It won’t be news to readers that weather is one of the biggest factors affecting the efficient use of fertilisers.
There can be too much rain or too little rain. It can be too cold or too hot. But on the positive side, Ireland’s temperate climate is one of our best assets.
Ireland has a climate that can grow high grass yields and produce some of the highest yields of cereals in the world but, as we all know, the weather varies hugely throughout the season and targeting fertiliser at appropriate times is an essential element of that success.
Nitrogen and phosphorus are the two elements of greatest concern regarding losses as they are currently the most expensive nutrients to purchase. So, ensuring efficient use and being confident in rate reduction is essential.
Watching the weather forecast is also essential. Otherwise money could literally be washed down the drain
With this in mind, it is of paramount importance that we target fertiliser application at the most appropriate timing possible. But watching the weather forecast is also essential. Otherwise money could literally be washed down the drain.
As we hear more and more about fertiliser reduction under the Farm to Fork strategy and Ag Climatise, we also hear more about nitrous oxide, ammonia and leaching. In this article, we take a look at some of the loss pathways of these nutrients and use this knowledge to help reduce those losses.
Phosphorus (P) is lost mainly through sediment (fine soil) loss into watercourses, as P attaches itself to soil particles. The main method of P loss is overland flow. Wet soils with poor permeability, along with heavy rain, create ideal conditions for P loss.
The new regulations under the nitrates directive aim to prevent sediment and P loss by cambering roadways away from watercourses
Overall, the majority of Irish soils are fairly susceptible to P loss, but there is huge variation in the capacity for loss across soil types.
One of the main ways to prevent P loss is to avoid application of organic manures or artificial fertilisers prior to heavy rainfall. The new regulations under the nitrates directive aim to prevent sediment and P loss by cambering roadways away from watercourses. For example, if animals or tractors leave debris on the roadway, it can easily wash into a watercourse. This is also a reason for moving water troughs away from waterways.
Preventing nitrate loss
Nitrate can also be lost with heavy rainfall, as it moves easily through the soil. It is mobile in the soil and is therefore easily leached away, and is also susceptible to runoff. However, if this nitrate is available when the crop needs it most for growth, then the risk of loss to the environment is much lower. This is why it is important to spread fertiliser, particularly nitrogen, when it is in greatest demand.
However, leaching can still occur during periods of high demand if recently spread nitrogen is followed by heavy rain.
It is important to remember that spread urea can also be leached
Large amounts of rain in a short period can cause nitrogen to be leached immediately, whereas steady and low levels of rain can assist with nutrient deployment and crop growth. Leaching is also much more likely when land is at field capacity.
It is important to remember that spread urea can also be leached, especially where heavy rain is on the way and temperatures are low.
Reducing nitrogen gas emissions
Ammonia emissions are receiving a lot of attention at the minute, and plans to reduce them include the use of low-emission slurry spreading and handling equipment and protected urea in the case of artificial fertilisers. However, neither of these can displace good application practice when spreading.
Ammonia is lost through volatilisation. This generally occurs when temperatures rise, but can decrease when soil moisture levels rise.
Where lime has been spread on land, urea should not be spread for at least three months
However, research has shown that loss can occur when temperatures are high and soil moisture levels allow the fertiliser to dissolve, but where moisture level is not high enough to prevent losses to the atmosphere.
Soil pH can also affect volatilisation. Where lime has been spread on land, urea should not be spread for at least three months. However, urea can be spread before lime application. Teagasc recommends 10 days between urea application and lime application.
Nitrous oxide losses generally occur where CAN is spread and the loss comes about as a result of denitrification. Low levels of oxygen in the soil can increase the risk of this loss where nitrate converts to nitrous oxide, so once again, applying fertiliser when it is needed most can help prevent losses.
As a decrease in oxygen levels can cause denitrification, the process is likely to increase where land is wet or heavy rain occurs. This is why CAN is not used early in the season for grass growth, but it should not be used in high temperatures either.
Increases in soil temperatures can lead to an increase in denitrification, which is one reason why fertiliser should not be applied in high temperatures.