Bear Kaufmann. Initially posted April 7, 2008. Updated August 5, 2008.

Images showing trial preparation and radish germination
(Select image to enlarge in Gallery.)

Materials/Methods

Char was Lazzari Brand mesquite BBQ char (due to availability), crushed and screened to 1/8".
No nutrients were added to the char itself or to the soil.
Soil was FoxFarm OceanForest Potting Soil.
Sand used was horticultural sand.
No mycorrhizal fungi were added.
Mixtures range from 0-100% sand, soil, and char in ~16% increments by volume. 90 pots total. 28 combinations with 3 pots each + 6 additional pots at 33%/33%/33% composition. Pots were placed randomly within the tray. Tray was rotated 180° occasionally.
Plants were watered daily by a drip irrigation system.
Plants were removed from pots ~1 month after first watering. Soil was rinsed from roots and roots were patted dry with a towel. Wet weight of roots+shoots was measured (Acculab VI-3mg, 0.001 g precision).

Figure 1. Box Plots Showing Effect of Composition Across Three Transects

Figure 2. Pictures of Radishes at Important Compositions

Results

Plant growth was stunted even for the best preforming plants, likely due to the small pot size. Leaf color varied across different compositions.
A mixture of 33% charcoal and 67% soil had the best growth (176% of pure soil). Aside from mixtures around this level (Figure 1b), high levels of charcoal showed a generally negative effect on plant growth (Figure 1c).

Discussion

The positive interaction effects of charcoal and soil (Figure 1a,1b) are interesting. Assuming charcoal itself provides no integral nutrients to the soil (eg. nitrogen), increasing amounts of charcoal displace nutrients available from the soil mixture. The effects at 33% char/67% soil, however, show beneficial effects. This could be explained by increased mineralization rates caused by the charcoal causing soil nutrients to be more available to plants. Beyond 33%, the Cation Exchange Capacity of the charcoal may have held the nutrients produced by mineralization, making them less plant available. Since the charcoal was not amended/soaked in a nutrient bearing solution it likely had a low Base Saturationi leading to adsorption of nutrients as they became available. Other potential explanations for increased growth along the soil/char transect include alterations to pH or limiting nutrients (eg potassium(?)) provided by the charcoal. The speculative mineralization/CECii model could also explain the effects seen along the sand/char transect. Here, since the sand lacks organic materials and bound nutrients for soil microorganisms to make plant available, the increasing unsaturated CEC may have made any nutrients less plant available.

Author: Bear Kaufmann bear@ursine-design.com

Agronomic values of greenwaste biochar as a soil amendment
K. Y. Chan, L. Van Zwieten, I. Meszaros, A. Downie,and S. Joseph
Australian Journal of Soil Research 45(8) 629–634, December 2007

Abstract

A pot trial was carried out to investigate the effect of biochar produced from greenwaste by pyrolysis on the yield of radish (Raphanus sativus var. Long Scarlet) and the soil quality of an Alfisol. Three rates of biochar (10, 50 and 100 t/ha) with and without additional nitrogen application (100 kg N/ha) were investigated. The soil used in the pot trial was a hardsetting Alfisol (Chromosol) (0–0.1 m) with a long history of cropping. In the absence of N fertiliser, application of biochar to the soil did not increase radish yield even at the highest rate of 100 t/ha. However, a significant biochar × nitrogen fertiliser interaction was observed, in that higher yield increases were observed with increasing rates of biochar application in the presence of N fertiliser, highlighting the role of biochar in improving N fertiliser use efficiency of the plant. For example, additional increase in DM of radish in the presence of N fertiliser varied from 95% in the nil biochar control to 266% in the 100 t/ha biochar-amended soils. A slight but significant reduction in dry matter production of radish was observed when biochar was applied at 10 t/ha but the cause is unclear and requires further investigation.

Significant changes in soil quality including increases in pH, organic carbon, and exchangeable cations as well as reduction in tensile strength were observed at higher rates of biochar application (>50 t/ha). Particularly interesting are the improvements in soil physical properties of this hardsetting soil in terms of reduction in tensile strength and increases in field capacity.

Keywords: charcoal, char, agrichar, soil strength, soil carbon sequestration, hardsetting soil, slow pyrolysis.
Australian Journal of Soil Research 45(8) 629–634
Submitted: 27 July 2007 Accepted: 2 November 2007 Published: 7 December 2007
Full text DOI: 10.1071/SR07109

See also:Assessing agronomic values of chars to an Australian hardsetting soil presentation to the International Agrichar Initiative conference, Australia, 2007.