Know your Onions – are you using too much N?

Onions are a crop with below average Nitrogen use efficiency, here we explore why that is and look at ways onion growers can get more from their inputs.

What is nitrogen use efficiency, and why does it matter?

 Nitrogen use efficiency or NUE, is the percentage of applied N that gets used by a crop. For most crops NUE is below 50%, so in agriculture more than half of the nitrogen fertiliser applied to crops ends up wasted.

Onions have a particularly poor NUE due to their shallow root system, where research by New Mexico State University shows that a mere 15-30% of N is recovered in a typical furrow irrigated US crop (Sharma et al 2012).

In other parts of the world NUE rates of less than 11% have been reported on sandy soils. Lets explore why this is.

Shallow root systems make onions poor at nutrient recovery.

Onions have shallow sparsely branched roots, which do not give much access to water and nutrients outside the topmost foot of soil. Furthermore the deeper onion roots go, the lower their density, becoming progressively less effective with depth.

This physiology is a ‘double-whammy’ for nutrient recovery, making them poor at accessing non-mobile nutrients like P, K nd Zn due to lack of surface area, but also making them poor at accessing mobile nutrients like nitrates which leach away faster than roots can pick them up.

Supplying nitrogen effectively is challenging on onion crops

 When we look at what form of N to use for best fertilisation of onion, it throws up some challenges.  For early growth stages (emergence until bulbing) onions are proven to respond better to ammonium and amine N than nitrate forms. However due to instability in the soil it is hard to supply these forms, as they transform in the soil faster than onions can feed on them (losing much of the N applied in the process).

 Onions need a good level of irrigation, so it is hard to supply N even with split applications without losing a large portion to leaching and still get enough water into the crop through the season.

In seasons with a wet spring much of the N applied at planting will wash out of the root zone, and the crop will have a low nitrogen status. However this can be reversed, as up to the five leaf stage nitrogen supply does not impact final bulb yield much. The real heavy lifting for yield is done during bulbing between 5 and 11 leaf stage.

Independent trials demonstrate LimiN is highly effective on onions

In 2016 a fully replicated independent field trial on onions was ran in the Netherlands (Flevoland region) using LimiN stabilised amine N. Ten 3L/Ha applications supplying 4.5Kg of N were compared with 150 Kg of conventional soil applied nitrogen.

There was no difference in yield between 4.5Kg of stabilised amine foliar applied through the season, and 150 Kg of conventional N.

 This is less surprising when we consider that conventional N has a low NUE on onions, so that 150Kg will only supply between 22 and 45 Kg to the crop. When we then factor in growth portioning, and the effect on root development of amine N we see that a little of the right product can go a long way.

 What this study shows is that growers looking at a stuttering crop at 3-leaf stage that has had a rough ride early season, should not give up. By using an efficient stabilised amine N source (like Lono) the crop can be supported and can still recover.