BIOCHAR - Fertilizers: Components, Uses in Agriculture and Environmental Impacts - page 114

Table 1. Numerical values established by IBI for biochar toxicity assessment reporting
Element/compound Maximum Allowable Threshold range †
PAHs 6 - 20 μg g -1 TM ‡
Dioxin/Furan (PCCD/F) 9 ng kg -1 I-TEQ §
PCBs 0.2 - 0.5 μg g -1 I-TEQ
Arsenic 12 - 100 μg g -1 DW ¶
Cadmium 1.4 - 39 μg g -1 DW
Chromium 64 - 100 μg g -1 DW
Cobalt 100 - 150 μg g -1 DW
Copper 63 - 1500 μg g -1 DW
Lead 70 - 500 μg g -1 DW
Molybdenum 5 - 75 μg g -1 DW
Mercury 1 - 17 μg g -1 DW
Nickel 47 - 600 μg g -1 DW
Selenium 1 - 100 μg g -1 DW
Zinc 200 - 2800 μg g -1 DW
† See text for clarification; ‡ Total mass; § International Toxicity Equivalent; ¶ Dry weight basis.
C ONCLUSION
Biochar is likely to have a positive and prolonged effect on soil properties, the effects of
which are yet to be fully appreciated given the relatively short-term studies being reported so
far. The agronomic value of biochar in improving soil fertility is highly process- and
feedstock-dependent. Biochar CEC decreases with increase in pyrolysis peak temperature
above ca. 350 - 400 °C, whereas porosity, anion exchange capacity, hydrophobicity, surface
area, and carbon stability increase. The increase in surface area and porosity may compensate
for the reduction in CEC while also providing habitat promoting microbial activity. In
addition, pyrolysis at higher temperature increases biochar liming capacity. Yet, feedstock
nutrient recovery and biochar hydrophility as well as plant available nutrient all decrease with
increase in pyrolysis temperature. Optimization is needed between increasing pyrolysis
temperature to enhance carbon stability on the one hand and producing biochar of high CEC
and nutrient content and availability on the other hand.
Biochar improves several soil quality indicators including CEC, bulk density, and carbon
content. The most profound effect of biochar on soil fertility and plant growth was shown to
be due to its CCE and the ability to increase the pH of dystrophic, acidic, and highly
weathered soils where biochar improvement of soil fertility has shown to be most significant
and consistent. Positive plant response to fertilizers application in biochar-amended soils
suggests that while improving growing conditions and ameliorating acid soils growth limiting
factors such as aluminum toxicity, nutrient content and availability become the main growth
limiting factor. Biochar from animal manure feedstock showed to maintain higher nutrient
content than biochar form herbaceous and woody feedstock and to, when applied at sufficient
application rate, provide for crop nutrient requirements. When produced as the main product
under slow pyrolysis conditions, biochar can be engineered to achieve desired characteristics;
conversely, when produced as a byproduct, post-production augmentation procedures to