How To Yield The Ultimate Onion: Soil Recomedations
Soil for growing onions needs to be fertile but it also needs to be porous. Be careful that the soil is not too wet or the bulbs may decay, and avoid planting in soil that contains too much acidity. Clay soils are probably too dense for the best bulb growth and loamy soil may retain too much moisture. If you want to grow a healthy onion, you will need a soil mixture which includes the benefits of all three major types: clay, loam, and sand.
Onions are Bulb Plants
Soil used for growing any type of bulb, including onions, needs to be fluffy or sandy, or a mixture of both. Loamy soil contains all of the nutrients that onions require, however it is often too dense for the bulbs to grow large. Loam is excellent for feeding roots, and contains many types of microorganisms required for plant growth, however needs to be treated before being used to grow onions. Bulbs are particular about the density of their soil and do not grow well if they are not able to expand easily.
Avoid Clay Soils for Onions
Clay contains a great deal of organic matter, but it has unique properties that inhibit the effectiveness of clay for growing any bulb or tuber. In order to make clay soils more acceptable for onions, fill a wheelbarrow halfway with sand and the remainder with compost or peat, and mix it into a 10 foot by 10 foot area. You want the benefits of the minerals found in clay soil, however need to loosen it up enough for bulb growth.
Conditioning Peat with Sand
Peat soil is nutrient rich, however is not well suited for growing onions without being treated. Peat is formed when nutrients and plant waste accumulate in a low-lying area such as a swamp, and this type of sedimentary soil may retain more water than onions can tolerate. To correct the problem, add 1 wheelbarrow of sand to every 100 square feet you are treating.
Moist Versus Wet
Onions are very high in moisture content and that moisture is taken directly from the soil. The goal is to have moist soil but not wet soil. To test the moisture content, take a handful of the soil and clench it in your fist. If water squeezes out between your fingers, the soil is too wet for onions to do well. The best treatment is to add sand to increase soil drainage and allow the area to dry out before trying again.
Onions and Potting Soil
Humus or potting soil are the optimum soils for growing onions. They have a relatively low density but are high in both macro and micro nutrients. If you are using pots, be sure that they have adequate drainage or are kept in an area where the amount of moisture can be closely controlled.