Some effects of plant ash on the chemical properties of soils and aqueous suspensions


Laboratory studies indicate that percolation of water through accretions of plant ash will markedly increase the pH of solutions entering the underlying soil. Ash derived from graminaceous straws raised the pH of an aqueous suspension (1 g ash: 500 ml H2O) to 10.6, compared to 9.3 for a saturated solution of CaCO3. However, on a weight basis these ashes possess only approximately 5–15% of the alkalising power of CaCO3 in terms of their ability to neutralise acid or elevate soil pH. The effect of ash on soil pH, and pH dependent soil properties, is determined by the amount and composition of the ash deposited and on the buffering capacity of the soil.

High field rates (2480–6750 kg ha−1) of ash added to a podzolic soil increased the pH of the soil solution by up to 3 units. A pH rise of 1 unit is sufficient to increase the solubility of native soil organic carbon (O.C.) in water (i.e. mobilize readily metabolizable microbial substrate) and stimulate respiration rate.

In more strongly buffered krasnozem soil, maximum rise in pH of the soil solution did not exceed 0.7 unit. However, even such small pH shifts significantly reduced the solubility of soil O.C. in water; probably by changing the character (and hence solubility) of organo-mineral chelation complexes. High rates of ash application also slightly decreased respiration rate in this soil type.

The application of neutralized ash, which added metal ions to the soil solution, had a similar but smaller effect to unneutralized ash on the solubility of soil O.C. in podzolic soil, but little effect in krasnozem soil.

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Raison, R.J., McGarity, J.W. Some effects of plant ash on the chemical properties of soils and aqueous suspensions. Plant Soil 55, 339–352 (1980).

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Key Words

  • Fire effects
  • Plant ash
  • Soil pH
  • Solubilization of soil organic matter