Cotton

How to get more from Nitrogen inputs
Managing the balance between vegetative growth (leafy) and reproductive growth (boll development) is the key to getting the best yields in cotton. Nitrogen is the biggest single influence on growth habit in cotton.

The key to getting the best from nitrogen applications on cotton is an understanding of how quantity, timing, and form affect supply, and how physiology affects demand. This gives clues as to how we can use less and get higher yields.

Both excess and deficiency of N reduce growth and yield in cotton, so high yield depends on getting it right.
If crops don’t have enough Nitrogen yield is reduced through premature senescence and boll shedding following flowering. However excess Nitrogen also reduces yield through boll shedding!
So how much nitrogen does cotton need? In the US the National Cotton Council calculate that cotton requires 60Lb per acre of Nitrogen for each bale of cotton produced. So a projected crop of 2 bales per acre would need 120Lb Nitrogen applied. However this is based on application of conventional N inputs.

These conventional inputs rely on over-application to ensure enough Nitrogen reaches the crop. In a typical cotton crop only around 33% of the N applied will end up in the crop, so in reality the crop requires only 20Lb per bale harvested – but farmers must apply 60Lb to get that much in the crop. In effect 20Lb for the crop, and 40Lb for the environment!

This waste occurs due to the instability of Nitrogen fertiliser in the environment, whereby nitrogen applied to crops is altered by soil bacteria, which compete with crops for the Nitrogen. Although farmers apply Nitrogen in different sources it is rapidly converted to nitrates, and along the way some is lost via volatilisation and leaching.

The take home here is:

1.    Whatever form of Nitrogen is applied, it gets taken up mostly as nitrate.

2.    Due to losses from leaching and volatilisation only about a third gets into the crop.


 Why does this matter? Well apart from wasting two thirds of farmers hard earned money, it also affects where the plant grows – altering ‘growth partitioning’.

To explain how Nitrogen dictates where plants grow we need to understand a little about how different forms of Nitrogen are metabolised by the crop.

Nitrates (NO3) are taken up by the plant and have to transported to leaves to be processed. In the leaf the crop uses Nitrate Reductase enzymes to convert the nitrate into amino acids. These amino acids are the building blocks for the proteins crops are made from. This process is energy intensive, and time consuming and leads to accumulation of nitrates in leaves awaiting processing (see below):

As nitrates accumulate in leaves the crop produces increased levels of auxin, a hormone that favours vegetative growth. This is why elevated Nitrogen leads to increasing vegetative growth habit.

By contrast, amine (NH2) nitrogen sources are processed by crops in the roots, and lead to an increase in cytokinin production, a different hormone that favours more reproductive growth (boll formation) and root development.

It is interesting to note that both too little and too much N both decrease yield via boll shedding. This is largely to do with crops reaching a ‘tipping point’ where extra N is detrimental due to overly vegetative growth leading to poor root systems, more leaf and boll shed.

The take home here is:

1.    Using Amine N can improve boll development

2.    Using conventional N gives mostly vegetative growth.


So now we have covered the level of N needed by cotton, and how the form can influence growth lets explore timings…

Nitrogen is used for different types of growth at different growth stages, and the requirement is not the same at these different growth stages.

This makes the timing of Nitrogen almost as critical as the quantity applied. Splitting applications is essential to reducing wasted Nitrogen and getting the best effect. Typically cotton crops only recover around 30% of nitrogen applied, with the rest wasted and lost to the environment.

The more time between Nitrogen application and its absorption by plants, the greater the potential for is for loss through leaching volatilisation, denitrification, and immobilisation. For this reason most nitrogen fertiliser used on cotton is split between PPA (pre plant application), FBA (First bloom application), and PBA (Peak bloom application).

Pre plant fertilisation is the most inefficient and should be minimised. Cotton needs very little Nitrogen at planting, as the crop takes up relatively small amounts until established, leading to excess applied Nitrogen being leached or volatilised and lost to the crop.

Research at Huazhong Agricultural University shows that Nitrogen absorbed by cotton crops during early growth is used primarily for vegetative growth, whereas Nitrogen absorbed later in the crops growth cycle is used for reproductive growth (Tang et al, 2012).

The aim for good yield should be establishing a good root system to support later growth, but research shows that while sufficient Nitrogen increases early root development excessive Nitrogen actually reduces it (Chen, 2018). Care then must be taken to only supply what is needed for good establishment, and additional Nitrogen applied later in the season.

Whilst the crop requires little Nitrogen early on, the requirement dramatically increases when reproductive growth commences. Once the crop starts to bloom, nitrogen requirement rapidly increases, with between 25% and 40% of all Nitrogen use occurring during the first two weeks of bloom. To meet this need the crop must take up around 4Lb of N every day during this period!

The lessons from this are:

1.    Pre Plant Nitrogen should be minimised

2.    Nitrogen supply during bloom is crucial to yield.

Getting so much Nitrogen into a crop during such a condensed time period is difficult and heavily influenced by soil conditions.

Research published in the Journal of Agronomy and crop Science shows that decreased root activity during bloom limits the ability of the crop to pick up sufficient nutrients for optimal growth, suggesting that it is the plants ability to take up, transport and process soil nitrogen that limits growth, rather than nitrogen availability (Gormuz, 2002). There is much research that suggests that foliar applications of nitrogen during bloom are a better way to provide N during this crucial period.

However growers looking for optimum quality and yield must also take into account the effect on quality. This is problematic, as the level of Nitrogen required for best fibre quality is higher than the level for best yield.

So putting on enough Nitrogen for good quality will reduce. This is due to the risk of triggering excessive vegetative growth, which leads to boll shed, and is heavily influenced by Nitrogen form (nitrates).

So we have now covered the effects of quantity, timing, form and placement on cotton yield. Once we understand these it can help to develop better ways to fertilise cotton.

Levity’s LimiN chemistry holds nitrogen in the amine (NH2) form, and products using this advanced chemistry are ideal for improving cotton cultivation.

Research on a wide range of crops has shown that Lono (and other products using Levity’s unique LimiN stabilised amine nitrogen) can be used to focus crops on reproductive growth. This makes Lono a useful tool for farmers to improve cotton yield and quality. Lets look at why…

Conventional nitrogen fertilisers are wasteful and typically only provide a third of their Nitrogen to the crop, with two thirds wasted. Using Lono eliminates the loss of Nitrogen to the environment, so less is needed.

Conventional Nitrogen fertilisers mainly provide Nitrogen as nitrate regardless of what form they are applied as they change form under the influence of microorganisms in the environment. Nitrate encourages vegetative growth, and creates a situation where too much Nitrogen actually reduces yield due to the crop growing in the wrong place.

By contrast Lono encourages root development, this allows a small dose to be used early to set the crop off to a good start. This eliminates the need for wasteful pre-plant Nitrogen and gives a robust root system and robust plant that can hold more bolls.

Use during bloom ensures proper boll development without creating vegetative growth and boll shed. The stabilised amine provided in Levity products provides Nitrogen in the form the crop can metabolise easily, using little energy and without creating excess vegetative growth.

Lono is ideal for foliar application to cotton as small applications go a long way, supplementing soil Nitrogen and focusing the crop on boll development.

Recommendations

  1. Alter the split of soil applied N to 10% at Pre Plant, 45% at first bloom, and 45% at peak bloom.
  2. Apply Lono at 3 leaf stage at 1 Pint/Ac, this helps the crop develop a robust root system and good canopy that can support a high yield.
  3. Apply Lono at first bloom at 1 Pint/Ac, then apply a further 2-3 1 Pint/Ac applications until peak bloom is over. These applications will keep the roots alive, and provide easily used amine Nitrogen during peak demand to build boll size, reduce boll shed and give good fibre development.
  4. Lono-K is also an excellent source of K, which is essential for good fibre development.

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Award winning scientist and experienced agronomist. With multiple patents, and proven track record of product development in biostimulants, pesticides and fertilisers.

Author: David Marks

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