Managing peat

The Waikato region contains about half of New Zealand’s peatland. Waikato peatlands cover approximately 94,000 hectares and contain about 2,700 million cubic metres of peat.

Most of this land has been developed. While drainage has greatly improved our ability to farm land profitably, too much drainage can speed up shrinkage of peat soils. A balance between drainage and cultivation is needed in order to protect our peat soils for future generations.

Peat forms from the build up of partially rotted plant material in wet environments. Waikato peatlands have taken over 18,000 years to form and are up to 11 metres deep.

Drainage has greatly improved our ability to farm peatlands, however too much drainage can lead to:

• increased shrinkage of peat soils
• large amounts of carbon dioxide (CO₂) being lost to the atmosphere – contributing to greenhouse gas emissions.

Waikato peatlands contain about 76 million tonnes of carbon. When peat is drained for development, the carbon in peat becomes exposed to air. The carbon is then able to bind with oxygen (O₂) in the air (oxidation) to form carbon dioxide gas (CO₂), a greenhouse gas. It’s estimated that developed peatland releases about 1.3 million tonnes of carbon dioxide each year.

Peat is a highly productive growing medium. Peat soils need drainage and cultivation to establish productive pastures and crops. However, this leads to irreversible shrinkage and oxidation (loss of carbon as carbon dioxide). Peat shrinkage is estimated to occur at about 20 cm per year after the initial cultivation, reducing to two centimetres per year as the peat becomes more compact and resilient.

Too much drainage of peatlands can also:

• cause shrinkage or loss of wetlands
• reduce water levels in neighbouring wetlands and peat lakes.

If we don’t manage our peat carefully, it will continue to shrink until eventually there will be no peat left.

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.

When managed properly peat is a valuable, highly productive resource. To be able to farm on peat soils long-term, farmers must find a balance between keeping the water table low enough for production, but high enough to minimise peat loss.

There are four key steps to maintaining a good water table in peat areas:

1. Avoid deep drainage

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, you’ll help keep the water table high enough to protect your peat.

2. Maintain the water table over summer

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

Water table management can be difficult to achieve on an individual farm. This is because groundwater is a resource that spans property boundaries. It may be useful for you to get together with your neighbours to discuss summer water table management. Better water table management will minimise shrinkage, allowing you to extend summer grass growth and farm your peat soils profitably for longer.

3. Fence drains and spray weeds

Controlling weeds and fencing drains to exclude stock reduces maintenance costs associated with machine cleaning drains.

Weeds should be sprayed in summer (January/February) before they seed. Note that glyphosate (‘RoundUp’) is the only herbicide approved for use over water.

Less machine cleaning of drains saves money. It also reduces impacts on water quality and the risk of drain deepening. Drains only need to be cleaned if their ability to function has been reduced by silt or weed growth.

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. A single electric wire is usually enough on most dairy farms.

4. Do not deepen drains during maintenance

Continual deepening leads to over-drainage, and makes pasture less productive. Keep your drains shallower and protect your valuable pasture.

Peat is naturally anaerobic (has no oxygen present) and is very acid (soil pH <4.5). Cultivation creates a surface layer of peat that’s aerated. When lime is added to peat, the pH is raised, making it less acidic. Aeration and lime make peat more suitable for a wider range of plants, such as pasture or crops.

However, 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.

When you do have to cultivate, use equipment that creates minimal disturbance. Try to:

• avoid chopping the peat too finely – this destroys the fibrous structure of the soil
• avoid using rotary hoes on peat soils
• try using disc ploughs, power harrows, chisel ploughs and paddle ploughs
• use no-till methods to renew pasture where possible – such as direct drilling.

Use your farm consultants to get the most up to date information on the best pasture species for your area. Some points to remember for peat soils:

• Maintaining a dense pasture sward is one of the best ways to protect your peat soil.
• Some herbicides behave differently on peat soils and may not be as effective.
• Overgrazing should be avoided – any bare patches of peat will shrink faster, resulting in an uneven surface.
• Minimise pugging during the wet winter months.