A significant proportion of crops’ susceptibility to disease and insects is a result of over-application of nitrogen, according to US regen crop consultant John Kempf.

Nitrogen mismanagement can degrade soil microbial communities, reduce protein content, increase lodging and ultimately reduce crop yield.

Excessive nitrogen applications can also cause a range of nutritional imbalances which lead to common storage and quality problems, particularly in root crops, fruits and vegetables.

See also: How crop nutrition can avoid insect and disease

Nitrogen use efficiency

“Plants have zero interaction with nitrogen,” he explains. “Instead, they interact with nitrates, ammonium, urea and organic nitrogen forms such as amino sugars, amino acids, peptides and proteins.”

Each of these compounds produces a very different physiological response inside the plant, which uses varying energy requirements.

“From an overall plant health and yield perspective these differences become very important,” explains John, who founded the agronomy firm Advancing Eco Ag which specialises in plant nutrition and soil microbial function.

He says organic nitrogen compounds such as amino acids are the most efficient form of nitrogen, as they are readily available to the plant. Urea is the second most efficient form of nitrogen, followed by ammonium and lastly nitrate.

“Nitrate costs plants the most amount of energy to use. We often think of nitrate as giving plants energy, but it actually creates yield drag,” he says.

This is because when plants absorb nitrate, it costs them a lot of photosynthetic energy to convert it into protein. This process also increases water requirements, as the nitrate conversion process requires three molecules of water for each molecule of nitrate.

“If you want to increase a plant’s drought susceptibility, it’s really easy: just add more nitrate,” he says.

Nitrate can produce weaker, more watery cell membranes which are prone to bacterial and fungal infections. It reduces absorption of trace minerals such as calcium and magnesium because it creates an alkaline environment surrounding the rhizosphere.

“The absorption of ammonium, on the other hand, acidifies rhizosphere pH and generally leads to better absorption of other plant nutrients such as manganese, zinc, iron and copper,” he says.

Apply N late, not early

So how can farmers manage agronomic systems to maximise nitrogen use efficiency? And what can be done to stabilise nitrogen in the soil profile?

First, John does not recommend up-front applications of nitrogen or potassium at drilling, and instead suggests microbial inoculants and biostimulant seed treatments to help establish microbial soil populations and kick-start crops into action.

“Our approach is to not use nitrogen or potassium at planting, unless we know for certain levels in the soil and plants are extremely deprived – in which case we may add a small amount.”

This is because the nitrogen requirement of a small seedling is very low. For many crops, the majority of N requirement – upwards of 70% – does not occur until the grain fill stage.

“We need to apply nitrogen late – we do not need it early,” John says. “Too much nitrogen too early results in an excess of vegetation. We have become acclimatised to the idea that lots of vegetative biomass is the foundation for producing high-yielding crops, so we tend to overuse nitrogen. Very frequently, the highest-yielding crops have less vegetation rather than more.”

The key is to establish microbial populations early in association with the root system, and to not suppress it or shut it down.

“If you want microbial inoculants and biostimulants to succeed, you cannot have nitrogen applications in close proximity to the microbial inoculant. It’s simply not going to work.”

© Tim Scrivener

John recommends using sap analysis throughout the growing season to evaluate crop nitrogen requirements at critical growth stages. Apply nitrogen in the form of foliar urea, which is more readily available to the plant and does not have the same effect of shutting down soil microbiology as granular applications.

A combination of sulphur, humic substances, molybdenum and soluble carbohydrates is added to the foliar spray to reduce leaching and facilitate conversion into plant-available form.