Soil quality monitoring in the Waikato region 2012

Report: TR 2015/02

Author: Matthew Taylor

About this report

This report provides baseline data and allows identification of the impacts of land use and associated key soil quality issues that have emerged over the last 10 years in the Waikato region. Waikato Regional Council monitors a variety of indicators to assess soil quality in the region to provide information about how particular land uses are affecting soils in the long-term as required by the Resource Management Act 1991 Section 35. Indicators are measurements of soil physical, chemical and biological properties that assess the ability of soil to perform the functions necessary for its intended and foreseeable uses. If these measurements fall within the range of values that is desirable for the soil to function, the soil is meeting targets. There are seven soil quality indicators, which are: pH, total carbon, total nitrogen, mineralisable N, Olsen P, bulk density and macroporosity. In addition there are two environmental indicators, aggregate stability and C:N ratio, that provide information to improve interpretation of the results but are not counted as soil quality indicators.

Layperson explanation

What is it about?

This report provides baseline data and allows identification of impacts of land use and associated key soil quality issues that have emerged over the last 10 years in the Waikato region.

Waikato Regional Council monitors a variety of indicators to assess soil quality in the region, to provide information about how particular land uses are affecting soils in the long term (as required by the Resource Management Act 1991 Section 35).

Indicators are measurements of soil physical, chemical and biological properties that assess the ability of soil to perform the functions necessary for its intended and foreseeable uses. If these measurements fall within the range of values that is desirable for the soil to function, the soil is meeting targets.

There are seven soil quality indicators: pH, total carbon, total nitrogen, mineralisable N, Olsen P, bulk density and macroporosity. In addition two environmental indicators not counted as soil quality indicators, aggregate stability and C:N ratio, provide information to improve interpretation of results.

The following land use classes were sampled:

• dairy pasture (pasture grazed with milking cows)
• other pasture (pasture used for cut and carry or grazed with animals other than milking cows)
• cropping (annual cultivation)
• horticulture (plants left in place)
• forestry (production pine forests)
• native (indigenous vegetation).

An additional land use class called “forest-to-pasture” was defined to encompass sites where the land use has recently changed from production forestry to dairy or other pasture.

Overall, soil quality in the region is declining for the 151 monitored sites. Results in 2012 showed:

• 13% of sites (14% of sites corrected for the amount of land) meet targets
• 33% of sites (30% of sites corrected for the amount of land) failed to meet one target
• 54% of sites (56% of sites corrected for the amount of land) failed to meet two or more targets.

The land use meeting most targets was production forestry (53% of sites). Dairy pasture and other pastoral land uses had the lowest proportion of sites meeting targets (2 and 0% of sites) and the highest proportion of sites failing to meet two or more targets (73 and 67% of sites).

Four key issues contributing to the degradation of the quality of the soil resource in the Waikato region were identified:

• surface compaction
• loss of soil organic matter
• excessive nutrients (above the needs of production)
• erosion.

Overall, surface compaction reduced steadily (improved) between 2003 and 2009, but declined markedly after 2009, and the rate of decline is increasing. This may be attributed to greater intensification, and the wet winter/spring periods over the last three years increasing vulnerability of land to compaction. Surface compaction remains a priority issue due to the large area of land affected and potential off-site effects including flooding, erosion, transport of contaminants, and sedimentation, and the continued decline in meeting targets.

However, soils recently converted from forest to dairy or other pasture, which were compacted by vehicles and thus damaged during the conversion process, have improved from 86% to 100%, meeting the target. This improvement can be attributed to improved pasture growth, with root expansion opening up the soil, while vegetation cover protects the surface from rain impact and reduces the pressure from animal hooves. These results show that these soils can be farmed without compaction being an issue.

Overall, loss of soil organic matter continued with a decline in average total C concentration from 9.9% to 9.5% since 2003 (equivalent to the loss of 7.2 Mt of carbon from the region). Much of this decline was from sites under annual cropping land use. In comparison, soil organic matter is increasing in soils recently converted from forest to dairy or other pasture. In 2009, these soils had total C and N concentrations significantly lower than those under other land uses. This is no longer the case as both total C and N concentrations have increased in the converted soils. If left undisturbed organic matter is expected to continue to increase in these soils to levels found under permanent dairy or other pasture.

In all land uses where fertiliser is applied, levels of nutrients, such as nitrogen and phosphorus are stable or trending upwards. Other pasture sites showed deficient nutrients.

A large proportion of forestry sites have high erosion risk because production forests tend to be situated on steeper land, especially if the trees are removed. The proportion of forestry sites meeting targets has recently increased with the conversion of some erosion-vulnerable forest sites to dairy pasture. However, the increased compaction may result in increased transport of contaminants and peak-flows causing localised flooding and bank erosion. In addition, some of the forest to dairy pasture sites still had bulk density measurements below targets, which indicates the soils are light and are easily eroded by wind or water. As a result, these sites may have a higher risk of eroding, especially between crops or at re-sowing when the land is bare and/or is on sloping ground.

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Soil quality monitoring in the Waikato region 2012 [PDF, 1.4 MB]