How a weather event in January affected fertiliser efficiency

We all know it. Weather plays a crucial role in the efficiency of our fertiliser use. Farmers avoid spreading fertiliser before heavy rain to reduce the risk of losses. But what happens when a heavy rain event comes along and nutrients have been spread?

At the end of January this year, in the open slurry spreading season, one inch of rain fell in the Ballycanew area of Co Wexford and 10% of the phosphorus (P), which leaves the catchment of 1,200ha on an annual basis, left in those 24 hours. Over the whole 12 months, 50% of the P left the catchment during high rainfall events. To put this into perspective. Over the year, approximately 1kg P/ha leaves the catchment, a relatively small amount from farms.

If every farmer cut their P use by 1kg/ha in the catchment the amount of P being lost is unlikely to be reduced

This water also makes its way into the sea quickly to water already high in P, so ecologically speaking it’s not the worst scenario, but it is of course a major issue that needs to be tackled. If the water was flowing into a lake, it would sit in that lake and cause eutrophication and algal blooms in the summer, which would have drastic environmental consequences.

The problem needs to be addressed, but it is not as simple as cutting fertiliser use. In fact, if every farmer cut their P use by 1kg/ha in the catchment the amount of P being lost is unlikely to be reduced.

The area is actually very low in P, the average P index of the catchment is 2, but grass growth is good and most farmers in the area are not pushing to build soil indexes. Fertiliser is another cost after all.

The catchment in Ballycanew being referred to is part of the Teagasc Agricultural Catchments Programme (ACP). This is a project running for over a decade in six different areas across the country, measuring water quality at one point in the main river every 10 minutes in all six areas.

One thing that the data from the catchments programme has displayed is the need for different advice on fertiliser use across the country.

As described above, Ballycanew in Co Wexford has a phosphorus problem and nitrates have never been an issue, but much of the national advice on fertiliser or organic manure use is based on nitrogen losses.

There’s never a problem with nitrogen loss in Ballycanew

Eddie Burgess, a specialist on the ACP, said: “In Ballycanew, over 60% of the water in the river came into the river by overland flow and because of that it’s going to be P risky and N safe.”

He noted that in areas like Ballycanew with a high clay content (a Macamore soil) the focus should be on P loss, not N loss, adding: “There’s never a problem with nitrogen loss in Ballycanew. The N level is very safe there.”

As a result of this, he noted that management advice needs to be altered to suit the region. Farmers in Ballycanew are being advised on slurry storage and application timing.

Much of the advice farmers hear on slurry is with regard to nitrogen losses, either to water or to the atmosphere.

For P loss, the advice needs to be focused on applying nutrients when soils are in good conditions and carefully managing storage and farmyard drainage.

“One set of rules across the country isn’t applicable and in somewhere like Ballycanew you need to be doing something to prevent P run-off,” he added.

The first step Eddie advised farmers to take is to avoid any rainwater getting into tanks which can reduce potential slurry storage capacity.

It’s easier said than done, but the next solution is to increase slurry storage capacity to avoid having to spread to free up tank space.

And the second is where storage space is tight to only spread enough slurry to free enough space in the tanks for the remainder of the housing period. No one has a crystal ball, but spreading the majority of slurry in better ground conditions, when soils can properly soak up the slurry can significantly reduce P loss.

Targeted buffer zones can also play a big role

Another essential point is to target where the organic manures on the farm are going. They should be going on to the fields that need P and potassium most and not just on to the dry fields which can be travelled on when the slurry spreading season opens.

Targeted buffer zones can also play a big role. Rather than a farmer fencing a buffer zone all around their land, the ACP can identify the critical source areas for P. If these areas are fenced off from livestock and from any applications of fertiliser or manure, then P loss could be significantly reduced and less land is lost to bufferzones, which may only be working in small areas along a watercourse.

In summary, Eddie noted that if farmers pay attention to the three Rs – right place, right time and right rate – they can significantly reduce P loss and improve on-farm efficiencies.

The monitoring of water in this particular catchment shows that one size does not fit all when it comes to fertiliser advice. What works to reduce the impact of nitrogen on our environment does not necessarily work for phosphorus.

This may mean that in areas at risk of P loss, farmers try to spread in a drier time of the year when soils are not saturated and on the land that needs it most.

What works to reduce gaseous emissions from nitrogen can lead to more P loss on certain soils so a balance needs to be struck.

Fencing off those critical source areas is one simple and effective way of improving water quality and works with the farmer.

Buffer zones

Buffer zones the length of the field take land out of production, perhaps where there is no need. That buffer may only be needed for a 5m length, but may need to be further out in the field.