BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 104

1991). While most cereal crops tend to accumulate higher Si compared to legumes, 1 to 3%
vs. <0.5%, respectively (Marschner, 1995), biochars from cereal crops are likely to result in
high ash content but of lower CCE value as relative contribution of SiO 2 to biochar CCE is
much lower than that of alkali or alkaline carbonate and/or oxides. Indeed, plants can differ
markedly in their basic cations content and ash alkalinity (Tang & Rengel, 2003). Legumes
(and those of temperate zones more than those of tropical ones) tend to accumulate excess
basic cations, resulting in much higher alkaline ash content than non-legume plants (Bolan et
al., 1991).
While base accumulation of cereal crops ranges from 25 to 75 cmol kg -1 , that in pasture
legumes plants ranges from 61 to 255 cmol kg -1 (Tang & Rengel, 2003). Producing biochar
from straw of different plant materials at 350 °C for 4 hours, Yuan et al. (2011a, 2011b)
found that biochar from legume feedstock resulted in higher biochar alkalinity, ranging from
217 to 326 cmol kg -1 (mung bean [ Vigna radiata ] > peanut > soybean > pea [ Pisum sativum ]>
faba bean [ Vicia faba ]) compared to biochar from non-legume feedstock which ranged from
120 to 191 cmol kg -1 (canola > corn > rice > wheat). Incubated in an Ultisol (pH 4.2) at a rate
of 10 g kg -1 for 50 days, biochar from legume straw increased the soil pH by 0.5 to 0.7, while
the biochar from non-legume straw increased the soil pH by only 0.1 to 0.4 (Yuan et al.,
2011b). At the end of the 50 d incubation, only pea and soybean straw biochars increased soil
pH above the pH achieved by their respective raw material applied to soil at 20 g kg -1 (Yuan
et al., 2011b).
Organic wastes of initially high ash content, e.g. chicken, dairy and hog manures, likely
result in biochar of relatively high CCE. Yet, incubating an Ultisol (pH 4.8) for eight weeks
with chicken manure biochar produced at 350 and 700 °C, at similar application rate as in
Yuan et al. (2011b), i.e. 10 g kg -1 , resulted in increases in soil pH by only 0.3 and 0.5 pH
units, respectively (Hass et al., 2012). Soil pH increased to 5.8 and 5.9 by the end of the
incubation at application rates of 20 and 40 g kg -1 of biochar produced at 700 and 350 °C,
respectively. Based on linear regression between biochar application and soil pH, the authors
calculated that 36 and 56 g kg -1 (i.e. equivalent to 73 and 112 Mg ha -1 ) of chicken manure
biochar produced at 700 and 350 °C, respectively, were needed to increase the soil pH to 6.4,
a level achieved with 3 g kg -1 (6 Mg ha -1 ) of dolomitic lime (Hass et al., 2012). Such high
biochar application rates may not be attractive replacement option for lime. Moreover,
calculated on a volume basis the spread between biochar and dolomitic lime application rate
more than double as bulk density of biochar is less than half that of lime. However, as
additional benefits associated with biochar application emerge such an investment might be
justified.
Figure 4 provides a conceptual illustrative summary for the effect for pyrolysis peak
temperature on selected biochar properties that discussed above. Initial mineral and organic
feedstock composition will impact biochar composition at each peak temperature, with
feedstock of high lignin to (hemi) cellulose ratio (i.e. woody biomass) likely to result in lower
carbon loss during pyrolysis, higher total carbon recovery in biochar, higher stability of the
recovered carbon, and higher surface area than that produced from feedstock of lower
lignin:(hemi)cellulose ratio (e.g. grass). Yet, such biochar likely will result in lower CEC,
lower mineral (nutrients, liming) content, and to exhibit a lower concentration of mineral
content during pyrolysis due to lower losses of initial feedstock carbon matrix.