Soil quality monitoring in New Zealand: trends and issues arising from a broad-scale survey

Interpretation of national-scale surveys is needed to determine long term impacts of land use on soil quality. We report the findings of a survey of soil quality in New Zealand conducted 1995–2001. The survey covered 511 sites representative of 98% of New Zealand’s land area. Land uses included: arable cropping and horticulture, mixed cropping, drystock pasture (sheep, beef or deer), dairy pasture, tussock grassland, plantation forestry, and indigenous forest, occurring on 12 soil orders. At each site, land-use and soil profile were described, and the topsoil (0–10 cm depth) analyzed for total C, total N, anaerobically mineralisable N, pH, Olsen P, bulk density and macroporosity. Pastoral land and indigenous forests had similar total C contents (56–67 mg cm−3). Mean N contents under pastures were 4.3–5.9 mg cm−3, whereas all other land uses were <3.5 mg cm−3. Olsen P was much greater under pastures and cropping (44–49 mg cm−3) than under plantation and indigenous forests (ca. 10 mg cm−3). Indigenous and plantation forest soils were the most acidic (mean pH 5.4), and cropping soils the most alkaline (mean pH 6.2). Indigenous forests had the lowest bulk density (0.76 Mg m−3), and mixed cropping soils the highest (1.22 Mg m−3). Macroporosity was variable (9.3–25.6% (v/v)). Overall, 80% of the soil properties fell within target ranges identified as desirable to maintain soil quality for production and environmental objectives. There was widespread but moderate compaction under pastures and cropping, depletion of total C under cropping, and nutrient imbalance (usually excess P) under cropping and dairy pastures. The study identified soil, land use and management combinations of concern for loss of soil quality, and provided benchmark data against which to measure change.