Root Development of Onions
Onions have a shallow, sparsely branched root system with most roots in the top foot of soil. Rooting density decreases with soil depth. The sparse, shallow rooting of onions has important implications for management of relatively immobile nutrients (Phosphorus (P), Potassium (K), and some micronutrients such as Zinc (Zn)). Therefore, onions are more susceptible than most crops to deficiencies of these nutrients. To make sure the plants have these nutrients at the beginning stages of growth use a balanced fertilizer such as 10-20-10 Onions Special Fertilizer which will help establish a viable root system. This will give the roots an advantage to take up nitrogen which in turn makes the plant grow better tops and bulb later.
The shallow root system of onions also is an important consideration for efficient management of mobile nutrients such as nitrate-N and sulfate-S. Mobile nutrients can be lost from the root zone by overirrigation. It is recommended for your onion plants to receive 1″ of water per week.
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Early development of roots three weeks after transplanting. Each plant has approximately 10-12 delicate, white, poorly branched roots down to a maximum depth of 12 inches and about 4” wide.
Half-grown plants approximately 2 months after planting have from 28 to 33 roots at a depth of up to 27 inches and a width of 20 inches.
Mature bulbs have 20-25 roots that mostly run outward at various angles and then turn downward. They extend out as far as 32 inches and downward as far as 39 inches. Interesting is that the older roots die as the onion reaches maturity.
Can I use mycorrhizae powder on my onion roots when planting?
Mycorrhizae powder is a natural fungus (“mycor-rhiza” meaning “fungus-root”) found in any soil that has plant growth. The job of this fungus is to help roots with collecting nutrients or protecting roots from disease. Studies have shown no real benefit with onion plants that have proper fertilization, watering, and planted in soil that has had plant growth. Benefits of mycorrhiza are more apparent if planted in pasteurized soil due to the soil not having mycorrhiza, or if the plant is lacking water and/or nutrients. Applying Mycorrhizae powder as directed will not do any harm it is more of an insurance if the onion plant needs protection from not enough water or nutrients. I would recommend fertilization and proper watering to prevent your onion plants from needing mycorrhiza but, having mycorrhiza in the soil if not already there would be a nice insurance to fall back on. Over application will show diminishing returns and reduce the plants tolerance. You know your soil and environment more that me so you can use this as a information and make your decision from there. Thank you for your question and I wish the best to your onion season.