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N and P for Oilseed Rape Establishment

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In the last few years it has become increasingly apparent that the concept of placing the autumn N and P fertiliser alongside the seed is being widely adopted and generally accepted as best practice for the establishment of winter oilseed rape.

However, is placing fertiliser in this scenario based on agronomically sound judgement and what are the other options available, asks Tim Kerr, Hutchinsons fertiliser manager.

“Phosphate is considered a very important nutrient for crop establishment; phosphate helps to fuel all the major processes in the plant that require energy. In the first 60 days from emergence oilseed rape will require around 15% of its total P requirement. The amount may not sound so significant; however the mechanics of P uptake are why we lend so much importance to P as a starter fertiliser,” he says.

Mr Kerr points out that phosphate is an immobile nutrient in the soil. “The plant relies on extracting that 15% of P requirement from a small fraction of the total soil.  During the first 60 days the roots will only reach around 5% of the topsoil, and P absorption relies on direct root contact with the soil for it to be taken up by the plant.”

Tim Kerr

“Therefore enriching the soil in the immediate rooting zone with water soluble phosphate or plant available P will help to maintain critical P levels in the aforementioned rooting zone over this period.”

The benefits from placing nitrogen close to the seed may appear less obvious, yet still there are advantages to the system, he adds.

“Most starter fertilisers that contain nitrogen and phosphate are based on ammonium phosphate. However, there is a proven synergistic effect on plant uptake of phosphate where nitrogen is present in the ammonium form. This may be partly down to a localised decrease in pH where NH3 and P are applied together.

“This effect is most pronounced in soils with a high pH - increased P absorption when it is applied in conjunction with nitrogen fertiliser, will be partly due to an enhanced physiological capacity of roots to absorb P, brought about by greater root development in the soil with a concentrated area of nutrients from an NP starter fertiliser.”

Mr Kerr says that better root establishment will improve a plant’s ability to forage for nutrients and water by giving it access to a greater proportion of the soil – and in turn this will ensure yield potential is maintained from the outset.

Conversely, he points out that in a phosphate shortfall scenario, the yield potential of the crop will be reduced irreparably. “Remembering that phosphate is effectively immobile in the soil and recognising the dual benefits of placing N and P together – there is a compelling case to opt for one of the starter fertiliser options available to growers.”

Primary_P_OSR

“DAP (18-46-0) is probably the most commonly used product for this purpose. This is an ammonium phosphate and therefore will offer the benefits discussed earlier, however the ratio is only really appropriate for soils at index 1 or below.”

At index 2 or above, Mr Kerr suggests alternatives that will supply less P without compromising on the amount of N applied. “If it is not possible to place the fertiliser, then incorporation into the seedbed will be the next best approach.”

“Micro granular fertilisers are an option based on ammonium phosphate, but it is worth checking that you are getting what you want or need.”

The main advantage of micro-granular products is the increased availability of nutrients through a much greater surface area, he adds. “Applying the same weight of a conventional granular fertiliser provides up to 400% less surface area of fertiliser. Consequently microgranular products can buffer the soil’s capacity to supply P much quicker than a standard 2-4mm sized granular fertiliser.“

He observes that five years ago it was not that easy to find seed drills with an option to apply fertiliser along with the seed. “However today the opposite is true – and the choice of application equipment is both wide and relatively low-cost.”

“When it comes to additional application equipment, the investment required to place liquid fertilisers can be the most significant. However, this is not always the case – and those used to handling liquid fertilisers may already have some of the necessary components.”

“Liquid fertiliser in solution is by definition 100% soluble and therefore rapidly available. Phosphate is again normally supplied in the ammonium phosphate form, offering the combined benefits already mentioned.”  

“Solutions are also available in different ratios, offering the flexibility to apply appropriate quantities of P along with the N.  Liquid fertiliser is normally applied in bands – delivering concentrated doses of N and P in and around the rooting zone.”

Whilst this is by no means an exhaustive list, he points out that the benefits of fertiliser application techniques based on a sound, science based understanding of nutrient availability are sufficiently well understood for the farming industry to invest in and adopt.