Onions are extremely sensitive to structural damage and moisture, which is why they should preferably be grown on plots of which the structure and de-watering are good. The soil must be easy to prepare in the spring, because an early planting period is a requirement for early harvest.
Early harvest is of great importance for onion cultivation, because the cultivation is generally focused on marketing fresh onions early. In the cultivation of 2nd year onion sets, the soil must be free from stem nematodes (Sclerotium cepivorum) and white rot (Ditylenchus dipsaci). In case of doubt about the health condition of the soil, soil investigation can provide the desired answers.
Crop rotation means growing different subsequent crops on a plot to prevent soil diseases. It isn't until several years later, that the same crop is grown on the plot. A lot of crops suffer from soil-borne diseases caused by nematodes, fungi and insects. To keep a plot that is not contaminated with stem nematodes and / or white rot healthy, extensive crop rotation is required. In this, a growing frequency of 1 to 5 or more is recommended in onion crops (such as seed onions, onion sets, pickles or pickled onions).
If there is inadequate crop rotation, or too tight rotation, Fusarium damage can render the cultivation of onions impossible. In the past, Dutch growers have suffered heavy damage to the onions after too tight crop rotation.
Soil preparation is generally only understood to mean the creation of a seed or plant bed. However, it covers more, because the ploughing that comes prior to that forms an essential part of the soil preparation.
Soil preparation is an aspect that requires a lot of attention. In all soil preparation, the goal must be: improving the structure and the retention thereof.
The fineness/coarseness of the soil particles can be controlled in seed bed /plant bed preparation by means of the speed of the harrow-pins and / or the driving speed. At plots where 2nd year onion sets must be grown, solid and timely ploughing is absolutely necessary, mainly on clay and sulphur soil. Only on mild sulphur soil, this preparation can be postponed to the spring. To be able to cover the onion sets with enough soil during planting, the soil must be loosened to a depth of 8-10 cm.
The soil must be sufficiently dry; preparing too wet soil usually results in too course cloddy plant bed. The result is insufficient coverage of the planted material with loose soil. If, in these cases, strongly drying circumstances arise after planting, this can have results for the success of the plants. In addition, clods can cause significant problems at harvest.
Driving across the soil for fertilization or preparation is best done with a tractor with tires with a maximum tire pressure of 0.5 bar. In case of frost, driving across the soil is ill-advised, because it might damage the onion sets.
Nitrogen ( N )
2nd year onion sets required a fair amount of nitrogen. Based on various tests in the famous onion cultivation areas in the Netherlands, issuing 160 kilos of Nitrogen (N) per hectare is to be advised during cultivation. These tests have also shown that it is useless to administer more nitrogen, and that it is even ill-advised. Excesses of nitrogen are not absorbed by the plant and can adversely affect its quality. As a result of administering too much Nitrogen (N) the percentage of bald onions will increase and the hardness decrease.
In the cultivation of winter onions, it is advised to administer 30-40 kilos of Nitrogen (N) in the fall, and to administer the remainder of the total required amount (110-120 kilos) spread across the spring (prior to June 1st ).
To achieve a quick start for spring cultivation, basic fertilization of 60-80 kilos of Nitrogen (N) is more than enough.
Shortage of phosphate in onions is rare. However a shortage could be responsible for the slow ripening of the crop. To be able to establish whether phosphate issuance is optimal, good insight into the phosphate level of the soil is necessary. Once the PW-number is known, the necessary amount of phosphate can be read from the adjacent table. This number is determined based on an extraction of phosphate from the soil. Mainly at the start of the 2nd year onion set cultivation, fertilization with an easily absorbable phosphate fertilizer is important. This, due to the fact that, when the soil is cold, little immediately absorbable phosphate is available for the plant. In case of higher soil temperature in May / June, more phosphate becomes available from the soil, due to which further fertilization is usually no longer necessary.
Just like phosphate deficiency, potassium deficiency is relatively rare in onions. A potassium deficiency is characterized by a deep dark green colour, and dead leaf points, with strong constriction at the transition from the green part to the dead part. The advised amount of potassium (adjacent table) depends on the potassium level of the soil, expressed in the potassium (K) number or the K-HCI number in loess soils. In the potassium fertilization at 2nd year onion sets, chlorine-containing potassium fertilizers can used without any issues.
On light calcareous (marine) clay soils and on soils rich in phosphate and / or organic substance, manganese deficiency can arise. An important symptom is a floppy crop, of which the leaf is more or less striped. In case of serious or long-term deficiency, the crop will have lagging development. Recovery usually occurs quickly after spraying with a 1,5% solution of manganese sulphate. As soon as the first symptoms are observed, the spraying should be initiated. It is recommended to use 1000 litres of water per hectare. In that case, the necessary concentration of manganese sulphate is 15 kilos per hectare. Due to the risk of damage to the leaf, spraying should take place during cloudy weather or in the evening. Where necessary, the spraying process can be repeated after a week.
As soon as the soil and weather conditions permit in early spring, the onion sets can be planted. To be able to harvest early, timely planting is recommended. In suitable circumstances, planting is sometimes even done as early as in February.
The hibernation varieties are preferably planted in the second or third week of October. The soil must be easy to prepare, so that a solid plant bed can be created, with sufficient loose soil to cover the onions. Onion sets are not very sensitive to frost. Provided they are properly covered with soil, they will not be easily damaged after planting, in case of night frost.
For weed control in 2nd year onion sets, both mechanical and chemical methods are available. The method to be selected depends on the circumstances, they determine the chance of success. Mechanical weed control is not possible in a moist soil or after a certain crop stage for instance, whereas chemical weed control may not work properly in dry soil, such as is the case in substances with soil preparation. In addition, they may not counter the specific weed occupation enough.
To minimize weed issues, it is recommended to begin cultivation as clean as possible.
The possibilities of hoeing depend on the soil and the weather conditions. Hoeing is possible from the moment the rows are clearly visible, until the leafs have grown so much that there is too little room to hoe without damage. Young weeds can be removed by hoeing. Optionally, the use of small harrow pins behind the hoes, the effect can increased.
For chemical weed control in 2nd year onion sets a number of substances are available. Briefly after planting, a number of soil herbicides can be used to stop weed from sprouting. This can be done in a weed-free soil. The soil must be moist, but if rain is expected shortly, that is fine too. A soil herbicide is never 100% effective, in addition, some weed can be present during the planting process. For this weed, a contact herbicide can be used, to control all weeds present. When using contact herbicide, please make sure that they are used before the crop emerges, otherwise, it will affect the onions as well. Always consult your pesticide advisor.
After the onions have developed and no new green foliage forms, the bulb will grow strongly in size and the neck will grow increasingly week. This weakening of the neck results on the leaves lowering at some point, depending on the wind strength. The lowering of the leaves is a clearly visible sign of ripening. After the lowering, the leaves will usually die partially. As soon as about 50% of the leaves have died and the onions have a yellow brown dry membrane covering the bulb, it is time to begin harvest. To reduce the risk of various fungal diseases, it is important to stop irrigation after the lowering of the leaves.
To be able to dry with less energy and to facilitate the process of moving the onions into and out of the storage area, the largest part of the leaves is cut off (haulm topping). The onions should only above the split of the leaves when the leaves are wind -dry. Too tight and moist topping increases the risk of fungal diseases.
After harvested (if harvested timely and in good weather) a field drying period of several days is recommended. A product harvested too late, in which the leaves have died entirely, must be stored inside immediately.
To prevent the onions from being damage, the equipment must be properly calibrated when harvesting and collecting. The sieve chains must be covered and one should aim at the lowest possible falling height of the onions.
DRYING AND STORAGE
For proper drying and storage, one must have a capacity of 150m³ air per m³ onions, at a counter-pressure of 300 Pascal. Immediately after arrival, continuous drying must be initiated (preferably in crates with an (open) mesh soil). At 30˚Celsius when the onions have been harvested under the right conditions (about 50% died leaf). If the leaves of the onions have almost fully died during harvest, it is recommended to dry a little cooler at a temperature of about 25˚Celsius to minimize the percentage of bald onions. The neck rot fungus, which grows fastest between 22 and 25˚ Celsius and that can enter the onion via the neck, cannot grow in dry tissue. For that reason, it is important to seal the entrance to the bulb as quickly as possible by drying, until the neck has fully tried and does no longer 'roll' between thumb and index finger.
As soon as the 2nd year onion sets have dried, they are usually sorted immediately, cooled, packed and exported. If the 2nd year onion sets must be stored after all, for instance for industrial processing, please contact us for tailored advice.