Soil

Climate change is projected to alter weather patterns in Waitaha including more extreme weather events such as storms, high winds and droughts.

Changes such as these could exacerbate natural soil erosion processes in many parts of the region as well as increase the risk of human-induced soil erosion.

The forestry sector is particularly affected by extreme weather events and can worsen the effects of extreme weather events on soil. In recent years, heavy rain inland of Tolaga Bay to the north of Gisborne/Tai Rāwhiti mobilised a large volume of sediment and wood debris. The damage caused significant deposition of sediment and wood debris over farmland as well as damage to property and infrastructure. Events of this scale have not occurred in the Waitaha region, although smaller scale events such as at Gebbies Pass, Banks Peninsula/Horomaka have occurred.

Soil loss can be prevented through a range of actions such as planting or retaining vegetation on sloping land, preventing livestock access to riverbanks, and adopting improved cultivation and livestock grazing strategies. Permanent forestry can also protect against erosion, once it is established, and can stabilise erosion prone land.

The Soil Conservation and Revegetation Programme (SCAR) is an initiative that aims to reduce erosion and sediment loss, and stabilise damaged hill country land in North Canterbury/Waitaha.

Cadmium is a soft heavy metal that is toxic at certain concentrations and can contaminate food and fodder crops when these are grown in soils with elevated cadmium levels.

This can negatively impact human and livestock health as well as international trade if food safety standards are exceeded.

Increasing concentrations of cadmium in soil are cumulative and persist for the foreseeable future which can limit the versatility of soils for different land uses. It is uncertain whether the methods available to address this are feasible.

Sources of cadmium in soil

Cadmium is a natural impurity in phosphate rock used to produce phosphate fertiliser and its concentration varies geographically. Phosphate fertilisers are the primary source of cadmium in soils, but it can be added through other sources such as compost, irrigation water, dairy farm effluent, or be present due to natural processes.

Between 2001 and 2005, approximately 30 tonnes of cadmium per annum were added to agricultural soils in New Zealand/Aotearoa through phosphate fertiliser use.

Land use is a key driver of cadmium concentrations in soils. Soils subject to intensive agriculture that require high phosphate inputs, such as dairy farming, tend to have higher cadmium concentrations compared to other land uses. Other land uses with higher than the national average are the production of kiwifruit and berries, orchards, market gardens, beef farms, and drystock pasture.

Cadmium concentrations in Waitaha

Cadmium concentrations in Waitaha soils vary across the region. From 2000 to 2020, 40 percent of monitored arable and pastoral sites showed increases in soil cadmium, while 11 percent decreased and 49 percent showed no change. As of 2020, all monitored arable and pastoral sites had soil cadmium concentrations within acceptable levels.

Research trials from 2015 to 2017 found some potatoes grown throughout the region exceeded food safety standards for cadmium despite being grown in soils that did not have unacceptably high cadmium concentrations. The current recommended management strategy for potato growers in Waitaha is to grow varieties which take up low levels of cadmium. The reasons for these exceedances are not clear and more information is needed to understand the causes and extent of these elevated cadmium concentrations in some food crops.

Extracting a soil core

Soil compaction refers to the compression of open pore spaces in soils through which gases, water and dissolved nutrients move. Slower infiltration and movement of water in compacted soils has the potential to lead to faster overland flow runoff.

Soil compaction also increases resistance to plant root growth, and this can impact on productivity and increase susceptibility to soil erosion.

Emissions of nitrous oxide, a greenhouse gas, are known to increase from compacted topsoil.

Heavy machinery trafficking and livestock treading can cause soil compaction, and the use of irrigation can lower the resistance of soils to it. Topsoil compaction is particularly evident under irrigated dairy pastures.

Monitoring soil compaction

We monitor for the symptoms of soil compaction by looking at soil properties such as bulk density, macro-porosity, and penetration resistance. Our arable and pastoral soil quality monitoring programme tells us that soil structure changes are an issue for our lowland soils in arable and pastoral use. A high proportion of our lowland monitoring sites indicate increased vulnerability to becoming less permeable, water erosion, and contaminant run-off.

Our arable and pastoral soil quality monitoring programme also tells us that nearly half of our continuous cropping monitoring sites and a quarter of our mixed arable monitoring sites show degraded levels of soil organic matter. Soil organic matter is vital for many soil functions including soil structure and the capacity of soils to hold onto nutrients and water, retaining them until they are needed by plants and organisms. Low levels of soil organic matter in arable land translate to soils being more vulnerable to structural degradation, a reduced ability to effectively cycle and store nutrients and increased reliance on inorganic fertiliser inputs to reach target yields.

The most recent trend analysis of monitoring data suggests that topsoil structure has been declining in arable and pastoral lowland soils. Researchers have picked up on the widespread compaction of pastoral soils in New Zealand/Aotearoa, and are seeking to better understand its effects, review potential mitigation practices, and identify knowledge gaps.

Climate change projections

It is challenging to predict the likely impacts of climate change on the soil and vegetation of the region, as it is quite probable some parts of the region will get drier and others wetter.  Climate change has the potential to increase both droughts and storm events in the region and both have the potential to affect soil health.