The essential requirements for a good soil for growing onions are plenty of organic material, sufficient drainage, and regular weeding. If a soil has the proper combination of sand and hummus and you can easily work it, it should hold moisture and nutrients well and should drain properly. Adding compost to your soil is a great idea and will always improve soil texture and drainage.
Sandy soils need to be worked with compost before growing onions for the first time. Break up the soil in the fall and work in the compost. If time permits, allow the soil to sit and plow again in the spring before planting your onion crop. Otherwise, you can break up the soil and add the compost just before planting in the spring. Just make sure the compost is fully decomposed. If you’re not sure how to compost correctly, please see this excellent compost guide from the University of Missouri Extension Center.
It is generally appropriate to grow onions where you have grown other crops before. In fact, if you’ve kept your garden weed free, this can help improve the texture of your soil. Corn, beans, and potatoes are suitable crops with which to precede onions. Cowpeas are also excellent for conditioning the soil before planting a large crop of onions.
Onions generally should be rotated with other crops each season. If you’re growing a large batch of onions, try out a leguminous crop for use as a green manure. Continuous cropping with onions in the same plot can attract pests and diseases to your garden over time.
The cultural requirements of onions are frequent shallow stirring of the soil and proper weeding. The feeding roots of the onion run close to the surface of the soil and should not be disturbed by deep cultivation. Sometimes a heavy rain after seeding will pack the surface so that the seeds can’t break through. Under these circumstances it may be necessary to slightly break up the soil with a steel rake or a rake-like attachment on a cultivator.
Mulching your onions after they start to form larger plants is an excellent way to retain soil moisture and keep weeds down in your vegetable garden.
In most cases, fertilizers aren’t really necessary to grow onions. A little compost added to the soil should be sufficient to grow a good onion crop. It is difficult to make the soil too rich for onions, provided that the compost is well decomposed and properly mixed in with the soil.
If you choose, an application of a natural or organic fertilizer during the growing period can also help your onions thrive without the potential hazards of using chemical fertilizers. Better yet, try out a natural Mycorrhizal fungi root-builder, which will help your plants make better use of the nutrients already found in the soil.
Whatever kind of fertilizer you decide to use, you should first educate yourself about the nutrient content of the product. Many gardeners over-fertilize their plants because they do not know what level of nutrients they need or how often to apply a fertilizer. The nutrient content of fertilizers is reflected by the NPK number. NPK stands for Nitrogen, Phosphorous, and Potash (Potassium).
The onion does best under rather cool conditions, with lots of moisture during the early stages of growth. A thin layer of mulch can help to retain soil moisture. For proper ripening of the bulbs, they will need a reasonable degree of heat together with both dry soil and drier air.
After they start to mature, onions often do quite well with just rainwater, depending on the level of rainfall in your area. However, supplemental water may be necessary in drier areas. In general, you can water every 5-7 days during the initial growth period and during dry spells.
Weeds are often a problem with onion crops because they compete for nutrients. When you cultivate for weeds, make sure to work only the top layer of soil or carefully pull out the weeds by hand. You may end up damaging the roots if you cultivate too deeply. Mulching your garden is an excellent way to keep weeds down in your vegetable garden. You can also consider a permeable weed barrier to reduce problems with weeds.
It is almost impossible to produce a crop of onions without a little hand weeding, especially after heavy rains. The work of hand weeding may be minimized by the use of specialty weeding tools.
Thinning is generally left until the time of the first hand weeding, when all thick bunches along the rows are thinned to a uniform stand of eight or ten plants to the foot. It is always a good idea, however, to allow for considerable loss of the plants, and unless the plants are so thick as to actually crowd, thinning won’t be entirely necessary.