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?