Is there a case for spring-applied DAP at soil index 2 for spring grass?

Published 13 February 15

Is there a case for spring-applied DAP at soil index 2 for spring grass?

It is well known that phosphorus (P) is most needed in spring, when root development is vital to the coming season’s plant growth. As a result, where P is required, recommendations usually include it in the first or second dressing, except for cutting where it will be applied per cut, as needed, says Elaine Jewkes, BGS director.

The recommended application for grazed grass according to the Fertiliser Manual (RB209) at soil index 2 is 20kg/ha P2O5 for the whole season. This level is considered to account for the P removed in the grazed grass, alongside the excreted returns from the animal. Being commonly applied early in the year, this is why there are various fertiliser compounds with an analysis (and names to match) that can provide N to establish a strong first grazing alongside the whole year’s P recommendation.

However, an approach that is used on some farms is to apply DAP (Diammonium Phosphate) as an early dressing, even at index 2 or 3, believing this aids greater root development and phosphorus levels in the plant. Even at a modest 100kg product/ha (0.8 bag/ac) this will supply 46kg/ha P2O5, more than twice the current recommended rate. Is there a case for this?

This level of application is rather high for soils that are likely not to be deficient in P. Phosphorus is concentrated in the top few centimetres of the soil profile and is primarily lost by erosion or transport of soil. Early grazing may result in losses of soil on hooves or by runoff after the surface is trampled. There is also evidence that treading by animals under wet conditions can push phosphorus out in water through fine pores in the soil. These pathways may seem small relative to the gullies formed in an over-tilled soil, but eutrophication (over-enrichment) of water by P is calculated in parts per billion (µg/l), not the parts per million (mg/l) that soil P is measured in. That is, tiny concentrations make a damaging difference to water ecology.  

That said, there may be some room for manoeuvre. In most situations, the rate recommended by the Fertiliser Manual will be sufficient, particularly as few intensively grazed pastures will be deficient in P. However, work from Northern Ireland suggested soils at the lower end of P index 2 may not be receiving quite enough to support good growth under intensive grassland use and they have now split the P index into 2-and 2. So if you feel P is needed, it may be a good idea to look not just at the index, but at the actual analytical result.  If the result is 16-20 mg/l, applying the recommended rate plus an extra 10-15 kg/ha may be acceptable (ie 30-35 kg/ha P2O5 for grazing) in the early dressing, while above this, 21-15 mg/l, the recommended rate should be adhered to. This way, the soil will gradually reach the higher part of the index, ensuring sufficient P supply to the grass.

At higher soil indices, there is no real reason to apply P. At soil index 3, losses start to increase and there is no real agronomic benefit. Nutrients applied in slurry should be accounted for in the fertiliser plan, whatever the soil index.  

So, in summary, there is some justification for building the index to the top of index 2 for spring grass, but you need an up-to-date soil test to judge this. There is little justification for high rates of P application, particularly when the soil is still quite wet, as this increases the risk of loss. Don’t forget – nutrients in slurry should be accounted for in any fertiliser plan.

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