Straight nitrogen – is it enough?


The past few seasons have offered a benefit to growers who placed early orders for nitrogen, and it’s likely the market will open above last year’s level. While it’s tempting to impulse buy large quantities of straight product, this can compromise crop potential and should be avoided warns Mark Tucker, Yara UK’s Head of Agronomy.

“First ask ‘how much do I really need?” says Mr Tucker. “Especially when NPKS and NS are also required, the need to combine with sulphur is so great that the answer will be very little.  With annual sulphur deposition now less than 15kg SO3/ha/year the majority of crops will respond to sulphur with both increased yields and improved quality. In fact, purchasing a granulated nitrate plus sulphate product can give a ROI of between £2 and £10 for every £1 invested – definitely worth considering.”

Sulphur is felt by many agronomists to be the second most important nutrient after nitrogen, as all crops require it for growth. It plays an essential role in the formation of plant proteins, amino acids, some vitamins and enzymes.

So how best to meet a potential sulphur shortfall? After sulphur in the atmosphere, there are three other sources: soil reserves, organic wastes and mineral fertilizers.

“Mineral fertilizers are by far the most reliable and convenient method for preventing deficiency,” says Mr Tucker, “but it is important to note that the sulphur applied should be sulphate which reaches the plant roots quickly and is easily absorbed. Elemental sulphur requires oxidisation before becoming available to plants. Applying a sulphate directly is more reliable and avoids losses.”

By comparison, organic sulphur also needs to mineralise and is often lost via leaching before the crop picks it up: organic sources of sulphur are also unpredictable in their release pattern. This issue has been demonstrated in grassland management where responses to sulphur from mineral fertilizer have been seen even where high levels of manures have been applied.

Unlike nitrogen, which mobilises easily in the plant, sulphur is less mobile and remains in the older leaves with deficiency showing in the youngest leaves first.  The correct strategy for application, therefore, is ‘little and often’ applying multiple, smaller, applications in line with crop demand through March, April and May, coincidental with the splits of nitrogen. This will ensure a maximum return from your investment.

“Trials have shown the yield increase from this approach to be approximately 4% higher than single, large applications early in the season at approximately 0.38 t/ha in wheat and 0.5 t/ha in oilseed,” says Mr Tucker. “Single, large applications can also create their own problems such as the interaction with molybdenum causing deficiency and a subsequent yield penalty of approximately 0.4 t/ha.”

On the evidence of the latest industry trials and advice, Yara recommends:

Apply ‘little and often’ to expect a 4% yield increase: 0.38 t/ha in wheat and 0.5 t/ha in OSR

March, April, May – same time as nitrogen

45 – 50 kg SO3/ ha in cereal crops

50-75 kg SO3/ha in oilseed

Applied in two applications for improved nutrient use efficiency

Avoiding negative nutrient interactions which occur towards 100 kg SO3/ha and above.

ROI – getting it right

So, what return on investment (ROI) can you expect from taking the correct approach of multiple applications and supplying the optimum rates?

“In cereal crop trials yield increases of 0.3 t/ha have been observed, whilst in oilseed this has been higher at 0.5 t/ha,” says Mr Tucker, “and using a granulated nitrate plus sulphate product will result in an ROI of between £2 and £10 return for every £1 invested.”

This yield benefit will only be expressed if sulphur is the limiting factor.  If other nutrients are an issue they need addressing as well. In early spring (February, March) when soils are cold and wet, phosphate and potassium can also be required to promote early spring growth of roots and shoots. This enables the plant to access the soil’s phosphate reserves whilst at the same time building biomass above ground – essential for attaining high yields.

“In this case, simply switch your first spring application to a ‘Spring Starter’ – NPKS true uniform compound fertilizer with the target to apply 35 – 50 kg P2O5 and K2O / ha along with 50 kg N/ha and 20-25 kg SO3/ha,” recommends Mr Tucker. “This will give arable crops the best possible start in the spring as they look to recover root and shoot growth lost over the winter.  Research into this approach have shown a consistent yield increase of a further 0.25 – 0.3 t/ha, above the expected sulphur response.”

The answer therefore to the question ‘How much straight nitrogen do you actually need?’ is indeed ‘very little’.  As a guide, 25% of your nitrogen should be in the form of an NPKS true uniform compound fertilizer and 50-75 % will be in the form of a nitrate plus sulphate fertilizer.

Image: Mark Tucker, Yara UK’s Head of Agronomy.



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