Seed weight affects growth rate
The rate of growth of onion seedlings seem to be dependent on seed weight, Plant & Food senior scientist, Bruce Searle, told the Onions New Zealand research seminar.
“They’re coming out of the ground at a different size already,” he said.
“We think that’s because of the seed weight and that does affect the rate of growth.”
He is based in Hawke’s Bay and has worked on looking into emergence of onion crops and their vigour for the last two years.
There was a lot of variability seen with weights of 153 grams to 464g per square metre due to environmental factors, with plants unable to catch that deficit up.
“We’re interested in when the radical pops out of the seed and starts to form the root because that’s a fairly consistent time,” he said.
“Temperature is one of the most important factors.”
An experiment was set up looking at different temperatures and field capacity to see how these factors changed onion seedling growth.
“As temperatures got hotter soil moisture was more important,” he said. At 25 degrees C growth slowed down so late onion plantings were going to need more moisture to get seeds out of the ground.
Searle said a tractor had been developed overseas which was able to sense soil moisture and plant sweetcorn seeds at the optimal depth for growth.
“I don’t know if you could do it with onions,” he said.
Soil compaction and crusting through lack of moisture could also interfere with onion emergence, he said.
“The plant is not going to recover so you’ve got to have the soil in good condition.”
Research over the last 25 years had shown a range of time before emergence took place, from 10 to 20 days.
“It’s much easier to manage a crop coming out of the ground fast, but you don’t have a lot of control over that,” he said.
“You do over soil conditions.” When rainfall statistics were overlaid on the emergence timing it was seen that there could be from five to 10 millimeters to 120mm received.
“Normally onion crops are getting enough moisture but in some years there was too much,” he said.
“If you’ve got a compaction layer 120mm is a problem and managing the soil is the only way to handle that.”
A root stunting abnormality had been seen in Hawke’s Bay where either the seed did not get out of the ground, or else it did and then died.
“It happens in seed production in temperate regions and we think it’s caused by seed that hasn’t reached physiological maturity,” he said.
“It’s worse when the soil gets wet and we don’t know why. The solution is to use primed seed.”
Searle said some work on seed characteristics had been carried out last summer, which found that small, dense seed was better than big, light seed.
Looking at vigour during establishment a recent trial studied the effect of field capacity. It found that onion seed grown in soil at 90% field capacity was twice the height of one grown in soil at 70% field capacity at the three-leaf stage.
“Keeping water up is very important,” he said.
“If the plant is twice the size at that stage it’s going to be a lot bigger at harvest.”
A light irrigation between emergence and the three-leaf stage if needed could help.
With nutrients he said to start with there wasn’t a lot of uptake by the plant, but then when the onion bulb is filling 80% of nitrogen is taken up, compared with 20% at earlier stages. This meant growers needed to apply nitrogen later on in the plant’s growth, but they might choose to make one application at an earlier stage if they were worried they would not get back on the paddock because of wet conditions. A Hawke’s Bay trial showed that virtually all the nitrogen applied early could be leached out of the top 30 centimetres of the soil with an application of 194mm of water.
“The onion roots are often not below 30cm so you can easily lose nitrogen if you are putting it on early,” he said.
“You might want to keep soil nitrogen levels low if you know you are going to have events where you will get leaching. A lot of nitrogen is not a good thing because it can cause diseases.”
A trial was now being carried out using nitrogen test strips to show if it was necessary to make further applications.
Searle said that work with barley seedlings had shown that a nitrogen deficiency early on was beneficial later as the plants were more resistant to aphids and thrips.
“There are good reasons to look at how much nitrogen you’re putting on and what you’re doing to your system,” he said. A United States trial had shown different responses with wheat and maize rotations, meaning something different was going on. But new tools were coming online to help scientists better understand what was happening.
“... some work on seed characteristics had been carried out last summer, which found that small, dense seed was better than big, light seed.”
It was also thought that small seeds growing in cold conditions could need more phosphate, he said.
Another trial had been carried out looking at whether there was a correlation between seed weight and seed area but this couldn’t be proved.
“Heavier seed doesn’t grow faster but those with the lowest seed weight had the lowest rate of growth and greatest variation in size,” he said.
And heavier seed produced a more even canopy at harvest.
“Understanding the microbiome and how to use nitrogen will be important for the future,” he said.
“We will know more about variability and will be able to produce more uniform crops.”
▴ Bruce Searle – big variations in onion seedling growth.