Protecting peat land pays in many ways

When managed properly, peat soils can also be a valuable, productive resource, but it must be acknowledged that use of these soils inevitably leads to peat loss and shrinkage through oxidation.

A key to successful long-term farming on peat soils is finding the right balance between keeping the water table low enough for production but high enough to minimise peat loss.

The Waikato region has about half of New Zealand's peatlands, with about 94,000ha of peatlands containing about 2.7 billion cubic metres of peat.

Drainage has greatly improved our ability to farm these lands but too much drainage can lead to increased shrinkage of peat soils and other environmental consequences.

When peat is drained, the carbon in the soil is exposed to air. The carbon is then able to bind with oxygen in the air to form carbon dioxide, a noted greenhouse gas. It's estimated that developed peatland releases about 1.3 million tonnes of carbon dioxide each year.

Drainage of peatlands for production can also cause a reduction of water levels in neighbouring wetlands and peat lakes, which are at-risk natural ecosystems.

And as peat shrinks the depth of fertile topsoil also decreases. This means that further drainage, cultivation and pasture renewal are needed to maintain productivity, increasing the cost to farmers and the impacts on the environment.

If we don't manage our peat carefully, it will continue to shrink until eventually there will be no peat left; a unique and valuable resource will be lost forever.

In some areas, the underlying soils that landowners will be left with may have poor fertility, requiring high inputs to maintain productivity. The flood risk and pumping costs in the low lying areas may increase substantially.

So how can we best strike a balance between keeping the water table on peatlands low enough for production but high enough to minimise peat losses?

The right drain depth is one of the keys. Deep drains in peat cause over-drainage and rapid subsidence of peat soils. As the peat dries it shrinks and cracks, making soils difficult to re-wet. Rainwater flows down into the subsoil through cracks in the peat. When peat dries it becomes waxy and doesn't reabsorb water easily. By keeping drains shallow, we can keep the water table high enough to protect the peat soil.

Keeping the water table high in drier periods is important for pasture growth and maintaining soil quality, for peat and mineral soils. This can be achieved by putting weirs or stopgates in the drains.

Better water table management will minimise shrinkage, allowing for extended summer grass growth and profitable farming of peat soils for longer.

Controlling weeds in drains and fencing them off to exclude stock reduces the cost of machine cleaning drains, which can also lead to them deepening.

Most silt in drains comes from stock damaging the drain banks. By fencing off your drains you'll greatly reduce the need to machine-clean them.

Keeping cultivation of peat soils to a minimum is also important. Peat is naturally anaerobic (has no oxygen present) and is very acid (soil pH <4.5). Cultivation creates a surface layer of aerated peat which accelerates oxidation and loss of the soil. The pH can be corrected by adding lime, but cultivation causes peat to shrink twice as fast as it does under pasture. So the less cultivation you do, the longer your peat soil will last.

The peat farmers' group, with AgResearch and other stakeholders, is organising a field day on peat soil management on April 12 at Orini, north of Hamilton. Waikato Regional Council will provide further details on this soon.

• Bala Tikkisetty is a sustainable agriculture advisor at Waikato Regional Council. Tel. 0800 800 401