How soil types can impact nitrate levels on farms
Typically Higher Heavy Lower And Will Have Levels, Clay Phosphorous 'our Soils Levels ' Nitrate
Some of the mysteries of agricultural water pollution were explained by Teagasc Agricultural Catchment Specialist, Eddie Burgess, on a recent Environment Edge podcast.
“The type of farm that is most problematic for nitrate probably doesn’t have a watercourse on it at all, because in a free-draining soil type, when it rains, the water percolates down through the soil, and there aren’t streams or ditches on it, and because the soil is so free-draining, that’s where nitrate is more than likely going to be coming from,” Eddie said.
“You could have a river going by your farm, and it could look in good condition. It could be deemed to be in good condition by the Environmental Protection Agency, but you could be losing a lot of nitrates,” remarked podcast host Cathal Somers.
The Agricultural Catchments Programme (ACP) is funded by the Department of Agriculture, Food and the Marine, and implemented by Teagasc in six small catchment areas countrywide.
Only one such catchment is being studied in Munster, the Timoleague catchment of eight square kilometres, from Ardgehane in Barryroe to Barryshall, just west of Timoleague. It is representative of some of the most intensively farmed dairying areas in Ireland. It is predominately grassland, on free-draining soil.
“Our free-draining catchments typically have high nitrate levels in them, and that’s not a reflection of the overall nitrogen application rate," said Eddie Burgess. "It’s not a reflection of the nitrogen use efficiency, it’s a reflection of the soil type”.
“Our heavy clay soils will typically have higher phosphorous levels, and lower nitrate levels. Typically, if you have a high clay content soil, a heavy soil, it will get wetter quicker, and most of the water ending up in the stream gets there via an overland flow pathway, because the soils are wet.
"Also, if the soils are wet, the bacteria in the soil are the anaerobic type which live on nitrate as a source of oxygen, so they’re denitrifying bacteria. They mitigate nitrate loss, if there’s a source of carbon, and temperatures are warm,” Eddie explained.
“In contrast to that, on a free-draining soil type, when it rains, most of the water ending up in the stream gets into it via groundwater-fed springs, and the water percolates through the soil before it ends up in the river.
"By doing that, any phosphorous that is being carried by that water is bound by the clay in the soil, and by the time the spring feeds into the stream, the phosphorous that’s left is a very, very small amount.”
The soil type greatly influences nutrient loss through water.
“Naturally, farm practice and farm intensity have an influence on the amount of nutrient that’s available to be lost, but how much actually gets lost is more often than not decided by the weather and the soil type,” Eddie explained. Those influences override the farming intensity.
Farmer participation in the ACP is voluntary. Initially, some farmers hesitated. “Landowners and farmers were apprehensive and cautious,” Eddie said. “We wouldn’t have been able to get established in any of the six catchment areas without the existing Teagasc advisory relationship that was built up in each of them”.
“There is an increasing interest in water quality. There’s a long way to go,” Eddie said.
“Another name for a catchment is a watershed. A catchment is like a bowl. It’s defined by the highest point around the edge, and all the water leaving a catchment comes from the rainfall that falls within that bowl shape or catchment area.
"It either leaves via a stream flowing out of the catchment, it leaves via evapo-transpiration from the leaves of plants that are growing in the catchment, or it could leave via groundwater flowing out underneath.”
Catchment scientists monitor water where it leaves the catchment, but also monitor groundwater, soil water, and the soils in every two hectares. "We do that continuously, and have been doing that for the last 15 years,” Eddie explained.
This long-term data set provides comprehensive understanding of the processes that influence water quality.
“We measure the quantity of nutrients in the water leaving the catchment. We also measure the volume of water. When you multiply the amount of water, how many cubic metres of water left over a given time period, by the concentration, we can calculate how many kilograms and how many tonnes of sediment, of nitrogen or phosphorous left the catchment.”
Specific nutrient losses, such as P per hectare can be determined, or, N and P nutrient losses over the closed period can be measured, and compared to the losses of those nutrients during the year as a whole.
“Typically, year in, year out, between 45% and 50% of the annual phosphorous that leaves the catchment does so during that closed period, which is only 25% of the year. And the same for nitrogen,” Eddie explained.
“The soil conditions during the closed period are usually at capacity. The soils are wet. Even if there hasn’t been rainfall, the day-length is shorter, and there aren’t the same evapotranspiration rates.
“Water quality is a measure of the conditions for supporting life at that particular location. Nutrients can affect that, sediment can affect it. Your nutrient and sediment could be pristine, but there could be a problem with a pesticide getting in.”
The shape of the river (its hydro-morphology) can also be a factor.
