar tends to have a pH of 12 - 13, and hardwood charcoal tends to have a minimum ash content of 2-10%. At 10% ash, the effect a tonne of charcoal might be equivalent to as much as 1/10 tonne of lime.  At the high end of the target biochar application range (50 MG/ha) (see 4.01), soil pH would increase equivalent to lime applied at 5 tonnes/ha, enough in some cases to increase soil pH by 1.0 unit. In my garden I applied a high rate of high ash content biochar and observed soil pH rise from 6.5 to over 8.0.  If you are applying substantial amounts of biochar you should test your soil pH and compare it to the ideal for your plants.


If your soil pH is below 6.0, and you are not trying to grow plants that need sub-6 pH (examples: Aechmea, Aspidistra, Camelia, Hydrangea (Blue), Orchid) you can rest assured that your soil's acidity level will improve quite significantly from the addition of biochar. At higher pH levels, the addition of thoroughly matured compost to the soil can enable so-called acid-loving plants to thrive in a soil of pH 7.  This is because the natural chelating effect of the organic matter allows it to maintain the availability of trace elements to plant roots. (Hendreck, 2002, Growing Media..)


Soil pH is a simplistic measure of soil dynamics. It can only measure an average value present in the soil complex. Soil pH guidelines are general at best. Accordingly, increasing your soil pH above the ideal range may not be a problem especially if 1) soil nutrients are both abundant and balanced and 2) the soil contains a substantial amount of thoroughly mature compost. 


No soil alkalinity problems associated with biochar have been reported by a confirmed or reviewed source. A widely held conviction is that alkalinity should should not be an issue with most soil types and with most biochar types.  However, suspicions are that it has affected at least one garden, and some potted plants, and if you suspect deleterious effects due to biochar induced alkalinity, we have more information on plant symptoms and responding to alkalinity.

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