Soil and Water Management

Soils need to be well structured and fertile to maximize growth and produce high yields. Seedbed condition is critical, particularly if crops are being grown from seed. A fine, consolidated seed zone is required for maximum germination and good establishment. 

Rooting is generally poor. Onions have sparse root systems with short lengths and few root hairs. Thus, it is important to maintain nutrient and soil moisture availability within the shallow rooting area.

Research suggests that water uptake is restricted to the top 25 cm of soil. While onions can survive long periods of drought stress, water availability is critical for growth and high yields of quality crops. Under drought stress, onions are more likely to split or form double and multiple bulbs.

Onions have a high water requirement, usually around 75 mm of water per week. However, late season irrigation can delay maturity and lead to skin cracking. When grown on particularly light soils, inter-row guard crops of barley or wheat, or the use of straw helps minimize erosion. Clods and stones will hinder growth, herbicide efficiency and mechanical harvesting, so heavy or stony soils are usually avoided. Soil pH is normally in the range of 6-7, but on organic soils, onions can be grown down to pH’s of 4. 

When pH drops below 5.5, magnesium and molybdenum availability drops and above 6.5, zinc, manganese and iron become deficient.

Onions are very sensitive to salinity and so saline soils and salty irrigation water should be avoided. In conditions where sodium chloride creates a soil solution with an osmotic pressure of 0.125MPa, growth is halved. Nitrate and sulfate forms of fertilizer are preferable; chlorides undesirable.

Nutrient Analysis

Soil analysis is used to provide baseline initial information for a fertilizer program – particularly an indication of potassium and phosphorus. It is also used to assess soil pH, and organic matter levels. 

Plant leaf analysis is important to continually monitor the balance between elements and fine-tune a fertilizer program that ensures quality crop production. Tissue analysis will also confirm visible deficiency symptoms and reveal hidden deficiencies where visual plant symptoms are not yet present. 

The type and position of the leaf selected for tissue analysis varies according to local practice. For young onion plants, whole shoots or leaf tops of the same physiological age are sampled. 

For plants in mid-growth to harvest, the youngest mature leaf or fully developed leaf blade is usually sampled. It is necessary to take into account the sampling method used and the time of sampling when interpreting the data.