The first issue; Economics which is why most of the commercially produced biochar is so expensive, currently around 2 dollars per pound, sold in tons. (one pound will effectively treat 1/2 cubic foot of soil to create terra preta)
bio char is not sold by the cubic yard at this time that I have found. 1 cu.yd. = 27cu.ft., 1 cu.ft. char = 2 lb., 1 cu.yd. = 54lb. x 2.00 = $108 (+taxes)
The second issue; Viability of the product, this means confirmation that the microorganisms are there and will remain alive not only during transit but after application.
The third issue; acceptability by people entrenched in "Modern method"
gardening and agriculture. this isn't "fertilizer" and the masses are saturated with the chemical companies advertising and most of the scientific community perpetuates the "grow it in dirt" mind set.
If you can get people out of this methodology and thinking more along the lines of "draught proofing your soil", then you will start to get them to consider the very real benefits of creating their own terra preta. (golf courses would benefit the most at this time)
Currently you have to buy your commercially produced, organism certified biochar at the rate of USD 2,000 + per ton.
Once you have it spread over the area to be treated, you have to incorporate the char into the soil (tillage) so it can work the magic.
In Trials, it generally takes one season for the effects to settle in and become stable.
Current recommendations (by the char selling companies) is to have a layer of char 1 foot or more thick, so for a 100 sq. ft. space you would use one ton of char.
If you add in the cost of producing compost/ biochar, you are looking at a price per pound of somewhere in the 3 dollar per pound range if the compost is 50% char.
Those are the obstacles to be overcome.
Redhawk