Farm Ireland

Monday 22 January 2018

Tillage: Winter rains pose big fertiliser issues

There is a temptation to try to get nitrogen out onto crops that have turned yellow.
There is a temptation to try to get nitrogen out onto crops that have turned yellow.
PJ Phelan

PJ Phelan

February is usually one of the driest months of the year. However, February this year gave 109mm of rain at my local Met station in Gurteen, Co Tipperary. This exceeded the maximum monthly rainfall since records commenced in Gurteen in March 2008.

We have also experienced the heaviest winter rainfall in recent years. Met Eireann described this winter as being wetter and milder than normal everywhere. Rainfall at all their monitoring stations was above their long term average and over half their stations reported the wettest winter on record.

It is likely that the mild winter has resulted in greater disease levels surviving in crops over the winter.

Disease control strategies will have to be planned carefully so as to keep both disease resistance and fungicide cost under control. A mild winter would also be likely to result in high levels of aphid transmitted virus.

However, rainfall levels appear to have kept aphid populations suppressed. In general winter barley crops looked surprisingly well up to the last week in February when many of them went yellow almost overnight.

The way land manages water is largely dependent on the soil topography and drainage characteristics. On free draining soils most of that rain moves down through the soil into groundwater; lands with poorer drainage are subject to run-off, over-land flow, or flooding.

Movement of water down through the soil profile is likely to carry nutrients and silt with it. Over-land flow will carry clay particles and the nutrients attached to the clay.

The big question now is do we need to adjust our fertiliser management programmes to allow for the increased winter rainfall? We generally associate fertiliser programmes with supplying the nutrients required for plant growth. What is equally important is the fact that most, if not all, nutrients impact on plant disease risk.

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While the importance of nutrition for disease prevention has long been recognised, very little research has been carried out while pesticides were able to do the job instead.

However with the advent of pesticide resistance, legislation prohibiting many active ingredients and ever increasing costs, more attention must be given to alternative ways of enhancing plant resistance.

It is unlikely that winter rainfall will have had a significant impact on the leaching of nitrogen as the level of soil nitrate nitrogen is low during the winter period and very little fertiliser nitrogen has been applied to date.

However, it is important to remember that application of fertiliser nitrogen to saturated or water-logged soils will result in denitrification - nitrogen becoming unavailable to plants.

The temptation to try to get nitrogen out on crops that have turned yellow can be hard to resist but is a fruitless exercise if land is subsequentially saturated due to rainfall.

Phosphorus does not leach in our mineral soils but is prone likely to be carried with clay particles when runoff takes place. Potassium is more mobile than Phosphorus but leaching out of the rooting zone is generally not an issue.

Analysis results of soil samples I took in January/February this year do not indicate any significant change in soil potassium levels over the winter period. Liming of acid soils can increase the retention of potassium fertiliser and decrease leaching risk.

Sulphur and boron are probably the first trace elements that come to mind when we think of trace elements and leaching. Sulphur is important for cereals and boron for brassicas, including oilseed rape and beet.

Most crops will require sulphur with the first application of nitrogen, while boron will have to be applied to oilseed rape crops as soon as soil conditions permit.

It is now too late to sow winter wheat and unless conditions improve rapidly the window for sowing spring beans is getting very tight.

Beans require a minimum of six months from sowing date to harvest so unless you are prepared to gamble on an October harvest, beans must be sown by the end of this week.

PJ Phelan is a tillage advisor based in Tipperary and is a member of the ACA and ITCA

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