This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
Sow them in a forcing-pit or hot-frame about the beginning of February. About six weeks thereafter place them in 3-inch pots - one in each pot - filled with rotten turf and leaf-mould - two parts of the turf to one of the mould; and grow them on in heat until about the end of April; then transfer them to a cold-frame, and keep it close for the first three days, except during sunshine, when the higher ends of the sashes ought to be raised 1 or 2 inches, according to the heat of the sun. Shut the frame about four o'clock, so as to husband the heat, and thus a higher temperature will be kept up in it during the night. After the first three days, the weather being favourable, open the frame half an hour earlier every day, and close half an hour later. The sashes should at the same time be raised a little higher every day until they are removed altogether; this may be accomplished in ordinary weather in the course of ten or twelve days, when the plants may be put out into a sheltered place, prepared in the following manner: - In autumn take out a trench, say, 8 inches deep and 9 inches wide, or thereby, then raise the subsoil with a pick 12 or 14 inches deep to carry off the superfluous water.
This being done, place the soil to be taken from the next trench on the top of the subsoil so raised. Continue to proceed in this way, raising the subsoil and turning the surface on the top of it until you have the quantity of ground required, then level the ground and remove the whole surface 1 inch deep, and place thereon a coating, 1 inch thick, of pure nightsoil, or, should such manure not be available, 2 inches of rotten cow, horse, or pig's dung may be used; after which spread the soil taken from the surface over the top of the manure. When thus prepared, give the whole a coating of strong salt in the proportion of 1 lb. of salt to 4 square yards. A little leaf-mould or rotten turf pointed into the ground immediately before planting would be useful in starting and rooting the plants. However, care must be taken, while this is being done, not to allow the manure to be buried too deep by the digging; nevertheless, it should be thoroughly mixed with the soil near the surface. Dung buried only 2 or 3 inches deep will produce a much better crop of Onions than if it were buried 8 or 10 inches. Many may be disinclined to believe this, but they have only to make one fair trial to become convinced.
Every horticulturist and agriculturist knows well that the roots of plants grow towards the manure; consequently, if the manure is deep, the roots of the plants will be deep also, and will in that way, to a very great extent, lose the benefit of the influence of the heat of the sun. On the other hand, when the dung is near to the top, the roots will not be far from it; and thus the plants will have all the advantages derived from the manure, as well as those from the heat and gases of the atmosphere, so essential to the growth of the plant. In planting out the Onions, knock them out of the pots, taking care not to break the balls. Plant them in rows, at least 14 inches apart and 9 inches between each plant, placing the under-side of the heads of the Onions on a level with the surface; then put a stake to each of them, and draw up the earth with a hoe on each side of the rows, and thus form ridges for the Onions to grow in. When the heads begin to develop, the ridges may be levelled and the stakes removed; then give another coating of salt in the proportion of 1/2 lb. to 4 square yards. In dry weather give them a good watering at least once a-week. The water may be mixed with old urine in the proportion of one of urine to ten of water.
In moist weather one of urine to five of water may safely be used. When urine is not to be had, 1/4 lb. of the best guano may be sown every three weeks, on a wet day, over 6 square yards of ground, prepared as aforesaid. In the absence of rain, use the watering-can with the rose on, so as to clean the plants of the guano and wash a portion of it into the soil. Some prefer using the guano in a liquid state, but I have always been most successful with it when used in the way of top-dressing. Those who have not prepared their Onion-ground in the autumn, can do so yet in the way recommended, taking care to use no manure but that which is thoroughly rotten; and instead of using 1 lb. of salt to 4 square yards, use now only half that quantity.
In proof of the advantages of growing Onions in the way referred to, I may mention that I grew one Onion of the Santa Anna Madeira variety to the weight of 1 lb. 14 oz., and that a few of the same variety were shown by the writer in a collection of vegetables at the last autumn show in Glasgow, which measured upwards of 15 inches in circumference. The six Onions shown by him which carried off the first prize for weight from the Renfrew Show were of the same sort, and Aveighed 9 pounds. Those that carried off the prizes from the Glasgow Show in September last were raised in heat and afterwards transplanted, the varieties being Muncham Park, Blood-red, and Dan-ver's Yellow. James Dobbie.