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Chemical and biological soil health results

Organic matter

Total organic carbon (a measure of soil organic matter) was lowest on cropping land and on tussock grasslands. Arable and mixed cropland soils often have lower organic matter than soils under pasture because the cropland soil is regularly cultivated.

On a national scale, horticulture in New Zealand (including vineyards, orchards and perennial crops) covers 1.6 per cent of the total land area, compared with 24 per cent in Europe, making environmental effects from cropping in New Zealand comparatively localised.

Because tussock grassland grows on drier soils, they are naturally low in organic carbon. Drier soils build up organic matter very slowly because the growing conditions result in lower productivity.

Monitored pasture topsoils all had high levels of organic matter in the top 10 centimetres of the soil (total carbon, total nitrogen and mineralisable nitrogen). This result was found to be similar or greater than organic matter levels under long-standing native vegetation. However, evidence in the soil profile down to 0.5 metres shows that over the past 25 years organic matter losses have occurred on intensively farmed pastures.

Nitrogen and phosphorus

Both total nitrogen and mineralisable nitrogen were greater under pastures than under other land uses. While a supply of mineralised nitrogen is essential for pasture productivity, there is a risk that when supply exceeds demand (when saturation is reached) any excess soluble nitrogen can be leached from the soil and adversely affect water quality. Excess soluble nitrogen is of particular concern given the high proportion of New Zealand land area in pasture.

The monitored dairy pastures and cropland soils exhibited high Olsen phosphate results (plant-available phosphate). This result reflects the regular application of superphosphate to New Zealand soils to stimulate the growth of introduced grasses, clovers, and arable crops.

Monitoring suggests continued soil compaction in pastures (macroporosity of less than 10 per cent) along with increased (possibly excess) levels of phosphate and nitrogen are likely to continue. Overall, results from soil health monitoring indicate that storage of nitrogen in organic forms, particularly in dairy pasture topsoils, is nearing saturation point.

Acidity

Soil pH (acidity or alkalinity) under exotic forests was found to be similar to that under native forests. Other agricultural land uses exhibit higher pH values (that is, they are more alkaline). Higher pH values under agricultural land reflect the widespread topdressing of lime following the clearance of forest land cover and the change in land use to more intensive pastoral farming.

What can you do?

  • Visit www.landcare.org.nz for information about sustainable land management and biodiversity initiatives in rural communities.
  • Composting kitchen and garden waste reinvigorates soil. Composting serves as a kind of carbon sink, storing carbon in the soil instead of in the atmosphere.

This information has come from the latest national state of the environment report Environment New Zealand 2007.

 

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Last updated: December 2007