Availability of water makes or breaks onion cultivation

It was perhaps surprisingly busy at the Rusthoeve experimental farm in Colijnsplaat on Thursday, August 25. For the 22^e once, the Onion Innovation and Knowledge Center (UIKC) organized the national Onion Day, a dusty edition with temperatures above 30 degrees. It is likely that many harvesters and loaders were standing still, so that arable farmers could still find time to visit the event, which could be described as pleasantly busy.

Only 132 millimeters
In four different tours, visitors were presented with quite a few test fields, with a strikingly relatively large number of tests with drip irrigation, which has been used at Rusthoeve for some time. "During the growing season - from May 1 to August 15 - only 132 millimeters of water fell here," says co-organizer Luc Remijn. Most of this fell in early May and hardly at all in the summer. Test fields that were not irrigated are therefore poor and appear dull, partly due to the high thrips pressure this season.

The fields with (drip) irrigation and fertigation look better. Especially when up to 250 millimeters of water has been given, the difference - in combination with heavy fertilization - is impressive. Supplier Van Iperen showed this. The test fields achieved yields of up to 80 tons per hectare. With sufficient moisture, the onions can make full use of the many hours of sunshine that 2022 has. Drip irrigation also makes a difference in a wetter year like 2021. Van Iperen's test field then reached a maximum of 74 tons, while traditional irrigation yielded 69 tons. Whether that extra 5 tons is worth the extra cost obviously depends on the season. In the dry year 2020, the yield difference was almost 37 tons. The difference between pure drip irrigation or a combined dose with fertilizers then resulted in a difference of 10 tons.

Salt water also helps
CZAV also showed visitors a test field with drip irrigation. In addition to fertilization, plant boosters have been administered to reduce the stress level in the plant. One test field was irrigated with brackish water with an EC of 8. These onions also looked fresher than untreated onions. The UIKC does not go that far in their own tests. They apply drip irrigation up to EC 5. 30, 60 or 90 millimeters are given to the crop during the season. Not very much when you consider that the crop evaporates 5 millimeters on a hot day. UIKC compared drip irrigation with tree irrigation. According to researcher Hans Moggré, 60mm of irrigation from a tree is comparable to 30mm via drip hoses.

“We have sown both single and double rows on a bed with drip hoses in between,” he explains. "With double rows it is clearly visible that the outer rows are lagging behind. A sowing system with single rows is advantageous. It is also possible to drip in the spring before emergence or to soften a crust."

Elevated cultivation system
Fusarium is also a common problem this season. This season, Rusthoeve has experimented with raised cultivation beds for the first time. In addition to traditional sowing, 75cm wide potato ridges and 150cm wide ridges were used. The land is cultivated and sown with primed seed at the same time. "What was immediately noticeable is that the onions on ridges and beds emerged four to five days later," says researcher Dominique Cammaert. "The crop later made up for that difference. What was noticeable in mid-August is that the ridges were nice and moist, despite the drought. With the flat field system, the soil is so hard and dry that you can hardly get into it. Also the development of the root system in ridge cultivation stands out in a positive way."

When it comes to disease pressure, Cammaert is cautiously positive. The fusarium infestation of onions on ridges and beds is considerably lower. "In the spring we worked the soil three times, after which 15 mm of rain fell in a short time. To break the crust, another operation with the speed harrow was carried out before sowing. The many operations probably resulted in a lesser structure, some fusarium in the hand works."

Despite alternative cultivation systems and drip irrigation, 2022 will once again be a difficult onion year for many Zeeland arable farmers. A relatively high price level does not change much. A meager balance offers little room for investment and it quickly pays to sow winter wheat, some growers indicate. Optimists hope for more resilient varieties and cultivation systems, or systems that can still provide moisture in small quantities. Growers and researchers are reluctant to irrigate with brackish water. Much is still unclear, especially about the long-term effect on the soil. Yet it is technological and cultivation innovations that must continue to make onion cultivation possible. Most arable farmers are still happy with growing with less chemicals, but without water, that is not an option.