Soil Quality and Trace Element Monitoring in the Waikato Region 2009

Report: TR 2011/13
Author: M Taylor

Abstract

This report provides an assessment of the current soil quality status of the soils of the Waikato region and interpretation of changes in soil characteristics over the last 7 years and reports on the trial of trace element indicators of defuse contamination.  The soil quality indicators meeting or not meeting targets are considered separately from the assessment of trace element indicators.

Results in 2009 showed that 26 per cent of sites meet soil quality targets. Of the rest, 34 per cent failed to meet one target and 40 per cent failed to meet two or more targets.  The land use meeting most targets was horticulture, followed by forestry and cropping. Dairy and other pasture had the lowest proportion meeting targets and the highest proportion failing to meet two or more targets, both as a proportion of sites and as land area.

There has been some improvement in meeting macroporosity (surface compaction) targets by pasture and in meeting lower (deficient in phosphorous) Olsen P targets by pasture and forestry over the last 7 years.  Conversely, the proportion of sites not meeting nitrogen targets is increasing.  While the proportion of sites meeting all existing indicator targets has increased over the last 7 years and the proportion of sites not meeting one indicator has decreased, the proportion of sites not meeting two or more indicator targets is static or increasing slightly.

Five key issues that cause loss of soil resource were distinguished.  One of these issues, soil compaction, shows a steady improvement in meeting targets but 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.  Loss of soil organic matter continues with a decline in average total C concentration from 9.9 per cent to 9.4 per cent over the last 7 years.  Much of this decline was from sites under annual cropping land use while, several indicators point to loss of soil organic matter from pine forest to pasture conversion during the conversion process. 

Excess nutrients, such as nitrogen and phosphorus are trending upwards or are stable at best under cropping, horticulture and most pasture sites.  Conversely, some forestry, conversion pasture and other pasture sites showed deficient nutrients, reflecting low carbon status (to hold nutrients) or worsening economic factors. 

A large proportion of forestry sites have high erosion risk, especially if the trees are removed.  It is a management practice to leave erosion prone soils in native bush or planted in production forestry to manage erosion.  In addition, some annual cropping and a few horticulture and pasture sites have a higher risk of eroding, especially between crops or at resowing when the land is bare and/or is sloping (> 7°).

Trialling of indicators for trace element monitoring clearly identified accumulation of diffuse soil contaminants. Cadmium (under horticulture, dairying and other pasture) and zinc (under dairying and other pasture) exceeded targets and the proportion doing so was increasing.  An unexpected observation was the significantly higher and upwards trending concentrations of chromium, mercury and nickel in soil under annual cropping.  These three elements remain well below targets but the cause of the accumulation needs to be identified before targets are exceeded.  Other diffuse contaminates, inked to fertiliser and agricultural chemical use, such as arsenic, copper and fluoride exceeded targets under annual cropping, horticulture, dairy pasture and other pasture land uses but trends are not yet clear due to insufficient data.  Lead had concentrations that were not of concern and levels are static or declining.

The soil quality status of soil in the Waikato region has improved in relation to the issue of compaction, although compaction remains a priority issue due the amount of land affected and its off-site impacts.  Similar improvements in relation to loss of soil organic matter, excessively high fertility levels, erosion risk, and accumulation of contaminants was not observed, and continuing efforts in education and enforcement are needed

Soil Quality and Trace Element Monitoring in the Waikato Region 2009 [PDF, 964 KB]