Nutrients and Peat Soils Research

Peat soils have been classified as high risk for effluent irrigation but some farmers dispute that. Little research has been done on the fate of effluent nutrients applied to peat soils, so a Sustainable Farming Fund project has begun in the Waikato looking at N and P movements. Initial trials have shown a dramatic response to N on raw peat soils but a lesser response on consolidated peat. Leaching losses will be examined over winter and spring.

Dairy farmers are being faced with strict rules around effluent irrigation and requirements for storage during wet weather when irrigation would result in excessive runoff and loss of nutrients to groundwater and waterways. Peat soils are generally classified as being high risk, meaning that storage ponds and effluent irrigation areas are large and therefore expensive. However, some farmers on peat disagree with the blanket “high risk” categorisation, arguing that peat soils vary considerably in content and character depending on the parent raw material and on how long the area has been farmed. Some areas have been in pasture for 50 years or more and it is argued that these consolidated peat soils behave more like mineral soils and constitute a less risky irrigation environment.

Raw peat soils are very high in fibrous organic matter, very acid and are subject to a fluctuating water table that is high in winter and spring. Consolidated peat that has been farmed for some decades is less acid, has more biological activity in the top 25mm and is generally well drained and may be less subject to high water table levels for long periods.

Ian Taylor, a farmer at Gordonton near Hamilton, argues that consolidated peat is not high risk and so does not require large effluent storage facilities.

“About three years ago I was looking at putting in an effluent lagoon to meet the new regulations and I found that my shallow peat soil was classified as high risk and so I was going to have to build a very large and expensive lagoon. So I got in touch with a soil scientist from AgResearch and we dug a few holes and he said it appears that your farm is not a high risk soil,” says Ian.

“There has been very little detailed research done on peat soils so by default all peat farms are classified as high risk. My view was that the consolidated peat on my farm was more like a peaty loam and was low risk. We agreed that research needed to be done to sort it out.”

Together with representatives from AgResearch, DairyNZ, Landcare Research, the Waikato Regional Council and Ballance Agri-Nutrients they put a research proposal to the MPI’s Sustainable Farming Fund and were eventually given a grant to look at the ways nutrients from fertilisers and effluent move through different peat soils and the ongoing contribution and effects of those nutrients reaching smaller waterways and possibly ending up in major rivers.

The farmer-led project started last July and involves a number of farms on consolidated peat that have been developed for over 40 years. These are being contrasted with Orini Downs Station where the soils are younger, the peat is fairly raw and has a lot of fibrous material.

The first step was to carry out some nitrogen response trials to determine the effect on pasture of increasing amounts of N – zero, 25, 50, 75 and 100 kgN/ha were applied. Dr Gina Lucci of AgResearch, the soil scientist leading the research, says that the results were surprising.

“We expected that as the application rate increased the response would taper off, and that certainly happened on the consolidated peat where there was a response at 25 and little more above 50 kg,” says Gina.

“However, on the raw peat site we continued to get a response up to 75 kgN. So it appears that the consolidated peat is behaving much more like a mineral soil where there is a significant background supply of N and pasture growth is already high. On the undeveloped peat you get much more bang for your buck from N fertiliser.”

The next step in the project is to look at leaching and losses to the drainage system. Currently the water table is at the seasonal low, perhaps 2m below the surface and so there is virtually no water flow to the drains.

“We plan to start this spring using lysimeters to measure nutrient losses from effluent through the soil profile and also measure nutrient levels in drains through to next winter. Theory suggests that P losses will be more of a problem than N but we need hard data to determine the true situation,” says Gina.

“We will also be looking at E.coli to see whether it moves differently through a peat soil than through a mineral soil. Ultimately we hope to tie all this information together in some sort of tool that farmers can test drive and give us feedback on.”

Concurrently, Landcare Research staff are analysing the physical and chemical characteristics of various peat soils to see how much they differ and to determine whether the raw material that the peats are formed from results in significant soil differences.

Ian Taylor hopes that the project will ultimately result in guidelines for best practice for application of both fertiliser and effluent on peat land.

“In terms of water quality we have four contaminants – nitrogen, phosphorus, sediment and microbes. We don’t think the sediment is too big an issue here because being flat country we don’t have as much run-off, and I suspect that our soils have reasonably high P retention so wouldn’t be leaching of a lot of P. Indications are that may be E.coli could be an issue but we need some hard science to confirm or deny it,” he says.

“One of the assumptions is that autumn is potentially the time of greatest leaching from our soils because the pasture is not growing so actively and so the root zone is not as active but I am sceptical about this and once again I guess we will get an answer once the research has been done.”