I now believe my abnormal increase in soil pH is due to a combination of low soil buffering capacity and the apparently high ash content of the biochar I manufacture. I use a modified open barrel approach and was estimating that I could achieve about 20% efficiency. i.e. about a pound of charcoal for every 5 pounds of dry wood. I anticipated getting about 10% ash at that rate. If my efficiency is instead lower, say 10%, it could have doubled my ash content, and thus the liming effect. It may not be the only reason for my quite surprising jump in soil pH, but certainly the first to consider.
I have updated
Gardening with Biochar FAQ per my experience:
5.0 What happens after biochar is in the soil?
5.01 Does biochar affect soil pH?
Raising soil pH is biochar's most important contribution to influencing soil quality. (
Source) Soil pH mostly influences the relative availability of nutrients. At low pH, aluminum toxicity is particularly harmful to plant growth. Aluminum toxicity is an extensive and severe soil problem and biochar is the most available and obvious solution that we have to combat it. Soil phosphorus availability is highly dependent on soil pH range, and thus biochar can be used to substantially increase phosphorus availability for soils that are below the ideal soil pH range of 6.5 to 7.0. (More on biochar and soil pH >>
biochar.pbwiki.com/Soil-pH)
Soil pH
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 pH 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, pH 6.0 to 7.0 is considered the ideal soil pH range for most garden plants.
Liming Effect of Biochar. The ash content of most biochars has a slight liming effect: it tends to increase a neutral or acidic soil pH to a more alkaline pH. Ash tends to have a pH of 12 - 13, and 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..)
Accordingly, adding so much biochar that you take your soil pH above the ideal range may not be a problem if 1) soil nutrients are both abundant and balanced and 2) the soil contains a substantial amount of thoroughly mature compost.
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.
Hypography note: Although the garden looked magnificently overgrown by the end, _all_ my plants were slow to develop and were reluctant to flower. Poor flower and fruit yield is a symptom of P deficiency. It is also a symptom of excessive N fertilization, which may have been a contributing factor. And while tomato and squash plants had great root masses by the end, not so with the beans, peas, strawberries, peppers, beets, and radishes which had very little below ground, indicative of limited phosphorus availability. Parsnips are still waiting to get pulled. The squash and tomato plants broke out of the doldrums and then never stopped growing when it got hot (positive biochar effect!) and eventually making for a very impressive looking garden. However the other (non-tomato, non-squash) plants never really kicked in like they should have. We felt that the peas and beans should have yielded at least twice as much, especially considering the ideal weather they had.
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 it out: 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. 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 pine-needles is purported to have an acidifying effect on alkaline soil.
- Use a high bio-oil condensate content biochar. This implies a lower temperature biochar as well as an effort to recover bio-oil condensates (example: wood vinegar) from the producer gas and returning it to the charcoal.
- Water processing can eliminate liming characteristic of charcoal: the alkaline constuents of charcoal are soluble. The downside is that ash-based nutrients (especially Ca, K, and S) are also removed.
- Increase applied organic matter. Peat can be especially effective in this regard. Peat applied at 2.5 lbs per square yard is capable of reducing pH by 1.0 unit in some soils.
- 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.
- Apply sulfur, an approach that requires time and microbial activity. To reduce 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.
