Peat soils need careful management

By Bala Tikkisetty

There are about 90,000 hectares of land covered by peat soils in the Waikato and about 80 per cent of this has been developed, mostly for farming.

Due to their low natural nutrient levels, effective use of fertilisers on these peat soils is necessary if peat soils are to be successfully farmed. It is very important to put on the right type of fertiliser at the right time and in the right amounts.

They also require different management practices than mineral soils to maximise their productivity, while farming peat land adjacent to lake and wetland areas requires very careful management to avoid damaging them.

Peat is formed from the remains of wetland plants in areas with high water tables. The high water table stops the rapid breakdown of the dead plant material. Consequently, peat soils have more than 20 per cent organic carbon in the topsoil.

While drainage and cultivation of peat is required to establish productive pasture, it leads to irreversible shrinkage of the peat and results in a continued subsidence of the land surface. Shrinkage is a result of consolidation following water removal and particle breakdown during cultivation, and also as a result of exposure to air. Shrinkage is estimated at about 200 millimetres per year after the initial cultivation, reducing to around 20mm per year as the peat becomes more consolidated.

As mentioned earlier, Waikato peat soils have low nutrient status. Applying the correct amounts of the right type of fertiliser will maintain good pasture for longer periods, reducing the need for frequent cultivation and pasture renewal. Lime is also required to increase the soil pH to an appropriate level for pasture and crop species. Soil and herbage tests are the ways to determine what nutrients your peat soil needs.

Timing of fertiliser application should aim to ensure plant uptake is maximised and any potential effects on the environment minimised. Fertiliser should be uniformly and evenly applied with none outside the target area. The precise placement of fertiliser depends on a number of factors. It requires careful integration of operator skills, sound equipment and appropriate formulation of fertiliser.

It is recommended farmers follow the Spreadmark Code of Practice for fertliser application, a programme governed by the Fertiliser Quality Council.

Another important issue is that development and drainage of peat soils can damage the nearby wetlands and peat lakes. The Waikato peat lakes are the largest remaining collection of such unique habitats in the country and have attracted international attention. Their unique ecosystems are highly dependent on the careful balancing of water levels.

Peat soils typically have a low anion storage capacity. This means the leaching of nutrients to ground water will be significant in peat soils. Increased leaching of nutrients can occur when water tables are near the ground surface.

The following strategies can be followed to minimise nutrient leaching when using peat soils for farming:

· Regular soil and herbage tests should be undertaken to match nutrient inputs to soil requirements.

· Use nutrient budgeting to ensure nutrient inputs match production and environmental goals.

· Apply nitrogenous fertilisers in split applications (not more than 30 kg of nitrogen per hectare in any one application).

· Avoid application if heavy rain is likely or when there are puddles on pasture.

· Allow a good buffer or margin between the fertilised area and the waterway.

· Because peat soils have a high water table, care should be taken when applying dairy effluent to pasture.

· If possible, use granulated fertilisers as they can be spread more evenly and accurately.

Riparian strips can also act as a filter to reduce the amount of contaminants that enter the water. Much of the loss of phosphorus to waterways occurs in association with sediment or dung.

Importantly, by having the right nutrient management programme for their property, farmers can reduce both cultivation and renewal costs, prolong the life of peat soils and help protect water quality.

· Bala Tikkisetty is a sustainable agriculture coordinator at Environment Waikato. Phone 0800 800 401 or email bala.tikkisetty@ew.govt.nz