Can biochar be used in effectively managing soil acidity?

What Causes Soil Acidity?

A recent article (Dai et al., 2017) reviewed how biochars can be used to decrease soil acidity. Approximately 50% of global arable land has a pH value < 5.5 and is considered acid. Soil acidification occurs by:

  • Acid deposition;
  • Removal of farm products; and
  • Application of ammonium-based fertilizers.

Soil acidity increases toxic metal availability, especially aluminum and manganese, and reduce nutrient availability including phosphorus, base cations (potassium, calcium, magnesium), and molybdenum.

Biochars Effect on Reducing Soil Acidity

Biochars have been shown to consistently elevate soil pH (Igalavithana et al., 2016). Biochar pH ranges from 5.9-12.3 with a mean of 8.9, the alkaline nature due to alkali salts determined by feedstock and pyrolysis temperature. As a rule, higher pyrolysis temperatures produce higher biochar pH, while manure-based biochars have higher pH values that lignocelluloses-based ones (Dai et al., 2017).

Criteria of how to use biochar for acid soil improvement. ++: recommend; +: could be considered; variable: inconsistent results reported. Source: Dai et al., 2017

As with any other amendments, appropriate application is required. Biochars with a pH > 7.0, have a positive effect on soil acidity with soils having a pH < 5.5, compared with application in alkaline soils. This is due more to biochar alkalinity rather than the pH value. Cations such as calcium, potassium, magnesium, sodium and silicon for either carbonates or oxides during pyrolysis which react with H+ and monomeric aluminum species in acid soils, increasing soil pH and decreasing exchangeable acidity.

References

Dai, Z., X. Zhang, C. Tang, N. Muhammad, J. Wu, P. C. Brookes, and J. Xu, 2017, Potential role of biochars in decreasing soil acidification – A critical review: Sci Total Environ.  Article in press.

Igalavithana, A., Y. S. Ok, A. R. A. Usman, M. Al-Wabel, P. Oleszczuk, and S. S. Lee, 2016, The Effects of Biochar Amendment on Soil Fertility, in M. Guo, Z. He, and S. M. Uchimiya, eds., Agricultural and Environmental Applications of Biochar: Advances and Barriers: SSSA Special Publication: Madison, WI, Soil Science Society of America, Inc., p. 123-144.

About the Author

Andrew Harley is a soil scientist and geochemist focused on managing acid soils with high metal contents.  Andrew is a founder of Biochar Solutions, Inc and Biochar Reclamation, with one of the first field scale biochar reclamation trials on a mine site.  For a free evaluation of your soil reclamation needs, email andrewharley@ascensionsoil.com.

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