An analysis of the Southport White Globe onion shows that 2,000 pounds of the mature bulbs contain 2.70 pounds of nitrogen, 0.92 pounds of phosphoric acid and 2.09 pounds of potash. The average legal weight of a bushel in the United States is about 56 pounds. A yield of 500 bushels an acre would, therefore, make a total of 28,000 pounds, and would require 37.80 pounds of nitrogen, 12.88 pounds of phosphoric acid and 29.26 pounds of potash. Although these figures have some value in indicating the need of the onion, it is generally recognized that the amounts used should be considerably in excess of that shown by chemical analysis. The fact is, that no other vegetable requires higher fertility than the onion. The plants must have a bountiful supply of available food until the bulbs are formed.
Stable manures are universally preferred to commercial fertilizers because of their influence on the physical properties of the soil. Poultry droppings, on account of their fineness and high percentage of nitrogen, possess the greatest value. When gardeners keep large flocks of chickens it will pay to collect the droppings at regular intervals and to preserve them so that there will be a minimum loss of nitrogen. All manures, however, are prized for this crop. Composting the coarser manures is regarded as essential to reduce them to the proper physical condition, to prevent excessive top growth at the sacrifice of bulb formation and to destroy weed seeds. Fresh manures may be applied to other crops the year before or spread in the fall before or after plowing. An excellent plan is to plow first and then apply and disk in the manure. Rotten or composted manure is used to the best advantage in the spring after plowing, thoroughly incorporating with the soil before sowing or planting the onion crop.
Hundreds of successful growers cultivating lands remote from supplies of manure must resort to the use of commercial fertilizers. The greatest differences prevail in regard to formulas and amounts used an acre. Voor-hees ("Fertilizers," p. 280) recommends, for sets, "50 pounds to the acre of nitrogen in organic forms, as dried blood, cottonseed meal or tankage; 60 of phosphoric acid, which may be partly in organic forms, as bone or tankage; and 100 of actual potash, derived from a muriate. The application of a fertilizer containing nitrogen 5 per cent, phosphoric acid 6 per cent and potash 10 per cent, at the rate of 1,000 pounds an acre, and well worked into the soil previous to planting, would furnish these amounts." He further recommends applications of nitrate of soda or sulphate of ammonia at intervals of about three weeks. The fertilizers usually employed range from 4 to 6 per cent of nitrogen, 5 to 8 per cent of phosphoric acid and 8 to 10 per cent of potash.
For use in southern states. Beattie (U. S. D. A. Farmers' Bulletin 354, p. 13) recommends:
For use in sections where cottonseed meal cannot readily be obtained:
Nitrate of soda, 14 to 16 per cent nitrogen........ 300
Dried blood .................................. 500
Acid phosphate ............................... 800
Muriate of potash, 50 per cent................. 400
When early maturity of large bulbs or bunching onions are desired, nitrate of soda should be used more freely. It is in general use among onion growers, the amount for dressing varying from 50 to 200 pounds an acre. The early applications are most valuable.
The total amount of commercial fertilizer an acre has a wide range among commercial growers. A very successful Massachusetts specialist never uses less than two tons. The application, however, usually varies from one-half to one ton an acre.
The great bulk of the onions grown in the United States is produced from seed sown in the open ground where the crop matures. This system is especially well adapted to conditions known to be highly favorable to the production of onions. Practically all of the American varieties, as Yellow Danvers, Yellow Globe, Southport Red Globe and Red Weathers-field, are grown from open ground seedlings.
Nitrate of soda, 14 to 16 per cent nitrogen......2,090
Cottonseed meal.............................. 750
Acid phosphate, 16 per cent..................... 750
Muriate of potash, 50 per cent.................. 300
Early seeding is regarded as of the greatest importance. As soon as the ground is fully prepared, the drills should be started. When wheel hoes are to be used it is customary to allow 12 to 14 inches between rows. When horse tools are employed in cultivating, the rows are made 24 to 30 inches apart. If the ground has been properly fitted for this crop, close planting and wheel hoe tillage will secure the largest profits. A small, well-trained mule can be used to draw the cultivator when the rows are only 18 inches apart.
In fairly heavy soil the seeds should be covered with not more than 1/2 an inch of soil. Three-fourths of an inch is sufficient in most soils, while 1 inch or more will do no harm in very sandy types.
When the rows are 12 inches apart, 4 1/2 pounds of good seed to the acre will generally give a satisfactory stand of plants. More seed should be used in heavy soils, because the percentage of germination will be lower. Some thinning is practiced by most onion growers, but the more skillful ones avoid this tedious operation to a great extent. They invariably make germination tests before sowing, and regulate the drill accordingly. It is customary to allow 8 to 12 plants to the foot of furrow. The thinning is frequently attended to at the first hand weeding.