Guidelines on Practical Aspects of Biochar Application to Field Soil ...

Figure 1. Left: Hand crushing biochar inside sacks in Honduras. Photo by J. Major. Right: Crushingmoistened biochar using a small compost shredder in Oregon, USA. Photo by D. Dalegowski.on biochar application rates according to soil types and crops. Also, biochar materials candiffer widely in their characteristics, thus the nature of a specific biochar material (e.g. pH,ash content) also influences application rate. In the published literature, several studieshave reported positive effects of biochar application on crop yields with rates of 5-50tonnes of biochar per hectare, with appropriate nutrient management. This is a largerange, but often when several rates are used, the plots with the higher biochar applicationrate show better results (Chan et al., 2007, 2008; Major et al., 2010b). Since the Ccontent of biochar materials varies, it may be appropriate to report application rates intonnes of biochar-C per hectare, as opposed to tonnes of bulk biochar material. A 10 t/haapplication of poultry manure biochar contains much less C (and more ash) than anequivalent application of wood waste biochar. However, “high-ash” biochar can constitute asource of various plant nutrients, and these should be taken into consideration whenmanaging soil fertility at the field level.Most biochar materials are not substitutes for fertilizer, so adding biochar withoutnecessary amounts of nitrogen (N) and other nutrients cannot be expected to provideimprovements to crop yield. Instances of decreasing yield due to a high biochar applicationrate were reported when the equivalent of 165 t of biochar/ha was added to a poor soil ina pot experiment (Rondon et al., 2007). At this high application rate, yields decreased tothe level of the unamended control. This is a very large amount that is unlikely to bepractically feasible in the field, at least for a one-time amendment. However, Asai et al.(2009) working in Laos reported greater upland rice yields with 4 t/ha biochar, but when 8or 16 t/ha were applied, yields were not different from the unamended control. A morerecent field study on a poor, acidic soil of the USA showed that peanut hull and pine chipbiochar applied at 11 and 22 t/ha could reduce corn yields below those obtained in thecontrol plots, under standard fertilizer management (Gaskin et al., 2010). The reasons for<strong>Guidelines</strong> for Biochar Application to Soil—International Biochar Initiative Page 9