Dear Nguyen Hue,

Biochar is one of the oldest soil amendments. A soil amendment is a product that when properly applied improves soil quality and fertility. Biochar is essentially charcoal that is produced in a special way from heating the air-dried plant material (biomass) in an oxygen depleted environment.One method of making biochar: pile up woody debris in a shallow pit in a garden bed; burn the brush until the smoke thins; damp down the fire with a one-inch soil covering; let the brush smolder until it is charred; put the fire out. The leftover charcoal will improve soil by improving nutrient availability and retention.One of the great sins of gardening. Cleaning up in fall, faced down a ton of seed-bearing foxtail, burdock and crabgrass. Sure, compost it hot to steam the weed seeds to death, but instead decided to try something different. dug a ditch, added the weeds and lots of woody pruning’s, and burned it, thus making bio char. It was my new way to improve soil—except the technique is at least 3,000 years old.

Poor quality sandy soils hamper productivity. Soils have depleted organic carbon contents that lead to poor nutrient retention, reduced aggregation, and low plant-available soil water retention. Past soil management used reduced tillage to increase organic carbon but it deteriorated quickly in the hot, humid environment. Biochars can provide an alternative recalcitrant carbon source. Since biochar varies widely in characteristics, it must be designed to fit the needs of the soil increased carbon, aggregation, nutrient retention, and plant-available water retention. Biochar design characteristics depend mainly on feedstock characteristics and method of pyrolysis. This review offers guidelines for designer biochar manufacture through feedstock selection and pyrolysis technique; it outlines potential usage to improve specific soil quality problems.

Biochar from sugar cane bagasse seems to be a promising sorbent and soil conditioner due to its high surface area, high surface acidity and microporous structure. This biochar can be applied to a wide pH range of soils for enhancing nutrient and water retention. On the other hand, the biochars from black wattle and vineyard possessing high concentrations of aromatic carbon, nutrients, and alkalinity are potential soil amendment agents. Black wattle biochar is more beneficial compared to biochar from vineyard due to its higher surface area, microporosity and cation exchange capacity.

Following method for enhancement of biochar.

1. Vacuum pyrolysis contributes to the development of C=O group on biochar surface.

2. Alkaline & acidic biochar are produced from the vacuum pyrolysis of the agriculture waste.

3. Black wattle, an invasive, has been found suitable for soil amendment purpose.

Regards,

Prem Baboo

Dr.Hue,you have raised a very good question but very difficult to answer(at least for me).Dr.Baboo has provided good background information and also attached good paper by Dr.Bayan .From the paper it appears that slow pyrolysis produced biochars are better than Fast pyrolysis ones.The biochar produced is also more in slow pyrolysis (25-31 %) than that produced by fast pyrolysis(12%of the original weight of biomass,air-dried).First major point is whether the biochars increase the pH of acid soils? If so to what extent?The biochars contain mineral matter and the pH of water saturated samples of slow pyrolysis biochars is in alkaline range.The biochars may to some extent  increase the nutrient availability in acidic soil but will they increase the pH of acid soils containing exchangeable Al and H ions(pH less than 5.5)?It was mentioned in the paper that the nutrient content of biochars is not enough to sustain optimal plant growth.The other benefits claimed are high surface area which can provide good conditions for microbes  by holding  more water and nutrients(biochars have surface charges at lower pH and can hold K,Ca and Mg ions).It appears that the application rates suggested,2-4% by weight appears to be very high.A 2% application rate corresponds to 17.4 short tons(800 lbs/1000 ft square area).The biochar at this rate is not recommended for Mollisols and Vertisols. Mollisols have already high organic matter and in Vertisols the biochar may increase the osmotic pressure in the rhizosphere .For increasing water holding power one has to apply biochar at 4% by weight rate (around 35 short tons).On fresh weight basis(before biochar preparation)the organic matter requirement would be more than 100 short tons.It is also not known how long the befit will last.My only concern is one has to go through previous work critically  before taking up new work on biochars especially for agricultural purpose. 

Thank you, all.

It seems to me that there is no magic bullet: We need to know our biochar AND our soil well before a good match can be realized.

BTW: The paper written by Dr. Bayan as provided by Dr. Baboo is quite informative. It is worth reading. Also, a good summary is provided by Dr. Rao here.

Cheers, NVH

Biochar production should be done in closed environment, otherwise it will cause severe pollution. There is always the alternative option to regularly apply compost. Biochar is especially an environmentally sound option when using it as byproduct of a pyrolysis.