If you think growing onions is easy and straightforward, you will end up with a failed crop. Fertilising onions or any vegetables for that matter, is not like baking a cake, where you can get a consistent result. You have to deal with different soils and also take into account the fertility of that soil.
Onions thrive in soil that has the right amounts of nutrients in the correct proportions. After that, you use nitrogen to manage the crop’s growth. The nitrogen is one of the most important factors in determining the success of a crop.
The amount of fertiliser you use varies from land to land
Base your initial fertilisation on a soil analysis. This approach will help maintain soil fertility at the desired level. In the early stages, you need just enough nitrogen to keep the colour of the leaves right. When you put down too much nitrogen at this stage, it can be very wasteful, because the nitrate goes into deeper layers with rain or irrigation and the root system of the young onion still has to develop for it to reach fertiliser that’s leached deeper down.
Once you have correctly fertilised the lands before planting, stimulate growth with nitrogen to bulk up plant volume. The swelling of the lower parts of the leaves produce the bulb. A small plant can’t produce a large bulb, so you need to bulk up the plant as fast as possible before bulb formation.
After this, use the leaf colour to reduce the nitrogen, which should change from lush dark green to medium green. This tells the plant that conditions are becoming less favourable and it must therefore concentrate on developing the bulb, and storing reserves to survive a dry season.
From this point, aim for a medium green as the plant concentrates on bulb formation rather than leaf growth. Monitoring leaf growth isn’t a switch you can turn on and off. This is where experience plays an important role and very few people will get it right the first time. However, you have to know about it so you can learn to judge when you should stop or reduce applying nitrogen. A lack of nitrogen during the main growth phase can render the plants more vulnerable to diseases, as would applying too much.
What happens when you add too much nitrogen
If there’s too much nitrogen at the beginning of bulb formation, the plant may short-circuit bulb development and go to seed. This is associated with thick necks that delay maturity and form poor bulbs. However, you could get away with overdoing nitrogen with one variety but not with another.
Too much nitrogen can also make the plants more vulnerable to diseases. So, pay close attention to the leaf colour of the crop throughout.
The amount of nitrogen to apply can vary from year to year due to leaching, so don’t base one year’s figures on the following year’s nitrogen requirements. Learn to ‘read’ the colour of the leaves instead. Also, onions are sensitive to a lack of zinc (Zn). Because zinc is not routinely added to fertilizer mixtures anymore, you will need to determine the zinc status of the soil. A zinc deficiency will show up as leaves curling at the tips like pigs’ tails.
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