Applying fertiliser to crops

Applying nitrogen is essential for improving crop yield potential.

Nitrogen is a major driver of dry matter production in winter cereals and yellowing of older leaves is a good indicator of nitrogen deficiency.

In a production system where there is a number of years of lucerne/legume pasture between cereal crops, it is possible to produce enough nitrogen for the following cereal crop, but for most situations, nitrogen needs to be applied to get optimum production. 

When to apply nitrogen

Nitrogen can be applied pre-plant or applied at sowing. 

To apply at sowing you will generally need to be able to place fertiliser away from the seed as the rate needed will burn the germinating seed, reducing germination. 

There are multiple methods available to apply additional nitrogen, aside from sowing time, at appropriate crop growth stages.

Each option requires a calculated approach to maximise the efficiency of each fertiliser application.

It is very common practice to apply nitrogen in-crop and the loss of applied nitrogen is less than previously thought, with recent studies revealing maximum losses of less than 30%.

Before applying fertiliser to crops

Take a soil test

The first step in applying any fertiliser is to do a soil test. 

When testing for nitrogen it is important to sample at depth as nitrogen is one of the most mobile nutrients in the soil. If you are in a situation where you have subsoil constraints such as acidity or sodicity, the nitrogen in the subsoil may not be available to the plant.

• Depth sampled is between 60 centimetres and 1 metre (dependent on the plant available root zone).
• Single sample or multiple samples (0-30, 30-60) show how much nitrogen is in the profile.

Taking samples through the profile allows you to see where the nitrogen is, which can help with timing as well as elimination the chance of a nitrogen bubble at depth, which would not become available until later in the growing season.

Work out nitrogen rate

Once you know how much nitrogen is in the soil, you can then work out your desired nitrogen rate. 

In a normal season, it would not be unrealistic for a dual-purpose wheat to grow 4t/ha of dry matter and 4t/ha of grain. 

This crop would require 224kg of nitrogen per hectare. Not all of this nitrogen would be removed. The majority would be returned through crop wastage, urine and dung. 

Unfortunately, the nitrogen in these waste products will not be available to the plant within the growing period.

Decide which form of nitrogen to use

There are a number of products that you could use to apply your nitrogen.

• Urea - Urea is the most common, but some producers will use sulphate of ammonia, especially if they would like to increase soil sulphur levels for other crops within the rotation.
• Sulphate of ammonia - Sulphate of ammonia is twice as acidifying as other nitrogen products, so should be avoided on low pH soils.
• Urea and ammonium nitrate - Liquid products such as urea and ammonium nitrate (UAN) can be faster acting but generally come at a greater cost per kg of nitrogen.

Choose fertiliser timing and application method 

• Prior to sowing, through broadcasting on a fallow paddock then incorporating with cultivation or Incorporated By Sowing (IBS).
• At sowing, fertiliser can be added with the seed or using a double chute system, where fertiliser can be banded below or to the side of the seed.
• After crop establishment, fertiliser can added through top dressing the crop and relying on rainfall for incorporation.
• Mid-Row Banding (MRB), where every second row is inserted with a fertiliser band under the soil surface.

How to apply fertiliser

Once the timing and method has been decided on, application can begin. 

Applying fertiliser before sowing

Incorporating nitrogen outside sowing is recommended to provide the best opportunity for crops to achieve their yield potential, while also reducing the risk of seedling damage from fertiliser. 

• Broadcasting fertiliser before sowing is an option.
• Decent rates of urea can be applied and then IBS, reducing risk of crop damage in a similar way to the use of pre-emergent herbicides.
• Tynes would be more appropriate than disc seeders in this circumstance, as they would achieve adequate soil throw out of the seed bed.
• Lower fertiliser rates could be used and then incorporated with cultivation.

Applying fertiliser at sowing time

In a perfect world growing a crop would entail placing all fertiliser requirements upfront with the seed at sowing and expecting that to meet all nutrient requirements. 

Issues

When using significant rates of fertiliser, sowing rigs need to be capable of placing fertiliser far enough away from the seed to avoid ‘burning’ seedlings. 

• 'Burning' seedlings is a process where the salt concentration of the fertiliser reduces the osmotic potential of seedlings and stops them from accessing soil moisture as well as causing.
• Ammonia toxicity is a concern in ammonia-based fertilisers like urea which are broken down in the soil and release ammonia gas. This can build up in concentration, impacting germination and early seedling development, particularly in broadleaf crops such as canola as high exposure to their taproots is often lethal. 

Studies have shown the toxicity zone to be 1 to 5 cm from the placement of the fertiliser. 

Generally, larger seeds have less damage potential than smaller seeds.

Below demonstrates the ranking of lowest tolerance to fertiliser to highest tolerance. Canola tolerating the least amount (Figure 1).

Deep banding fertiliser

Deep banding of fertiliser during sowing can assist in minimising crop damage by creating distance between the seed and fertiliser. This option is useful to a point, as it is dependent on crop selection and fertiliser concentration. 

Issues

Root growth can be severely inhibited when the tap root comes into contact with the toxicity zone from the urea, compared to a control treatment. 

Banding fertiliser at sowing generally requires a double chute system with a high draft and breakout force to deal with the increased placement depth, which may be a limiting factor.

Top dressing

Top dressing is used to reduce the risk and expense of applying a large amount of fertiliser upfront at sowing, particularly if the season turns poor. Leaving a significant amount of fertiliser unused in the soil and at risk to some loss to the N cycle, a point discussed a little later.

Nitrogen when top dressed is generally applied in the form of urea, as it can be applied in the largest swarf. In cereals, applications of fertiliser can occur at different growth stages depending if your aim is to increase yield or grain protein concentration. 

• Applications from tillering through to the start of stem elongation impact yield and can lead to a dilution in grain protein concentration.
• Applications from flag leaf emergence onwards heavily impacts grain protein.
• Applying N after flowering has limited effectiveness and is not recommended.

Top dressing requires a significant rainfall event to incorporate nitrogen into the root zone. A minimum 10 mm dependant on soil type and the type of rain received is suggested.

Issues

• Inadequate amounts of rain cause the fertiliser to fail to move off the soil surface and it begins to breakdown, gassing off into the atmosphere, this is called volatilisation.
• Volatilisation occurs when ammonium fertilisers are applied to the soil surface. Ammonium converts to ammonia gas and is then able to blow away.
• Volatilisation rates increase with increased moisture (wet soil surface) and increased temperature.
• The greater the soil moisture at application the increased risk of volatilisation.
• Other influences which impact risk include soil pH, temperature, wind speed and product selection.

It is preferable to apply products like urea prior to a rainfall event so that the ammonium moves into the soil with water.

Mid-row banding fertiliser (MRB)

MRB uses a three-point linkage mounted banding bar with twin disc openers on every second row, and press wheels to close the furrow.

Studies have found that applying nitrogen below the soil surface can reduce the risk of volatilisation.

GPS guidance is essential and urea is placed 25-35 mm below the soil surface.

Benefits

• This method allows for applications of nitrogen at required growth stages, similarly to top dressing, but with a reduced risk of volatilisation.
• Another benefit of this process is the reduced requirement for follow-up rainfall compared to top dressing. 

Issues

• This system may be limited by narrow row spacing and the requirement for new equipment.

Unused nitrogen

Nitrogen can be lost from the soil by:

• ammonia volatilisation
• nitrate leaching
• nitrate denitrification.

In the past these processes were thought to cause heavy losses of N (>50%) to applied fertiliser. Recent studies have actually found that the loss of N from unused fertiliser is a lot less significant than first thought.

Though there may be a risk for over fertilising with higher rates, this has been found to be utilised by the following crop and is reflected in the following gross margin.