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Eva Brod and Trond Knapp Haraldsen Bioforsk Soil and environment, Ås
Wood ash An alternative fertiliser for agriculture
Context
• Biomass fuel → wood ash = residual product
• 30 kt today disposed on landfills
• Contains K, P, Mg and other nutrients essential for plants
Which wood ashes can be used as fertiliser?
• Regulated by gjødselvareforskriften • Determining: heavy metal concentration
0 I II III
Cd 0.4 0.8 2 5
Pb 40 60 80 200
Ni 20 30 50 80
Zn 150 400 800 1500
Cu 50 150 650 1000
Cr 50 60 100 150
Table. Maximum limits of heavy metals (mg/kg DM) for quality classes 0 - III
> Agriculture: 0 – II > Urban greening: 0 - III > Bottom wood ash < fly ash > Limiting: Cd, Ni, Zn
Which wood ashes are suited as fertilisers?
• High nutrient contents (K, P, Mg, Ca, micronutrients) > 6 g K/100g DM > 1 g P/100g DM
• pH increase = yearly demand > CaCO3 + H+ > Ca2+ + CO2 + OH- or CaO + H+ > Ca2+ + OH- > Ca/K < 3
+ N
Are nutrients plant-available?
• Complex chemistry of wood ashes • Study one nutrient and exclude others!
1. K fertiliser 2. P fertiliser
Barley (year 1) and wheat (year 2) 35 or 70 kg K/ha Wood ash + mineral N or organic N and P
Ryegrass, 4 harvests 30 kg P/ha Wood ash + all other nutrients
Wood ashes as K fertiliser
• Effect in year 2 → Nutrient uptake → Yield → Soluble K in soil • Stronger effects
expected the following years
• K is soluble and plant-available! (Erich 1991; Etiegni et al. 1991; Ohno 1992… Brod et al. 2012; Brod et al. 2014)
Wood ashes as P fertiliser
• 70 – 80% of fertilisation effect of mineral fertiliser
• Delayed P availability • Soil pH: Variable
influence
Example: Solid-state 31P-MAS NMR
1. Wood ash 2. Wood/cereal ash
Various Ca phosphates Struvite
Various Ca phosphates
Pyro-phosphates
Summary
• Wood ash = promising alternative fertiliser
• Quality requirements > Heavy metal/nutrient ratio
> bottom wood ash vs. fly ash
• Implementation? > How to find and separate «high quality wood ashes»?
> How to establish alternative fertilisers on fertiliser market?