Did you know that onions are the most cultivated crop in The Netherlands? and that it is the most widely cultivated species of the genus Allium worldwide? This crop is an herbaceous bulbous plant that is cultivated for bulb production either as biannual or perennial, depending on the cultivation conditions. Onions are among the most widely adapted vegetable crops that can be grown under a wide range of climates from temperate to tropical. World production is nearly 106 million tons of bulbs on some 5.7 million ha.
Onions are vegetables that thrive in mid climates. The best performance can be obtained without extreme cold or heat and excessive rainfall, but it can also endure temperatures below 0 degrees. For optimum growth, the crop requires a relative humidity of around 70% and a mean daily temperature between 15 and 20°C. In normal conditions, onions grow bulbs in the first season of growth and flowers in the second. During the vegetative stage, onion crops need lower temperatures and shorter daylength. Moreover, for a good crop establishment the right amount of water is beneficial in this stage. However, during the maturity stage and development of the bulb dry, warm weather and longer daylength are needed for a higher yield and quality. The critical daylength to produce the bulb, varies from 11 to 16 hours depending on variety. Therefore, the selection of a proper cultivar that can be adapted to the local climate in terms of daylength requirements, is essential.
While other common crops are planted directly from the seeds, onions can also grow from sets. Onion sets are small onions that look like small bulbs and once they mature, they develop into full size bulbs. This last method is known to have better success rate than direct seed sowing. Onions can be planted in spring or autumn, and they need typically about 100 days of growing to produce decent sized bubs.
The best soil to successfully grow onions is deep, friable loam, with good drainage and moisture-holding capacity and sufficient organic matter content. Compacted soils will strongly affect the development of the bulb. The optimum pH ranges around 6 and 7, but onions can also be grown in medium alkaline soils.
Since onions are heavy feeders, it requires more mineral fertilizers than other vegetables for a bulb and shot growth. Due to its sparse and shallow rooting system, this crop calls for careful management of water and nutrients to ensure an appropriate supply. Nutrients like P, K and some macronutrients are immobile and therefore not easily accessed by the plant while other mobile nutrients like the N is easily reachable.
The demand of nutrients varies along the development of the crop. Therefore, the timing of the application of nutrient is also crucial. For instance, the lack of nitrogen available will severely suppress growth but the excess in late growing season is believed to delay maturity and cause double centres. Or for the case of potassium, where applying it during the development of the bulb, increases it size and quality and low levels makes plants more susceptible to cold.
Consequently, a proper soil monitoring can help the farmer to make better decisions and manage the crop more efficiently. This can be performed by two soil testing solutions provided by AgroCares: Scanner or Lab-in-a-Box.