This section is from the book "A Dictionary Of Modern Gardening", by George William Johnson, David Landreth. Also available from Amazon: The Winter Harvest Handbook: Year Round Vegetable Production Using Deep Organic Techniques and Unheated Greenhouses.
Soot, a chief constituent of which is charcoal, lias long been known as a very effective fertilizer; and burning has still longer been known as a mode of reducing stubborn soils to prompt productiveness. But both these sources of fertility might owe their efficiency to other causes than their affording carbon to plants; and it is only within these very few months that anything like a general knowledge has been diffused that mere charcoal is one of the best of manures. The fact has been known for many years to individuals, as, for example, to Mr. Barnes, of Bicton; but it is only very lately that gardeners generally have learned, and I am happy in being able to join my voice to that excellent cultivator's in announcing, that - charcoal is a most efficient manure to all cultivated plants, especially to those under glass. Heaths, rhododendrons, cucumbers, roses, orchidaceous plants, hydrangeas, camellias, melons, and pine apples, have been the subjects of extended and most successful experiments. The advocates are too well known to require more than naming, for among them are Dr. Lindley, Mr. Barnes, Mr. Maund, Mr. Snow of Swin-ton Gardens, Mr. Stewart of Stradsett Hall Gardens, and Mr. Rivers. I think no cultivated plant would be unbene-fited by having charcoal applied to the soil in which it is rooted.
It should be broken into small pieces, about the size of a nut, and for potted plants may be mixed in the proportions of one part charcoal to twenty parts earth. If applied to the open ground, one-fourth of a bushel may be sown over a square rod, and dug in just before inserting the crop. The reason of charcoal being so useful as a manure is very apparent. MM. Sennebier, Ruckert, Saussure, and others, have demonstrated that plants are rendered much more luxuriant and productive, by having carbonic acid applied to their roots, than other plants to whose roots no such application was made. Now charcoal kept moist, as when buried in the soil, slowly combines with oxygen, and emits carbonic acid - in fact, it slowly dissolves. I am sorry to differ from such an authority as Liebig, who broadly asserts that "Carbon never combines at common temperatures with oxygen, so as to form carbonic acid." This was long since shown to be otherwise by Count Rumford; and may easily be demonstrated to be incorrect, by confining a few ounces of fresh and moistened charcoal powder, mixed with earth, in a glass receiver full of oxygen, over lime water; carbonate of lime will form, showing the gradual evolution or carbonic acid.
"A piece of ground that was cropped with coleworts last autumn, (1843,) was cleared early, and the refuse trenched in during the winter. 95 feet in length and 10 feet in width, was planted with small onions on the 14th of February, which onions had been sown the second week of September in the previous autumn. They were planted in rows one foot apart, and six inches from plant to plant - with the intention of drawing every alternate one for use through the summer - but the whole nine rows did not get entirely thinned. The following is the weight when ripe for storing on the 1st of August.
"Five rows grown where 4 lbs. of bone-dust to each row had been sown in a drill drawn 3 inches deep and filled up, and the onions planted over it - producing 420 lbs. weight of onions - each row yielding from 82 to 88 lbs.
"The other 4 rows had applied to them of fresh dry charred refuse and ashes, made from the garden rubbish-heap, two common buckets full, weight 14 lbs. They produced 366 lbs. of onions, the rows weighing respectively 99, 89, 95, and 83 lbs. The last row being injured by a row of red cabbage growing near.
"Many of the foregoing onions, which were a mixture of the Globe, Deptford, and Reading, measured in circumference from 14 to 16.V inches, and weighed as many ounces. I weighed 12 together, that turned the scale at 12 lb. 9 oz. I can only fancy what a wonderful saving and benefit it would be to the country, to char the refuse of old tan, chips, sawdust, ditch scourings containing sods, weeds, bushes, and refuse. By keeping the surface of the earth well stirred, no crops appear to suffer by drought that are manured by charrings, but continuo in the most vigorous health throughout the season, never suffering materially by either drought or moisture".
On spring sown onions and on turnips, Mr. Barnes finds charred or carbonized vegetable refuse equally beneficial. Three rows, each 95 feet long, of the white globe onion, manured with bone-dust, weighed 251 lbs.; whilst three similar rows of the same variety, and grown under precisely similar circumstances, but manured with char-rings, weighed 289 lbs.