Soil quality for dairy and converted dairy sites in the Auckland region 2014 changes after 18 years

E Chibnall, F Curran-Cournane
Auckland Council Research and Evaluation Unit, RIMU
Publication date:  

The land and soil in the Auckland region are important, valuable and non‐renewable resources. Soil supports the growing population by providing food, a place to live and work, and recreational and tourism opportunities. Some soil and landform combinations also have cultural and historical significance to different groups of people. Auckland Council is required by the Resource Management Act 1991 section 35 to report on the state of the environment in part, or in whole, no longer than every five years. Monitoring of soil samples collected at a variety of land use sites is conducted every five years by the Auckland Council which concentrates on sampling one land use per year. Auckland Council monitors soil quality to determine any changes in the ability of the soil to sustain biological production, maintain environmental quality or promote plant and animal health.
Dairy sites were the focus of 2014 soil sampling. Land use in rural Auckland is undergoing rapid change, with much rural land being converted to residential and lifestyle living. While the number of dairy farms in Auckland has been decreasing over the last 14 years in Auckland, dairy farming has become more intensive with growing herd sizes.

Twenty dairy soil sites were first identified and sampled between 1996 and 2000 (i.e. <2000) and subsequently repeated in 2009. These 20 dairy and ‘converted‐dairy’ sites were sampled again in 2014, adding to an increasingly valuable dataset for determining changes in soil quality. The ‘converted dairy’ sites included those farms that converted to drystock, rural lifestyle living and horse farming activities in rural Auckland. The 2014 sampling period also included sampling at an additional 5 new dairy sites across the region contributing to a total 25 dairy and ‘converted dairy’ sites as part of the annual soil quality monitoring programme.

Twenty‐percent of all sites (n=5) sampled in 2014 failed to meet one soil quality indicator with 40% (n=10), 32% (n=8) and 8% (n=2) failing to meet two, three and four indicators, respectively. The soil quality indicator of most concern was soil macroporosity (‐10 kPa) with 23 sites falling below the recommended macroporosity guidelines. Macroporosity measures the large pore space in soil and is an indicator of soil compaction. Compact soils are more prone to pollutant loss to surface waters via surface runoff and it can also affect pasture yields.

Concentrations of Olsen P and total nitrogen were also of concern with 60% and 40% of sites exceeding the upper recommended limits, respectively. When in excess of crop requirements, both phosphorous and nitrogen can be lost from soil to water via surface runoff and cause increased risk of eutrophic conditions. There were significant differences between mean concentrations of Olsen P, acroporosity (‐5kPa) and bulk density when the three sampling periods were compared. Of these changes, continuous low soil macroporosity was the indicator of most concern. With increasing land use change it is important to continue to monitor the quality of long term soil sites and to investigate potential changes over time.

Auckland Council technical report, TR2015/020

November 2015