In 1842, English nurseryman William Paul experimented with a variety of different soil additives on roses grown in heavy soil. He found that the most beneficial additive was "burnt earth", what we would probably call biochar. He found that roses grown with burnt earth bloomed better and survived the following Winter's frost better than roses grown with various kinds of organic fertilizers.
He wrote about it in his book "The Rose Garden", which was first published in 1848 and which became the most successful gardening book in the English language. It remained in print for more than 50 years. In the chapter on soils, Paul described the benefits of "burnt earth" and his process for making it. His process involved covering burning brush and weeds with a layer of earth:
"Earth may be burnt at any season of the year. It has been the custom here, for some years past, when the operations of pruning, grafting, &c., are ended, instead of suffering the rough branches to lie about, presenting an untidy appearance, to collect them in a heap. A wall of turf, about three feet high, of a semi-circular form, is then built round them. The branches are set on fire, and when about half burnt down, seed-weeds, and such rubbish as collects in every garden and will not readily decompose, are thrown on the top, and earth is gradually cast up as the fire breaks through. During the first two or three days great care is requisite to keep the pile on fire. Here is the point where many fail. They allow the flame to break through and expend itself before the heap is thoroughly kindled. Constant watching is necessary at this juncture. As the fire breaks through, the heap should be opened and a layer of bushes and weeds should be added, and then a layer of earth. Follow up this plan, and the fire will spread through the whole heap ; and any amount of earth may be burnt, by continually adding to those places where the fire appears the strongest.
The soil burnt here is the stiffest loam that can be found within our limits, and which is of rather a clayey nature ; also turf from the sides of ditches and roads, in itself naturally sour and full of rank weeds. Burnt earth has been found beneficial in every instance where applied, and two or three annual dressings of it have worked wonders on moist heavy soils."